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Chapter 6 Impacts andAdaptation

In its June 2001 report, the Committeeon the Science of Climate Change,which was convened by the National

Research Council (NRC) of theNational Academy of Sciences, con-cluded that “[h]uman-induced warmingand associated sea level rises areexpected to continue through the 21stcentury.” The Committee recognizedthat there remains considerable uncer-tainty in current understanding of howclimate varies naturally and willrespond to projected, but uncertain,changes in the emissions of greenhousegases and aerosols. It also noted thatthe “impacts of these changes will becritically dependent on the magnitudeof the warming and the rate with whichit occurs” (NRC 2001a).

SUMMARY OF THE NATIONAL ASSESSMENT

To develop an initial understandingof the potential impacts of climatechange for the United States during the

One of the weakest links in our knowledge is the connection between global and regionalpredictions of climate change. The National Research Council’s response to the President’srequest for a review of climate change policy specifically noted that fundamental scientif-ic questions remain regarding the specifics of regional and local projections (NRC 2001a).Predicting the potential impacts of climate change is compounded by a lack of under-standing of the sensitivity of many environmental systems and resources—both managedand unmanaged—to climate change. (See Chapter 1, page 6.)

Uncer ta in t ies in Regiona l and Loca l Pro jec t ions o f C l imate Change

While current analyses are unable to predict with confidence the timing, magnitude, orregional distribution of climate change, the best scientific information indicates that ifgreenhouse gas concentrations continue to increase, changes are likely to occur. The U.S.National Research Council has cautioned, however, that “because there is considerableuncertainty in current understanding of how the climate system varies naturally and reactsto emissions of greenhouse gases and aerosols, current estimates of the magnitude offuture warmings should be regarded as tentative and subject to future adjustments (eitherupward or downward)” (NRC 2001a). Moreover, there is perhaps even greater uncertaintyregarding the social, environmental, and economic consequences of changes in climate.(See Chapter 1, page 4, “The Science” box.)

Uncer ta in t ies in Es t imates o f the Timing, Magni tude, and Dis t r ibut ion o f Future Warming

82 ■ U . S . C L I M AT E A C T I O N R E P O RT 2 0 0 2

21st century, the U.S. Global ChangeResearch Program has sponsored awide-ranging set of assessment activi-ties since the submission of the SecondNational Communication in 1997.These activities examined regional, sec-toral, and national components of thepotential consequences for the envi-ronment and key societal activities inthe event of changes in climate consis-tent with projections drawn from theIntergovernmental Panel on ClimateChange (IPCC). Regional studiesranged from Alaska to the Southeastand from the Northeast to the PacificIslands. Sectoral studies considered thepotential influences of climate changeon land cover, agriculture, forests,human health, water resources, andcoastal areas and marine resources. Anational overview drew together thefindings to provide an integrated andcomprehensive perspective.

These assessment studies recog-nized that definitive prediction ofpotential outcomes is not yet feasible asa result of the wide range of possiblefuture levels of greenhouse gas andaerosol emissions, the range of possibleclimatic responses to changes in atmos-pheric concentration, and the range ofpossible environmental and societalresponses. These assessments, there-fore, evaluated the narrower questionconcerning the vulnerability of theUnited States to a specified range ofclimate warming, focusing primarily onthe potential consequences of climatescenarios that project global averagewarming of about 2.5 to almost 4ºC(about 4.5–7ºF). While narrower thanthe IPCC’s full 1.4–5.8ºC (2.5–10.4ºF)range of estimates of future warming,the selection of the climate scenariosthat were considered recognized that itis important to treat a range of condi-tions about the mid-range of projectedwarming, which was given by the NRCas 3ºC (5.4ºF). Similarly, assumption ofa mid-range value of sea level rise ofabout 48 cm (19 inches) was near themiddle of the IPCC’s range of 9–88 cm(about 4–35 inches) (2001d).

Because of these ranges and theiruncertainties, and because of uncertain-

ties in projecting potential impacts, it isimportant to note that this chapter can-not present absolute probabilities ofwhat is likely to occur. Instead, it canonly present judgments about the rela-tive plausibility of outcomes in theevent that the projected changes in cli-mate that are being considered dooccur. To the extent that actual emis-sions of greenhouse gases turn out to belower than projected, or that climatechange is at the lower end of the pro-jected ranges and climate variabilityabout the mean varies little from thepast, the projected impacts of climatechange are likely to be reduced ordelayed, and continued adaptation andtechnological development are likely toreduce the projected impacts and costsof climate change within the UnitedStates. Even in this event, however, thelong lifetimes of greenhouse gasesalready in the atmosphere and themomentum of the climate system areprojected to cause climate to continueto change for more than a century.Conversely, if the changes in climateare toward the upper end of the pro-jected ranges and occur rapidly or leadto unprecedented conditions, the levelof disruption is likely to be increased.Because of the momentum in the cli-mate system and natural climate vari-ability, adapting to a changing climateis inevitable. The question is whetherwe adapt poorly or well. With eitherweak or strong warming, however, theU.S. economy should continue to grow,with impacts being reduced if actionsare taken to prepare for and adapt tofuture changes.

Although successful U.S. adaptationto the changing climate conditions dur-ing the 20th century provides somecontext for evaluating potential U.S.vulnerability to projected changes, theassessments indicate that the challengeof adaptation is likely to be greater dur-ing the 21st century than in the past.Natural ecosystems appear to be themost vulnerable to climate changebecause generally little can be done tohelp them adapt to the projected rateand amount of change. Sea level rise atmid-range rates is projected to cause

additional loss of coastal wetlands, par-ticularly in areas where there areobstructions to landward migration, andput coastal communities at greater riskof storm surges, especially in the south-eastern United States. Reduced snow-pack is very likely to alter the timing andamount of water supplies, potentiallyexacerbating water shortages, particu-larly throughout the western UnitedStates, if current water managementpractices cannot be successfully alteredor modified. Increases in the heat index(which combines temperature andhumidity) and in the frequency of heatwaves are very likely. At a minimum,these changes will increase discomfort,particularly in cities; however, theirhealth impacts can be amelioratedthrough such measures as the increasedavailability of air conditioning.

At the same time, greater wealth andadvances in technologies are likely tohelp facilitate adaptation, particularly forhuman systems. In addition, highly man-aged ecosystems, such as crops and tim-ber plantations, appear more robust thannatural and lightly managed ecosystems,such as grasslands and deserts.

Some potential benefits were alsoidentified in the assessments. For exam-ple, due to increased carbon dioxide(CO2) in the atmosphere and anextended growing season, crop and for-est productivities are likely to increasewhere water and nutrients are sufficient,at least for the next few decades. As aresult, the potential exists for an increasein exports of some U.S. food products,depending on impacts in other food-growing regions around the world.Increases in crop production in fertileareas could cause prices to fall, benefit-ing consumers. Other potential benefitscould include extended seasons for con-struction and warm-weather recreation,and reduced heating requirements andcold-weather mortality.

While most studies conducted todate have primarily had an internalfocus, the United States also recognizesthat its well-being is connected to theworld through the global economy, thecommon global environment, sharedresources, historic roots and continuing

Impacts and Adaptation ■ 83

family relations, travel and tourism,migrating species, and more. As a result,in addition to internal impacts, theUnited States is likely to be affected,both directly and indirectly and bothpositively and detrimentally, by thepotential consequences of climatechange on the rest of the world. To bet-ter understand those potential conse-quences and the potential for adaptationworldwide, we are conducting and par-ticipating in research and assessmentsboth within the United States and inter-nationally (see Chapter 8). To alleviatevulnerability to adverse consequences,we are undertaking a wide range of activ-ities that will help nationally and inter-nationally, from developing medicinesfor dealing with infectious disease topromoting worldwide developmentthrough trade and assistance. Asdescribed in Chapter 7, the UnitedStates is also offering many types ofassistance to the world community,believing that information about andpreparation for climate change can helpreduce adverse impacts.

INTRODUCTION This chapter provides an overview of

the potential impacts of climate changeaffecting the United States. The chapteralso summarizes current measures andfuture adaptation and response optionsthat are designed to increase resilience toclimate variations and reduce vulnerabil-ity to climate change. The chapter is notintended to serve as a separate assess-ment in and of itself, but rather is drawnlargely from analyses prepared for theU.S. National and IPCC Assessments,where more detailed consideration andspecific references to the literature canbe found (see NAST 2000, 2001 andIPCC 2001d, including the review ofthese results presented in NRC 2001aand IPCC 2001a).

As indicated by the findings pre-sented here, considerable scientificprogress has been made in gaining anunderstanding of the potential conse-quences of climate change. At the sametime, considerable uncertainties remainbecause the actual impacts will dependon how emissions change, how the cli-

mate responds at global to regionalscales, how societies and supportingtechnologies evolve, how the environ-ment and society are affected, and howmuch ingenuity and commitment soci-eties show in responding to the poten-tial impacts. While the range ofpossible outcomes is very broad, allprojections prepared by the IPCC(2001d) indicate that the anthro-pogenic contribution to global climatechange will be greater during the 21stcentury than during the 20th century.Although the extents of climate changeand its impacts nationally and region-ally remain uncertain, it is generallypossible to undertake “if this, then that”types of analyses. Such analyses canthen be used to identify plausibleimpacts resulting from projectedchanges in climate and, in some cases,to evaluate the relative plausibility ofvarious outcomes.

Clear and careful presentation ofuncertainties is also important. Becausethe information is being provided topolicymakers and because the limitedscientific understanding of theprocesses involved generally precludesa fully quantitative analysis, extensiveconsideration led both the IPCC andthe National Assessment experts toexpress their findings in terms of therelative likelihood of an outcome’soccurring. To integrate the wide varietyof information and to differentiatemore likely from less likely outcomes, acommon lexicon was developed toexpress the considered judgment of theNational Assessment experts about therelative likelihood of the results. Anadvantage of this approach is that itmoves beyond the vagueness of ill-defined terms, such as may or might,which allow an interpretation of thelikelihood of an outcome’s occurring torange from, for example, 1 to 99 per-cent, and so provide little basis for dif-ferentiating the most plausible from theleast plausible outcomes.

In this chapter, which uses a lexiconsimilar to that developed for theNational Assessment, the term possible isintended to indicate there is a finitelikelihood of occurrence of a potential

consequence, the term likely is used toindicate that the suggested impact ismore plausible than other outcomes,and the term very likely is used to indicatethat an outcome is much more plausiblethan other outcomes. Although thedegree of scientific understandingregarding most types of outcomes is notcomplete, the judgments included herehave been based on an evaluation of theconsistency and extent of available sci-entific studies (e.g., field experiments,model simulations), historical trends,physical and biological relationships,and the expert judgment of highly qual-ified scientists actively engaged in rele-vant research (see NAST 2000, 2001).Because such judgments necessarilyhave a subjective component, the indi-cations of relative likelihood maychange as additional information isdeveloped or as new approaches toadaptation are recognized.

Because this chapter is an overview,it generally focuses on types of out-comes that are at least considered likely,leaving discussion of the consequencesof potential outcomes with lower likeli-hood to the more extensive scientificand assessment literature. However, it isimportant to recognize that there arelikely to be unanticipated impacts ofclimate change that occur. Such “sur-prises,” positive or negative, may stemfrom either (1) unforeseen changes inthe physical climate system, such asmajor alterations in ocean circulation,cloud distribution, or storms; or (2)unpredicted biological consequences ofthese physical climate changes, such aspest outbreaks. For this reason, the setof suggested consequences presentedhere should not be considered compre-hensive. In addition, unexpected socialor economic changes, including majorchanges in wealth, technology, or polit-ical priorities, could affect society’s abil-ity to respond to climate change.

This chapter first describes theweather and climate context for theanalysis of impacts, and then provides asummary of the types of consequencesthat are considered plausible across arange of sectors and regions. The chapter then concludes with a brief


summary of actions being taken at thenational level to learn more about thepotential consequences of climatechange and to encourage steps toreduce vulnerability and increaseresilience to its impacts. Although thefederal government can supportresearch that expands understandingand the available set of options and thatprovides information about the poten-tial consequences of climate change andviable response strategies, many of theadaptation measures are likely to beimplemented at state and local levelsand by the private sector. For these rea-sons and because of identified uncertain-ties, the results presented should not beviewed as definitive. Nonetheless, themore plausible types of consequencesand impacts resulting from climatechange and the types of steps that mightbe taken to reduce vulnerability andincrease adaptation to climate variationsand change are identified.

WEATHER AND CLIMATE CONTEXT

The United States experiences awide variety of climate conditions.Moving across from west to east, the cli-mates range from the semi-arid and aridclimates of the Southwest to the conti-nental climates of the Great Plains andthe moister conditions of the easternUnited States. North to south, the cli-mates range from the Arctic climate ofnorthern Alaska to the extensive forestsof the Pacific Northwest to the tropicalclimates in Hawaii, the Pacific Islands,and the Caribbean. Although U.S. soci-ety and industry have largely adapted tothe mean and variable climate condi-tions of their region, this has not beenwithout some effort and cost. In addi-tion, various extreme events each yearstill cause significant impacts across thenation. Weather events causing the mostdeath, injury, and damage include hurri-canes (or more generally tropicalcyclones) and associated storm surges,lightning, tornadoes and other wind-storms, hailstorms, severe winter storms,deep snow and avalanches, and extremesummer temperatures. Heat waves,floods, landslides, droughts, fires, land

subsidence, coastal inundation and ero-sion, and even dam failures also canresult when extremes persist over time.

To provide an objective and quantita-tive basis for an assessment of the poten-tial consequences of climate change, theU.S. National Assessment was organizedaround the use of climate model scenar-ios that specified changes in the climatethat might be experienced across theUnited States (NAST 2001). Ratherthan simply considering the potentialinfluences of arbitrary changes in tem-perature, precipitation, and other vari-ables, the use of climate model scenariosensured that the set of climate condi-tions considered was internally consis-tent and physically plausible. For theNational Assessment, the climate scenar-ios were primarily drawn from resultsavailable from the climate models devel-oped and used by the United Kingdom’sHadley Centre and the Canadian Centrefor Climate Modeling and Analysis. Inaddition, some analyses also drew onresults from model simulations carriedout at U.S. centers, including theNational Center for AtmosphericResearch, the National Oceanic andAtmospheric Administration’s (NOAA’s)Geophysical Fluid Dynamics Labora-tory, and the National Aeronautics andSpace Administration’s (NASA’s) God-dard Institute for Space Studies.

Use of these model results is notmeant to imply that they provide accu-rate predictions of the specific changes inclimate that will occur over the next100 years. Rather, the models are con-sidered to provide plausible projections ofpotential changes for the 21st century.1

For some aspects of climate, all models,as well as other lines of evidence, are inagreement on the types of changes tobe expected. For example, compared tochanges during the 20th century, all cli-mate model results suggest that warm-ing during the 21st century across thecountry is very likely to be greater, that

sea level and the heat index are going torise more, and that precipitation is morelikely to come in the heavier categoriesexperienced in each region. Also,although there is not yet close agree-ment about how regional changes in cli-mate can be expected to differ fromlarger-scale changes, the model simula-tions indicate some agreement in pro-jections of the general seasonal andsubcontinental patterns of the changes(IPCC 2001d).

This consistency has lent some con-fidence to these results. For someaspects of climate, however, the modelresults differ. For example, some mod-els, including the Canadian model,project more extensive and frequentdrought in the United States, whileothers, including the Hadley model, donot. As a result, the Canadian modelsuggests a hotter and drier Southeastduring the 21st century, while theHadley model suggests warmer andwetter conditions. Where such differ-ences arise, the primary model scenar-ios provide two plausible, but differentalternatives. Such differences provedhelpful in exploring the particular sensi-tivities of various activities to uncertain-ties in the model results.

Projected Changes in the Mean Climate

The model scenarios used in theNational Assessment project that thecontinuing growth in greenhouse gasemissions is likely to lead to annual-average warming over the United Statesthat could be as much as several degreesCelsius (roughly 3–9ºF) during the 21stcentury. In addition, both precipitationand evaporation are projected toincrease, and occurrences of unusualwarmth and extreme wet and dry con-ditions are expected to become morefrequent. For areas experiencing thesechanges, they would feel similar to anoverall northern shift in weather

1 For the purposes of this chapter, prediction is meant to indicate forecasting of an outcome that will occur as a result ofthe prevailing situation and recent trends (e.g., tomorrow’s weather or next winter’s El Niño event), whereas projectionis used to refer to potential outcomes that would be expected if some scenario of future conditions were to come about(e.g., concerning greenhouse gas emissions). In addition to uncertainties in how the climate is likely to respond to achanging atmospheric concentration, projections of climate change necessarily encompass a wide range because ofuncertainties in projections of future emissions of greenhouse gases and aerosols and because of the potential effectsof possible future agreements that might limit such emissions.


enced more periods of very wet or verydry conditions, and most areas experi-enced more intense rainfall events.While warming over the 48 contiguousstates amounted to about 0.6ºC (about1ºF), warming in interior Alaska was asmuch as 1.6ºC (about 3ºF), causingchanges ranging from the thawing ofpermafrost to enhanced coastal erosionresulting from melting of sea ice.

Model simulations project that min-imum temperatures are likely to con-tinue to rise more rapidly thanmaximum temperatures, extending thetrend that started during the 20th cen-tury. Although winter temperatures areprojected to increase somewhat morerapidly than summer temperatures, thesummertime heat index is projected torise quite sharply because the risingabsolute humidity will make summerconditions feel much more uncomfort-able, particularly across the southernand eastern United States.

Although a 0.6ºC (1ºF) warming may

systems and climate condition. Forexample, the central tier of states wouldexperience climate conditions roughlyequivalent to those now experienced inthe southern tier, and the northern tierwould experience conditions much likethe central tier. Figure 6-1 illustrateshow the summer climate of Illinoismight change under the two scenarios.While the two models roughly agree onthe amount of warming, the differencesbetween them arise because of differ-ences in projections of changing sum-mertime precipitation.

Recent analyses indicate that, as aresult of an uncertain combination ofnatural and human-induced factors,changes of the type that are projectedfor the 21st century were occurring tosome degree during the 20th century.For example, over the last 100 yearsmost areas in the contiguous UnitedStates warmed, although there wascooling in the Southeast. Also, duringthe 20th century, many areas experi-

not seem large compared to daily varia-tions in temperature, it caused a declineof about two days per year in the num-ber of days that minimum temperaturesfell below freezing. Across the UnitedStates, this change was most apparent inwinter and spring, with little change inautumn. The timing of the last springfrost changed similarly, with earlier ces-sation of spring frosts contributing to alengthening of the frost-free season overthe country. Even these seemingly smalltemperature-related changes have hadsome effects on the natural environment,including shorter duration of lake ice, anorthward shift in the distributions ofsome species of butterflies, changes inthe timing of bird migrations, and alonger growing season.

With respect to changes in precipi-tation, observations for the 20th cen-tury indicate that total annualprecipitation has been increasing, bothworldwide and over the country. Forthe contiguous United States, totalannual precipitation increased by anestimated 5–10 percent over the past100 years. With the exception of local-ized decreases in parts of the upperGreat Plains, the Rocky Mountains, andAlaska, most regions experiencedgreater precipitation (Figure 6-2). Thisincreased precipitation is evident indaily precipitation rates and in thenumber of rain days. It has caused wide-spread increases in stream flow for alllevels of flow conditions, particularlyduring times of low to moderate flowconditions—changes that have gener-ally improved water resource condi-tions and have reduced situations ofhydrologic drought.

For the 21st century, models project acontinuing increase in global precipita-tion, with much of the increase occur-ring in middle and high latitudes. Themodels also suggest that the increasesare likely to be evident in rainfall eventsthat, based on conditions in each region,would be considered heavy (Figure 6-3).However, estimates of the regional pat-tern of changes vary significantly. Whilethere are some indications that winter-time precipitation in the southwesternUnited States is likely to increase due to

F IGURE 6-1 Potent ia l E f fec ts o f 21st-Century Warming on the Summer C l imate o f I l l ino is

This schematic illustrates how the summer climate of Illinois would shift under two plausibleclimate scenarios. Under the Canadian Climate Centre model’s hot-dry climate scenario, thechanges in summertime temperature and precipitation in Illinois would resemble the currentclimatic conditions in southern Missouri by the 2030s and in Oklahoma by the 2090s. For thewarm-moist climate scenario projected by U.K.’s Hadley Centre model, summer in Illinoiswould become more like current summer conditions in the central Appalachians by the 2030sand North Carolina by the 2090s. Both shifts indicate warming of several degrees, but the sce-narios differ in terms of projected changes in precipitation.

Note: The baseline climatic values are for the period 1961–90.

Source: D.J. Wuebbles, University of Illinois Urbana-Champaign, as included in NAST 2000.

Average Summer Temperatures (°F)Total Summer Precipitation (inches)

70°F

75°F

80°F

10"

10"15"

15"2030s

2090s

20"

Canadian Model

70°F

75°F

80°F

10"

10"15"

15"

20"

Hadley Model

2030s

2090s


FIGURE 6-2 Observed Changes in Prec ip i ta t ion : 1901–1998

The geographical pattern of observed changes in U.S. annual precipitation during the 20thcentury indicates that, although local variations are occurring, precipitation has beenincreasing in most regions. The results are based on data from 1,221 Historical ClimatologyNetwork stations. These data are being used to derive estimates of a 100-year trend foreach U.S. climate division.

+10

+20

+40

-10

-20

-40

Trends (%/100 Years)

F IGURE 6-3 Pro jec ted Changes in the In tens i ty o f U.S. Prec ip i ta t ion fo r the 21st Century

The projected changes in precipitation over the United States as calculated by two models indicate that most of the increase is likely to occur inthe locally heaviest categories of precipitation. Each bar represents the percentage change of precipitation in a different category of storm inten-sity. For example, the two bars on the far right indicate that the Canadian Centre model projects an increase of over 20 percent in the 5 percentmost intense rainfall events in each region, whereas the Hadley Centre model projects an increase of over 55 percent in such events. Becauseboth historic trends and future projections from many global climate models indicate an increase in the fraction of precipitation occurring duringthe heaviest categories of precipitation events in each region, a continuation of this trend is considered likely. Although this does not necessarilytranslate into an increase in flooding, higher river flows are likely to be a consequence.

Source: Byron Gleason, NOAA National Climatic Data Center (updated from NAST 2000).

-10

0

10

20

30

40

50

60

0% 20% 40% 60% 80%100%

Tren

ds (%

cha

nge

in p

reci

pita

tion

per

cen

tury

) Canadian Model

Hadley Model

Lightest 5%

Moderate

Heaviest 5%

2.6

-6.0 -6.2

0.1

-4.7-6.5

2.54.0 3.1 2.9 1.5

4.7 5.1 4.56.5 7.9 8.6

6.58.8 7.9

6.48.3

10.6

22.8

-4.3

-10.0

-5.4-6.9

-9.0-9.0-5.7

-7.7

-3.8 -3.00.3

1.33.7

11.6

20.3

57.2

warming of the Pacific Ocean, changesacross key U.S. forest and agriculturalregions remain uncertain.

Soil moisture is critical for agricul-ture, vegetation, and water resources.Projections of changes in soil moisturedepend on precipitation and runoff;changes in the timing and form of theprecipitation (i.e., rain or snow); andchanges in water loss by evaporation,which in turn depends on temperaturechange, vegetation, and the effects ofchanges in CO2 concentration on evapotranspiration. As a result of themany interrelationships, projectionsremain somewhat uncertain of howchanges in precipitation are likely toaffect soil moisture and runoff, althoughthe rising summertime temperature islikely to create additional stress by sig-nificantly increasing evaporation.

Note: All stations/trends are displayed, regardless of statistical significance.

Sources: Groisman et al. 2001; NOAA National Climatic Data Center.


Projected Changes in Climate Variability

As in other highly developed nations,U.S. communities and industries havemade substantial efforts to reduce theirvulnerability to normal weather and cli-mate fluctuations. However, adaptationto potential changes in weather extremesand climate variability is likely to bemore difficult and costly. Unfortunately,projections of such changes remain quiteuncertain, especially because variationsin climate differentially affect differentregions of the country. Perhaps the best-known example of a natural variation ofthe climate is caused by the ElNiño–Southern Oscillation (ENSO),which is currently occurring every sev-eral years. ENSO has reasonably well-established effects on seasonal climateconditions across the country. For exam-ple, in the El Niño phase, unusually highsea-surface temperatures (SSTs) in theeastern and central equatorial Pacific actto suppress the occurrence of Atlantichurricanes (Figure 6-4) and result inhigher-than-average wintertime precipi-tation in the southwestern and south-eastern United States, and inabove-average temperatures in the Mid-west (Figure 6-5). During a strong ElNiño, effects can extend into the north-ern Great Plains.

During the La Niña phase, which ischaracterized by unusually low SSTs offthe west coast of South America, higher-than-average wintertime temperaturesprevail across the southern half of theUnited States, more hurricanes occur inthe tropical Atlantic, and more torna-does occur in the Ohio and Tennesseevalleys (Figures 6-4 and 6-5). During thesummer, La Niña conditions can con-tribute to the occurrence of drought inthe eastern half of the United States.

Other factors that affect the inter-annual variability of the U.S. climateinclude the Pacific Decadal Oscillation(PDO) and the North Atlantic Oscilla-tion (NAO).

The PDO is a phenomenon similarto ENSO, but is most apparent in theSSTs of the North Pacific Ocean. ThePDO has a periodicity that is on theorder of decades and, like ENSO, has

The frequency at which various numbers of hurricanes struck the United States during the 20thcentury has been found to depend on whether El Niño or La Niña events were occurring.Because of this observed relationship, changes in the frequency and intensity of these eventsare expected to affect the potential for damaging hurricanes striking the United States.

F IGURE 6-4 Likelihood of Hurricanes to Strike the United States Based on El Niño and La Niña Occurrence

Chan

ce o

f Occ

uren

ce (%

)

La Niña

El Niño

Number of Hurricanes per Year

0

20

40

60

80

100

1 ormore

2 ormore

3 ormore

4 ormore

5 ormore

6 ormore

7 ormore

90

76

67

27

39

0 0 0 0 0

19

83 1

Source: Bove et al. 1998.

Source: Florida State University, Center for Ocean–Atmospheric Prediction Studies. <http://www.coaps.fsu.edu>

F IGURE 6-5 C l imat ic Tendenc ies across Nor th Amer ica dur ing E l N iño and La Niña Events

Temperature and precipitation across North America have tended to vary from normal wintertime conditions as a result of El Niño (warmer-than-normal) and La Niña (colder-than-normal) events in the equatorial eastern Pacific Ocean. For many regions, the state of oceantemperatures in the equatorial Pacific Ocean has been found to be the most importantdeterminant of whether winter conditions are relatively wet or dry, or relatively warm orcold. For example, winters in the Southeast tend to be generally cool and wet during El Niño(warm) events, and warm and dry during La Niña (cold) events.

Dry

Very Dry

Wet

Very Wet

COLD

Cold-Event Winter (La Niña)

WARM

COLD

Warm-Event Winter (El Niño)

WARM

COLD


two distinct phases—a warm phase anda cool phase. In the warm phase,oceanic conditions lead to an intensifi-cation of the storm-generating AleutianLow, higher-than-average winter tem-peratures in the Pacific Northwest, andrelatively high SSTs along the PacificCoast. The PDO also leads to dry win-ters in the Pacific Northwest, but wet-ter conditions both north and south ofthere. Essentially, the opposite condi-tions occur during the cool phase.

The NAO is a phenomenon that dis-plays a seesaw in temperatures andatmospheric pressure between Green-land and northern Europe. However,the NAO also includes effects in theUnited States. For example, whenGreenland is warmer than normal, theeastern United States is usually colder,particularly in winter, and vice-versa.

Given these important and diverseinteractions, research is being intensifiedto improve model simulations of naturalclimate variations, especially to improveprojections of how such variations arelikely to change. Although projectionsremain uncertain, the climate model ofthe Max Planck Institute in Germany,which is currently considered to providethe most realistic simulation of theENSO cycle, calculates stronger andwider swings between El Niño and LaNiña conditions as the global climatewarms (Timmermann et al. 1999), whileother models simply project more ElNiño-like conditions over the easterntropical Pacific Ocean (IPCC 2001d).Either type of result would be likely tocause important climate fluctuationsacross the United States.

Using the selected model scenariosas guides, but also examining the poten-tial consequences of a continuation ofpast climate trends and of the possibil-ity of exceeding particular thresholdconditions, the National Assessmentfocused its analyses on evaluating thepotential environmental and societalconsequences of the climate changesprojected for the 21st century, asdescribed in the next section.

POTENTIAL CONSEQUENCES OF AND ADAPTATION TO CLIMATE CHANGE

Since the late 1980s, an increasingnumber of studies have been undertakento investigate the potential impacts ofclimate change on U.S. society and theenvironment (e.g., U.S. EPA 1989, U.S.Congress 1993) and as components ofinternational assessments (e.g., IPCC1996a, 1998). While these studies havegenerally indicated that many aspects ofthe U.S. environment and society arelikely to be sensitive to changes in cli-mate, they were unable to provide in-depth perspectives of how various typesof impacts might evolve and interact. In1997, the interagency U.S. GlobalChange Research Program (USGCRP)initiated a National Assessment processto evaluate and synthesize availableinformation about the potential impactsof climate change for the United States,to identify options for adapting to cli-mate change, and to summarize researchneeds for improving knowledge aboutvulnerability, impacts, and adaptation(see Chapter 8). The findings were alsoundertaken to provide a more in-depthanalysis of the potential time-varyingconsequences of climate change for con-sideration in scheduled internationalassessments (IPCC 2001a) and to con-tribute to fulfilling obligations under sec-tions 4.1(b) and (e) of the UnitedNations Framework Convention on Cli-mate Change.

The U.S. National Assessment wascarried out recognizing that climatechange is only one among many poten-tial stresses that society and the environ-ment face, and that, in many cases,adaptation to climate change can beaccomplished in concert with efforts toadapt to other stresses. For example, cli-mate variability and change will interactwith such issues as air and water pollu-tion, habitat fragmentation, wetlandloss, coastal erosion, and reductions infisheries in ways that are likely to com-pound these stresses. In addition, anaging national populace and rapidlygrowing populations in cities, coastalareas, and across the South and West are

social factors that interact with and insome ways can increase the sensitivity ofsociety to climate variability and change.In both evaluating potential impacts anddeveloping effective responses, it istherefore important to consider interac-tions among the various stresses.

In considering the potential impactsof climate change, however, it is alsoimportant to recognize that U.S. cli-mate conditions vary from the cold ofan Alaskan winter to the heat of a Texassummer, and from the year-round near-constancy of temperatures in Hawaii tothe strong variations in North Dakota.Across this very wide range of climateconditions and seasonal variation,American ingenuity and resources haveenabled communities and businesses todevelop, although particular economicsectors in particular regions can experi-ence losses and disruptions fromextreme conditions of various types. Forexample, the amount of property dam-age from hurricanes has been rising,although this seems to be mainly due toincreasing development and populationin vulnerable coastal areas. On theother hand, the number of deaths eachyear from weather extremes and fromclimatically dependent infectious dis-eases has been reduced sharply com-pared to a century ago, and total deathsrelating to the environment are cur-rently very small in the context of totaldeaths in the United States, eventhough the U.S. population has beenrising. In addition, in spite of climatechange, the productivity of the agricul-ture and forest sectors has never beenhigher and continues to rise, withexcess production helping to meetglobal demand.

This adaptation to environmentalvariations and extremes has been accom-plished because the public and privatesectors have applied technologicalchange and knowledge about fluctuatingclimate to implement a broad series ofsteps that have enhanced resilience andreduced vulnerability. For example, thesesteps have ranged from better design andconstruction of buildings and communi-ties to greater availability of heating in


winter and cooling in summer, and frombetter warnings about extreme events toadvances in public health care. Becauseof this increasing resilience to climatevariations and relative success in adapt-ing to the modest changes in climatethat were observed during the 20th cen-tury, information about likely future cli-mate changes and continuing efforts toplan for and adapt to these changes arelikely to prove useful in minimizingfuture impacts and preparing to takeadvantage of the changing conditions.

With these objectives in mind, theU.S. National Assessment process,which is described more completely inChapter 8, initiated a set of regional, sec-toral, and national activities. This pagepresents an overview of key nationalfindings, and the following subsectionselaborate on these findings, coveringboth potential consequences and thetypes of adaptive steps that are underwayor could be pursued to moderate or dealwith adverse outcomes. The subsectionssummarize the types of impacts that areprojected, covering initially the potentialimpacts on land cover; then the potentialimpacts on agriculture, forest, and waterresources, which are key naturalresource sectors on which societydepends; then potential impacts associ-ated with coastal regions and humanhealth that define the environment inwhich people live; and finally summa-rization of the primary issues that arespecific to particular U.S. regions. A fulllist of regional, sectoral, and nationalreports prepared under the auspices ofthe U.S. National Assessment processand additional materials relating toresearch and assessment activities canbe found at http://www.usgcrp.gov.

Potential Interactions with Land Cover

The natural vegetative cover of theUnited States is largely determined bythe prevailing climate and soil. Wherenot altered by societal activities, climateconditions largely determine whereindividual species of plants and animalscan live, grow, and reproduce. Thus, thecollections of species that we are familiarwith—e.g., the southeastern mixed

Increased warming is projected for the 21st century—Assuming continued growth inworld greenhouse gas emissions, the primary climate models drawn upon for the analysescarried out in the U.S. National Assessment projected that temperatures in the contiguousUnited States will rise 3–5°C (5–9°F) on average during the 21st century. A wider range ofoutcomes, including a smaller warming, is also possible.

Impacts will differ across regions—Climate change and its potential impacts are likely tovary widely across the country. Temperature increases are likely to vary somewhat amongregions. Heavy precipitation events are projected to become more frequent, yet someregions are likely to become drier.

Ecosystems are especially vulnerable—Many ecosystems are highly sensitive to the pro-jected rate and magnitude of climate change, although more efficient water use will helpsome ecosystems. A few ecosystems, such as alpine meadows in the Rocky Mountainsand some barrier islands, are likely to disappear entirely in some areas. Other ecosystems,such as southeastern forests, are likely to experience major species shifts or break up intoa mosaic of grasslands, woodlands, and forests. Some of the goods and services lostthrough the disappearance or fragmentation of natural ecosystems are likely to be costlyor impossible to replace.

Widespread water concerns arise—Water is an issue in every region, but the nature of thevulnerabilities varies. Drought is an important concern virtually everywhere. Floods andwater quality are concerns in many regions. Snowpack changes are likely to be especiallyimportant in the West, Pacific Northwest, and Alaska.

Food supply is secure—At the national level, the agriculture sector is likely to be able toadapt to climate change. Mainly because of the beneficial effects of the rising carbon diox-ide levels on crops, overall U.S. crop productivity, relative to what is projected in theabsence of climate change, is very likely to increase over the next few decades. However,the gains are not likely to be uniform across the nation. Falling prices are likely to cause dif-ficulty for some farmers, while benefiting consumers.

Near-term forest growth increases—Forest productivity is likely to increase over the nextseveral decades in some areas as trees respond to higher carbon dioxide levels by increas-ing water-use efficiency. Such changes could result in ecological benefits and additionalstorage of carbon. Over the longer term, changes in larger-scale processes, such as fire,insects, droughts, and disease, could decrease forest productivity. In addition, climatechange is likely to cause long-term shifts in forest species, such as sugar maples movingnorth out of the country.

Increased damage occurs in coastal and permafrost areas—Climate change and theresulting rise in sea level are likely to exacerbate threats to buildings, roads, power lines,and other infrastructure in climate-sensitive areas. For example, infrastructure damage isexpected to result from permafrost melting in Alaska and from sea level rise and stormsurges in low-lying coastal areas.

Adaptation determines health outcomes—A range of negative health impacts is possiblefrom climate change. However, as in the past, adaptation is likely to help protect much ofthe U.S. population. Maintaining our nation’s public health and community infrastructure,from water treatment systems to emergency shelters, will be important for minimizing theimpacts of water-borne diseases, heat stress, air pollution, extreme weather events, anddiseases transmitted by insects, ticks, and rodents.

Other stresses are magnified by climate change—Climate change is very likely to modifythe cumulative impacts of other stresses. While it may magnify the impacts of some stress-es, such as air and water pollution and conversion of habitat due to human developmentpatterns, it may increase agricultural and forest productivity in some areas. For coral reefs,the combined effects of increased CO2 concentration, climate change, and other stressesare very likely to exceed a critical threshold, causing large, possibly irreversible impacts.

Uncertainties remain and surprises are expected—Significant uncertainties remain in thescience underlying regional changes in climate and their impacts. Further research wouldimprove understanding and capabilities for projecting societal and ecosystem impacts.Increased knowledge would also provide the public with additional useful informationabout options for adaptation. However, it is likely that some aspects and impacts of climatechange, both positive and negative, will be totally unanticipated as complex systemsrespond to ongoing climate change in unforeseeable ways.

Sources: NAST 2000, 2001.

Key Nat iona l Find ings Adapted f rom the U.S. Nat iona l Assessment


deciduous forest, the desert ecosystemsof the arid Southwest, the productivegrasslands of the Great Plains—are pri-marily a consequence of present climateconditions. Past changes in ecosystemsindicate that some species are sostrongly influenced by the climate towhich they are adapted that they arevulnerable even to modest changes inclimate. For example, alpine meadows athigh elevations in the West exist wherethey do entirely because the plants thatcomprise them are adapted to cold con-ditions that are too harsh for otherspecies in the region. The desert vegeta-tion of the Southwest is adapted to theregion’s high summer temperatures andaridity. Similarly, the forests in the Easttend to have adapted to relatively highrainfall and soil moisture; if drought con-ditions were to persist, grasses andshrubs could begin to out-compete treeseedlings, leading to completely differ-ent ecosystems.

To provide a common base of infor-mation about potential changes in vege-tation across the nation for use in theNational Assessment (NAST 2000), spe-cialized ecosystem models were used toevaluate the potential consequences ofclimate change and an increasing CO2concentration for the dominant vegeta-tion types. Biogeography models wereused to simulate potential shifts in thegeographic distribution of major plantspecies and communities (ecosystemstructure). And biogeochemistry modelswere used to simulate changes in basicecosystem processes, such as the cyclingof carbon, nutrients, and water (ecosys-tem function). Each type of model wasused in considering the potential conse-quences of the two primary model-basedclimate scenarios. These scenarios repre-sented conditions across much of theUnited States that were generally eitherwarmer and moister, or hotter and drier.The results from both types of modelsindicated that changes in ecosystemswould be likely to be significant.

Climate changes that affect the landsurface and terrestrial vegetation willalso have implications for fresh-waterand coastal marine ecosystems thatdepend on the temperature of runoff

water, on the amount of erosion, andon other factors dependent on the landcover. For example, in aquatic ecosys-tems, many fish can breed only in waterthat falls within a narrow range of tem-peratures. As a result, species of fishthat are adapted to cool waters canquickly become unable to breed suc-cessfully if water temperatures rise. Asanother example, because washed-offsoil and nutrients can benefit wetlandspecies (within limits) and harm estuar-ine ecosystems, changes in the fre-quency or intensity of runoff eventscaused by changes in land cover can be important. Such impacts aredescribed in the subsections dealingwith climate change interactions withwater resources and the coastal envi-ronment, while issues affecting terres-trial land cover are covered in thefollowing subsection.

Redistribution of Land CoverThe responses of ecosystems to pro-

jected changes in climate and CO2 aremade up of the individual responses oftheir constituent species and how theyinteract with each other. Species in cur-rent ecosystems can differ substantiallyin their tolerances to changes in tem-perature and precipitation, and in theirresponses to changes in the CO2 con-centration. As a result, the ranges ofindividual species are likely to shift atdifferent rates, and different species arelikely to have different degrees of suc-cess in establishing themselves in newlocations and environments. Whilechanges in climate projected for thecoming hundred years are very likely toalter current ecosystems, projectingthese kinds of biological and ecologicalresponses and the structure and func-tioning of the new plant communities isvery difficult.

Analyses of present ecosystem dis-tributions and of past shifts indicatethat natural ecosystems are sensitive tochanges in surface temperature, precip-itation patterns, and other climateparameters and changes in the atmos-pheric CO2 concentration. For example,changes in temperature and precipita-tion of the magnitude being projected

are likely to cause shifts in the areasoccupied by dominant vegetation typesrelative to their current distribution.Some ecosystems that are already con-strained by climate, such as alpine mead-ows in the Rocky Mountains, are likelyto face extreme stress and disappearentirely in some places. Other morewidespread ecosystems are also likely tobe sensitive to climate change. Forexample, both climate model scenariossuggest that the southwestern UnitedStates will become moister, allowingmore vegetation to grow (Figure 6-6).Such a change is likely to transformdesert landscapes into grasslands orshrublands, altering both their potentialuse and the likelihood of fire. In thenortheastern United States, both cli-mate scenarios suggest changes mainlyin the species composition of the forests,including the northward displacementof sugar maples, which could lead to lossin some areas. However, the studies alsoindicate that conditions in this regionwill remain conducive to maintaining aforested landscape, mainly oak and hick-ory. In the southeastern United States, however, there was less agreementamong the models: the hot-dry climatescenario was projected to lead to conditions that would be conducive tothe potential breakup of the forest land-scape into a mosaic of forests, savannas,and grasslands; in contrast, the warm-moist scenario was projected to lead to anorthward expansion of the southeast-ern mixed forest cover. (See additionaldiscussion in the Forest subsection.)

Basically, changes in land cover wereprojected to occur, at least to somedegree, in all locations, and thesechanges cannot generally be preventedif the climate changes and vegetationresponds as much as projected.

Effects on the Supply of VitalEcosystem Goods and Services

In addition to the value of naturalecosystems in their own right, ecosys-tems of all types, from the most naturalto the most extensively managed, provide a variety of goods and servicesthat benefit society. Some products ofecosystems enter the market and


contribute directly to the economy. Forexample, forests serve as sources of tim-ber and pulpwood, and agro-ecosystemsserve as sources of food. Ecosystemsalso provide a set of unpriced servicesthat are valuable but that typically arenot traded in the marketplace. Althoughthere is no current market, for example,for the services that forests and wet-lands provide for improving water qual-ity, regulating stream flow, providingsome measure of protection fromfloods, and sequestering carbon, someof these services are very valuable tosociety. Ecosystems are also valued forrecreational, aesthetic, and ethical rea-sons. For example, the bird life of thecoastal marshes of the Southeast andthe brilliant autumn colors of the NewEngland forests are treasured compo-nents of the nation’s regional heritagesand important elements of our qualityof life.

Based on the studies carried out,changes in land cover induced by climate change, along with an increasedlevel of disturbances, could have variedimpacts on ecosystem services, includ-ing the abilities of ecosystems tocleanse the air and water, stabilize land-scapes against erosion, and store car-bon. Even in such regions as theSouthwest, where vegetation isexpected to increase as a result of

increased rainfall and enhanced plantgrowth due to the rising CO2 concen-tration, an important potential conse-quence is likely to be a heightenedfrequency and intensity of fires duringthe prolonged summer season. In-creased fire frequency would likely be athreat not only to the natural landcover, but also to the many residentialstructures being built in vulnerable sub-urban and rural areas, and later wouldincrease vulnerability to mudslides as aresult of denuded hills. Considering thefull range of available results, it is plau-sible that climate change-induced alter-ations to natural ecosystems couldaffect the availability of some ecosys-tem goods and services.

Effects of an Increased CO2Concentration on Plants

The ecosystem models used in theNational Assessment considered thepotential effects of increases in theatmospheric CO2 concentration be-cause the CO2 concentration affectsplant species via a direct physiologicaleffect on photosynthesis (the process bywhich plants use CO2 to create new bio-logical material). Higher CO2 concen-trations generally enhance plant growthif the plants also have sufficient waterand nutrients (such as nitrogen) that areneeded to sustain this enhanced growth.

For example, the CO2 level in commer-cial greenhouses is sometimes boosted tostimulate plant growth. In addition toenhancing plant growth, higher CO2levels can raise the efficiency with whichplants use water and reduce their suscep-tibility to damage by air pollutants.

As a result of these various influences,different types of plants respond at dif-ferent rates to increases in the atmos-pheric CO2 concentration, resulting in adivergence of growth rates. Most speciesgrow faster and increase biomass; how-ever, the nutritional value of some ofthese plants could be altered. Bothbecause of biochemical processing andbecause warming temperatures increaseplant respiration, the beneficial effects ofincreased CO2 on plants are also pro-jected to flatten at some higher level ofCO2 concentration, beyond which con-tinuing increases in the CO2 concentra-tion would not enhance plant growth.

While there is still much to belearned about the CO2 “fertilization”effect, including its limits and its directand indirect implications, many ecosys-tems are projected to benefit from ahigher CO2 concentration, and plantswill use water more efficiently.

Effects on Storage of CarbonIn response to changes in climate and

the CO2 concentration, the biogeo-

Both the Hadley and the Canadian models project increasing wintertime precipitation in the U.S. Southwest toward the end of the 21st century and a conversion of desert ecosystems to shrub and grassland ecosystems.

F IGURE 6-6 Potent ia l E f fec ts o f Pro jec ted C l imate Change on Ecosystem Dis t r ibut ion

Canadian Model Hadley ModelCurrent Ecosystems

Alpine

Forest

Savanna

Shrubland

Grassland

Arid LandSource: R.P. Neilson, USDA Forest Service, Corvallis, Oregon, as presented in NAST 2000.


chemistry models used in the NationalAssessment generally simulated in-creases in the amount of carbon storedin vegetation and soils for the continen-tal United States. The calculatedincreases were relatively small, however,and not uniform across the country. Forexample, one of the biogeochemistrymodels, when simulating the effects ofhotter and drier conditions, projectedthat the southeastern forests would losemore carbon by respiration than theywould gain by increased photosynthesis,causing an overall carbon loss of up to20 percent by 2030. Such a loss wouldindicate that the forests were in a state ofdecline. The same biogeochemistrymodel, however, when calculating thepotential effects of the warmer andmoister climate scenario, projected thatforests in the same part of the Southeastwould likely gain between 5 and 10 per-cent in carbon over the next 30 years,suggesting a more vigorous forest.

Susceptibility of Ecosystems to Disturbances

Prolonged stress due to insufficientsoil moisture can make trees more sus-ceptible to insect attack, lead to plantdeath, and increase the probability offire as dead plant material adds to anecosystem’s “fuel load.” The biogeogra-phy models used in this analysis simu-lated at least part of this sequence ofclimate-triggered events in ecosystemsas a prelude to calculating shifts in thegeographic distribution of major plantspecies.

For example, one of the biogeogra-phy models projected that a hot, dryclimate in the Southeast would be likelyto result in the replacement of the cur-rent mixed evergreen and deciduousforests by savanna/woodlands andgrasslands, with much of the changeeffected by an increased incidence offire. Yet the same biogeography modelprojected a slight northward expansionof the mixed evergreen and deciduousforests of the Southeast in response tothe warm, moist climate scenario, withno significant contraction along thesouthern boundary. Thus, in thisregion, changes in the frequency and

intensity of such disturbances as fire arelikely to be major determinants of thetype and rapidity of the conversion ofthe land cover to a new state.

As explained more fully in the sec-tions on the interactions of climatechange with coastal and waterresources, aquatic ecosystems are alsolikely to be affected by both climatechange and unusual disturbances, suchas storms and storm surges.

Potential Adaptation Options toPreserve Prevailing Land Cover

The National Assessment concludedthat the potential vulnerability of natu-ral ecosystems is likely to be moreimportant than other types of potentialimpacts affecting the U.S. environmentand society. This importance arisesbecause in many cases little can be doneto help these ecosystems adapt to theprojected rate and amount of climate change. While adjustments inhow some systems are managed canperhaps reduce the potential impacts,the complex, interdependent webs thathave been naturally generated over verylong periods are not readily shiftedfrom one place to another or easilyrecreated in new locations, even toregions of similar temperature andmoisture. Although many regions haveexperienced changes in ecosystems as aresult of human-induced changes inland cover, and people have generallybecome adapted to—and have evenbecome defenders of—the altered con-ditions (e.g., reforestation of New Eng-land), the climate-induced changesduring the 21st century are likely toaffect virtually every region of thecountry—both the ecosystems wherepeople live, as well as those in the pro-tected areas that have been created asrefuges against change.

Potential Interactions with Agriculture

U.S. croplands, grassland pasture,and range occupy about 420 millionhectares (about 1,030 million acres), or nearly 55 percent of the U.S. landarea, excluding Alaska and Hawaii(USDA/ERS 2000). Throughout the

20th century, agricultural productionshifted toward the West and Southwest.This trend allowed regrowth of someforests and grasslands, generally enhanc-ing wildlife habitats, especially in theNortheast, and contributing to seques-tration of carbon in these regions.

U.S. food production and distribu-tion comprise about 10 percent of theU.S. economy. The value of U.S. agri-cultural commodities (food and fiber)exceeds $165 billion at the farm leveland over $500 billion after processingand marketing, Because of the produc-tivity of U.S. agriculture, the UnitedStates is a major supplier of food andfiber for the world, accounting for morethan 25 percent of total global trade inwheat, corn, soybeans, and cotton.

Changes in AgriculturalProductivity

U.S. agricultural productivity hasimproved by over 1 percent a year since1950, resulting in a decline in both pro-duction costs and commodity prices,limiting the net conversion of naturalhabitat to cropland, and freeing up landfor the Conservation Reserve Program.Although the increased production andthe two-thirds drop in real commodityprices have been particularly beneficialto consumers inside and outside theUnited States and have helped toreduce hunger and malnourishmentaround the world, the lower prices havebecome a major concern for producersand have contributed to the continuingdecline in the number of small farmersacross the country. Continuation ofthese trends is expected, regardless ofwhether climate changes, with continu-ing pressures on individual producers tofurther increase productivity andreduce production costs.

On the other hand, producers con-sider anything that might increase theircosts relative to other producers or thatmight limit their markets as a threat totheir economic well-being. Issues ofconcern include regulatory actions,such as efforts to control the off-siteconsequences of soil erosion, agricul-tural chemicals, and livestock wastes;extreme weather or climate events; new


pests; and the development of pestresistance to existing pest controlstrategies.

Future changes in climate areexpected to interact with all of theseissues. In particular, although some fac-tors may tend to limit growth in yields,rising CO2 concentrations and continu-ing climate change are projected, onaverage, to contribute to extending thepersistent upward trend in crop yieldsthat has been evident during the secondhalf of the 20th century. In addition, ifall else remains equal, these changescould change supplies of and require-ments for irrigation water, increase theneed for fertilizers to sustain the gain incarbon production, lead to changes insurface-water quality, necessitateincreased use of pesticides or othermeans to limit damage from pests, andalter the variability of the climate to which the prevailing agricultural sector has become accustomed. How-ever, agricultural technology is cur-rently undergoing rapid change, andfuture production technologies andpractices seem likely to be able to con-tain or reduce these impacts.

Assuming that technological ad-vances continue at historical rates, thatthere are no dramatic changes in federalpolicies or in international markets, thatadequate supplies of nutrients are avail-able and can be applied without exacer-bating pollution problems, and that noprolonged droughts occur in majoragricultural regions, U.S. analyses indi-cate that it is unlikely that climatechange will imperil the ability of theUnited States to feed its population andto export substantial amounts of food-stuffs (NAAG 2002). These studiesindicate that, at the national level, over-all agricultural productivity is likely toincrease as a result of changes in theCO2 concentration and in climate pro-jected for at least the next severaldecades. The crop models used in thesestudies assume that the CO2 fertiliza-tion effect will be strongly beneficialand will also allow for a limited set ofon-farm adaptation options, includingchanging planting dates and varieties,in res-ponse to the changing condi-

tions. These adaptation measures con-tribute small additional gains in yields ofdry-land crops and greater gains inyields of irrigated crops. However,analyses performed to date have neitherconsidered all of the consequences ofpossible changes in pests, diseases,insects, and extreme events that mayresult, nor been able to consider the fullrange of potential adaptation options(e.g., genetic modification of crops toenhance resistance to pests, insects, anddiseases).

Recognizing these limitations, avail-able evaluations of the effects of anticipated changes in the CO2 concen-tration and climate on crop productionand yield and the adaptive actions byfarmers generally show positive resultsfor cotton, corn for grain and silage,soybeans, sorghum, barley, sugar beets,and citrus fruits (Figure 6-7). The pro-ductivity of pastures may also increase asa result of these changes. For othercrops, including wheat, rice, oats, hay,sugar cane, potatoes, and tomatoes,yields are projected to increase undersome conditions and decrease underothers, as explained more fully in theagriculture assessment (NAAG 2002).

The studies also indicate that not allU.S. agricultural regions are likely to beaffected to the same degree by the pro-jected changes in climate that have beeninvestigated. In general, northern areas,such as the Midwest, West, and PacificNorthwest, are projected to show largegains in yields, while influences on cropyields in other regions vary more widely,depending on the climate scenario andtime period. For example, projectedwheat yields in the southern Great Plainscould decline if the warming is notaccompanied by sufficient precipitation.

These analyses used market-scaleeconomic models to evaluate the overalleconomic implications for various crops.These models allow for a wide range ofadaptations in response to changing pro-ductivity, prices, and resource use,including changes in irrigation, use offertilizer and pesticides, crops grownand the location of cropping, and a vari-ety of other farm management options.Based on studies to date, unless there is

inadequate or poorly distributed precip-itation, the net effects of climate changeon the agricultural segment of the U.S.economy over the 21st century are gen-erally projected to be positive. Thesestudies indicate that, economically, con-sumers are likely to benefit more fromlower prices than producers suffer fromthe decline in profits. Complicating theanalyses, however, the studies indicatethat producer versus consumer effectswill depend on how climate changeaffects production of these crops else-where in the world. For example, forcrops grown in the United States, eco-nomic losses to farmers due to lowercommodity prices are offset under someconditions by an increased advantage ofU.S. farmers over foreign competitors,leading to an increased volume ofexports.

Because U.S. food variety and sup-plies depend not only on foodstuffs pro-duced nationally, the net effect ofclimate change on foods available forU.S. consumers will also depend on theeffects of climate change on global pro-duction of these foodstuffs. Theseeffects will in turn depend not only oninternational markets, but also on howfarmers around the world are able toadapt to climate change and other fac-tors they will face. While there are likelyto be many regional variations, experi-ence indicates that research sponsoredby the United States and other nationshas played an important role in promot-ing the ongoing, long-term increase inglobal agricultural productivity. Furtherresearch, covering opportunities rangingfrom genetic design to improving thesalt tolerance of key crops, is expectedto continue to enhance overall globalproduction of foodstuffs.

Changes in Water Demands by Agriculture

Within the United States, a keydeterminant of agricultural productivitywill be the ongoing availability of suffi-cient water where and when it isneeded. The variability of the U.S. cli-mate has provided many opportunitiesfor learning to deal with a wide range ofclimate conditions, and the U.S. regions


FIGURE 6-7 E f fec ts o f Potent ia l Changes in C l imate on U.S Crop Yie lds

Results for 16 crops, given as the percentage differences between future yields for two periods (2030s and 2090s) and current yields indicate thatwarmer climate conditions are likely to lead to increased yields for most crops. The results consider the physiological responses of the crops tofuture climate conditions under either dry-land or irrigated cultivation, assuming a limited set of reasonable adaptive response by producers.Climate scenarios are drawn from two different climate models that are likely to span the range of changes of future conditions, ranging from thewarm-moist changes projected by the U.K.’s Hadley Centre model (version 2) to the hot-dry changes projected by the Canadian Climate Centremodel. The most positive responses resulted when conditions were warmer and wetter in key growing regions (e.g., cotton), when frost occur-rence was reduced (e.g., grapefruit), and when northern areas warmed (e.g., silage from pasture improvement).

Source: NAAG 2002.

-30% 0% 30% 60% 90% 120% 150%

0%

Cotton

Corn

Soybeans

Spring Wheat

Winter Weat

Sorghum

Rice

Barley

Oats

Hay

Sugar Cane

Sugar Beets

Potatoes

Tomatoes,Processed

Oranges,Processed

Grapefruit,Processed

1%

36%122%

56%102%

1%9%

23%33%

23%40%

-1%-6%

7%10%

-1%0%

8%16%

22%8%

22%21%

7%4%

9%18%

3%-16%

28%40%

3%

24%23%

2%

6%

23%22%

7%

-6%-15%

17%33%

7%16%

39%42%

-4%-21%

-1%-8%

10%44%

10%17%

13%

15%

120%

112%

28%49%

29%53%

Canadian Scenario, 2090s

Canadian Scenario, 2030s

Hadley Scenario, 2090s

Hadley Scenario, 2030s


where many crops are grown havechanged over time without disruptingproduction. In addition, steps to build upthe amount of carbon in soils—which islikely to be one component of any car-bon mitigation program—will enhancethe water-holding capacity of soils anddecrease erosion and vulnerability todrought, thereby helping to improveoverall agricultural productivity. Forareas that are insufficiently moist, irriga-tion has been used to enhance crop pro-ductivity. In addition, about 27 percentof U.S. cultivated land is currently underreduced tillage. Several projects, such asthe Iowa Soil Carbon Sequestration Pro-ject, that are underway to promote con-servation tillage practices as a means tomitigate climate change will have theancillary benefits of reducing soil ero-sion and runoff while increasing soilwater and nitrogen retention.

Analyses conducted for the NationalAssessment project that climate changewill lead to changes in the demand forirrigation water and, if water resourcesare insufficient, to changes in the cropsbeing grown. Although regional differ-ences will likely be substantial, modelprojections indicate that, on average forthe nation, agriculture’s need for irriga-tion water is likely to slowly decline. Atleast two factors are responsible for thisprojected reduction: (1) precipitationwill increase in some agricultural areas,and (2) faster development of crops dueto higher temperatures and an increasedCO2 concentration is likely to result in ashorter growing period and consequentlya reduced demand for irrigation water.Moreover, a higher CO2 concentrationgenerally enhance a plant’s water-use effi-ciency. These factors can combine tocompensate for the increased transpira-tion and soil water loss due to higher airtemperatures. However, a decreasedperiod of crop growth also leads todecreased yields, although it may be pos-sible to overcome this disadvantagethrough crop breeding.

Changes in Surface-Water Quality due to Agriculture

Potential changes in surface-waterquality as a result of climate change is an

issue that has only started to be investi-gated. For example, in recent decades,soil erosion and excess nutrient runofffrom crop and livestock production haveseverely degraded Chesapeake Bay, ahighly valuable natural resource. In simu-lations for the National Assessment,loading of excess nitrogen from cornproduction into Chesapeake Bay is pro-jected to increase due to both the changein average climate conditions and theeffects of projected changes in extremeweather events, such as floods or heavydownpours that wash large amounts offertilizers and animal manure into surfacewaters. Across the country, changes infuture farm practice (such as no-till orreduced-till agriculture) that enhancebuildup and retention of soil moisture,and better matching of the timing of a crop’s need for fertilizer with the timing of application are examples ofapproaches that could reduce projectedadverse impacts on water quality. In addi-tion, the potential for reducing adverseimpacts of fertilizer application and soilerosion by using genetically modifiedcrops has not yet been considered.

Changes in Pesticide Use by Agriculture

Climate change is projected to causefarmers in most regions to increase theiruse of pesticides to sustain the productiv-ity of current crop strains. While thisincrease is expected to result in slightlypoorer overall economic performance,this effect is minimal because pesticideexpenditures are a relatively small shareof production costs. Neither the poten-tial changes in environmental impacts asa result of increased pesticide use nor thepotential for genetic modification toenhance pest resistance have yet beenevaluated.

Effects of Changes in ClimateVariability on Agriculture

Based on experience, agriculture isalso likely to be affected if the extent andoccurrence of climate fluctuations andextreme events change. The vulnerabilityof agricultural systems to climate andweather extremes varies with locationbecause of differences in soils, produc-

tion systems, and other factors. Changesin the form (rain, snow, or hail), timing,frequency, and intensity of precipitation,and changes in wind-driven events (e.g.,wind storms, hurricanes, and tornadoes)are likely to have significant conse-quences in particular regions. For exam-ple, in the absence of adaptive measures,an increase in heavy precipitation eventsseems likely in some areas to aggravateerosion, water-logging of soils, andleaching of animal wastes, pesticides,fertilizers, and other chemicals into sur-face and ground water. Conversely,lower precipitation in other areas mayreduce some types of impacts.

A major source of U.S. climate vari-ability is the El Niño–Southern Oscilla-tion (ENSO). The effects of ENSOevents vary widely across the country,creating wet conditions in some areasand dry conditions in others that canhave significant impacts on agriculturalproduction. For example, over the pastseveral decades, average corn yield hasbeen reduced by about 15–30 percentin years with widespread floods ordrought. Better prediction of such vari-ations is a major focus of U.S. and inter-national research activities (e.g.,through the International ResearchInstitute for Climate Prediction)because, in part, such information couldincrease the range of adaptive responsesavailable to farmers. For example, givensufficient warning of climate anomalies(e.g., of conditions being warm and dry,cool and moist, etc.), crop species andcrop planting dates could be optimizedfor the predicted variation, helping toreduce the adverse impact on yields andoverall production. Because long-termprojections suggest that ENSO varia-tions may become even stronger asglobal average temperature increases,achieving even better predictive skill inthe future will be especially importantto efforts to maximize production inthe face of climate fluctuations.

Potential Adaptation Strategies for Agriculture

To ameliorate the deleterious effectsof climate change generally, such adap-tation strategies as changing planting


dates and varieties are likely to help tosignificantly offset economic losses andincrease relative yields. Adaptive meas-ures are likely to be particularly criticalfor the Southeast because of the largereductions in yields projected for somecrops if summer precipitation declines.With the wide range of growing condi-tions across the United States, specificbreeding for response to CO2 is likelyto be required to more fully benefitfrom the CO2 fertilization effectdetected in experimental crop studies.Breeding for tolerance to climatic stresshas already been exploited, and vari-eties that do best under ideal condi-tions usually also out-perform othervarieties under stress conditions.

Although many types of changescan likely be adapted to, some adapta-tions to climate change and its impactsmay have negative secondary effects.For example, an analysis of the poten-tial effects of climate change on wateruse from the Edward’s aquifer regionnear San Antonio, Texas, foundincreased demand for ground-waterresources. Increased water use from thisaquifer would threaten endangeredspecies dependent on flows fromsprings supported by the aquifer.

In addition, in the absence ofgenetic modification of available cropspecies to counter these influences,pesticide and herbicide use is likely toincrease with warming. Greater chemi-cal inputs would be expected toincrease the potential for chemicallycontaminated runoff reaching prairiewetlands and ground water, which, ifnot controlled by on-site measures,could pollute rivers and lakes, drinking-water supplies, coastal waters, recre-ation areas, and waterfowl habitat.

As in the past, farmers will need tocontinue to adapt to the changing con-ditions affecting agriculture, andchanging climate is likely to become anincreasingly influential factor. Presum-ing adaptation to changing climateconditions is successful, the U.S. agri-cultural sector should remain strong—growing more on less land whilecontinuing to lower prices for the con-sumer, exporting large amounts of food

to help feed the world, and storing car-bon to enhance resilience to droughtand contribute to the slowing of climatechange.

Potential Interactions with Forests

Forests cover nearly one-third of theUnited States, providing wildlife habi-tat; clean air and water; carbon storage;and recreational opportunities, such ashiking, camping, and fishing. In addi-tion, harvested products include timber,pulpwood, fuelwood, wild game, ferns,mushrooms, berries, and much more.This wealth of products and servicesdepends on forest productivity and bio-diversity, which are in turn stronglyinfluenced by climate.

Across the country, native forests are adapted to the local climates inwhich they developed, such as the cold-tolerant boreal forests of Alaska, thesummer drought-tolerant forests of thePacific Northwest, and the drought-adapted piñon-juniper forests of theSouthwest. Given the overall impor-tance of the nation’s forests, the poten-tial impacts from climate change arereceiving close attention, although it isonly one of several factors meritingconsideration.

A range of human activities causeschanges in forests. For example, signifi-cant areas of native forests have beenconverted to agricultural use, andexpansion of urban areas has frag-mented forests into smaller, less con-tiguous patches. In some parts of thecountry, intensive management andfavorable climates have resulted indevelopment of highly productiveforests, such as southern pine planta-tions, in place of the natural land cover.Fire suppression, particularly in south-eastern, midwestern, and westernforests, has also led to changes in forestarea and in species composition. Har-vesting methods have also changedspecies composition, while plantingtrees for aesthetic and landscaping pur-poses in urban and rural areas hasexpanded the presence of some species.In addition, large areas, particularly inthe Northeast, have become reforested

as forests have taken over abandonedagricultural lands, allowing reestablish-ment of the ranges of many wildlifespecies.

Changes in climate and in the CO2concentration are emerging as impor-tant human-induced influences that areaffecting forests. These factors areinteracting with factors already causingchanges in forests to further affect thesocioeconomic benefits and the goodsand services forests provide, includingthe extent, composition, and produc-tivity of forests; the frequency andintensity of such natural disturbances asfire; and the level of biodiversity(NFAG 2001). Based on model projec-tions of moderate to large warming,Figure 6-8 gives an example of the gen-eral character of changes that couldoccur for forests in the eastern UnitedStates by the late 21st century.

Effects on Forest ProductivityA synthesis of laboratory and field

studies and modeling indicates that thefertilizing effect of atmospheric CO2will increase forest productivity. How-ever, increases are likely to be stronglytempered by local conditions, such asmoisture stress and nutrient availability.Across a wide range of scenarios, mod-est warming is likely to result inincreased carbon storage in most U.S.forests, although under some of thewarmer model scenarios, forests in theSoutheast and the Northwest couldexperience drought-induced losses ofcarbon, possibly exacerbated byincreased fire disturbance. Thesepotential gains and losses of carbonwould be in addition to changes result-ing from changes in land use, such asthe conversion of forests to agriculturallands or development.

Other components of environmen-tal change, such as nitrogen depositionand ground-level ozone concentra-tions, are also affecting forestprocesses. Models used in the forestsector assessment suggest a synergisticfertilization response between CO2and nitrogen enrichment, leading tofurther increases in productivity(NFAG 2001). However, ozone acts in


the opposite direction. Current ozonelevels, for example, have importanteffects on many herbaceous species andare estimated to decrease production insouthern pine plantations by 5 percent,in northeastern forests by 10 percent,and in some western forests by evenmore. Interactions among these physi-cal and chemical changes and othercomponents of global change will beimportant in projecting the future stateof U.S. forests. For example, a higherCO2 concentration can tend to sup-press the impacts of ozone on plants.

Effects on Natural DisturbancesNatural disturbances having the

greatest effects on forests includeinsects, disease, non-native species, fires,droughts, hurricanes, landslides, windstorms, and ice storms. While some treespecies are very susceptible to fire, others, such as lodgepole pine, aredependent on occasional fires for suc-cessful reproduction.

Over millennia, local, regional, and

global-scale changes in temperatureand precipitation have influenced theoccurrence, frequency, and intensity ofthese natural disturbances. Thesechanges in disturbance regimes are anatural part of all ecosystems. However,as a consequence of climate change,forests may soon be facing more rapidalterations in the nature of these distur-bances. For example, unless there is alarge increase in precipitation, the sea-sonal severity of fire hazard is projectedto increase during the 21st century overmuch of the country, particularly in theSoutheast and Alaska.

The consequences of droughtdepend on annual and seasonal climatechanges and whether the current adap-tations of forests to drought will offerresistance and resilience to new condi-tions. The ecological models used inthe National Assessment indicated thatincreases in drought stresses are mostlikely to occur in the forests of theSoutheast, southern Rocky Mountains,and parts of the Northwest. Hurricanes,

ice storms, wind storms, landslides,insect infestations, disease, and intro-duced species are also likely to be climate-modulated influences thataffect forests. However, projection ofchanges in the frequencies, intensities,and locations of such factors and theirinfluences are difficult to project. Whatis clear is that, as climate changes, alter-ations in these disturbances and in theireffects on forests are possible.

Effects on Forest BiodiversityIn addition to the very large influ-

ences of changes in land cover, changesin the distribution and abundance ofplant and animal species are a result ofboth (1) the birth, growth, death, anddispersal rates of individuals in a popula-tion and (2) the competition betweenindividuals of the same species and otherspecies. These can all be influenced inturn by weather, climate, contaminants,nutrients, and other abiotic factors.When aggregated, these processes canresult in the local disappearance or

Note: All cases were calculated using the DISTRIB tree species distribution model, which calculates the most likely dominant types of vegetation for the given climatic conditions,assuming they have persisted for several decades.

Source: A.M. Prasad and L. R. Iverson, Northeastern Research Station, USDA Forest Service, Delaware, Ohio, as reported in NAST 2000.

Both the warm-moist climate change scenario from the Hadley climate model and the hot-dry scenario from the Canadian climate model sug-gest a significant northward shift in prevailing forest types. For example, the maple-beech-birch forest type is projected to shift north intoCanada and no longer be dominant in the late 21st century in the northeastern United States.

F IGURE 6-8 Potent ia l E f fec ts o f Pro jec ted C l imate Change on Dominant Forest Types

White-Red-Jack Pine

Spruce-Fir

Longleaf-Slash Pine

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Oak-Pine

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Elm-Ash-Cottonweed

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Aspen-Birch No Data

Hadley Scenario � 2070�2100Canadian Scenario � 2070�2100Current � 1960�1990


introduction of a species, and ultimatelydetermine the species’ range and influ-ence its population.

Although climate and soils exertstrong controls on the establishment andgrowth of plant species, the response ofplant and animal species to climatechange will be the result of many inter-acting and interrelated processes operat-ing over several temporal and spatialscales. Movement and migration rates,changes in disturbance regimes and abi-otic environmental variables, and inter-actions within and between species willall affect the distributions and popula-tions of plants and animals.

Analyses conducted using ecologicalmodels indicate that plausible climatescenarios are very likely to cause shiftsin the location and area of the potentialhabitats for many tree species. Forexample, potential habitats for treesacclimated to cool environments arevery likely to shift northward. Habitatsof alpine and sub-alpine spruce-fir inthe contiguous United States are likelyto be reduced and, possibly in the longterm, eliminated as their mountainhabitats warm. The extents of aspen,eastern birch, and sugar maple arelikely to contract dramatically in theUnited States and largely shift intoCanada, with the shift in sugar maplecausing loss of syrup production innorthern New York and New England.In contrast, oak/hickory and oak/pinecould expand in the East, and Pon-derosa pine and arid woodland commu-nities could expand in the West. Howwell these species track changes in theirpotential habitats will be strongly influ-enced by the viability of their mecha-nisms for dispersal to other locationsand the disturbances to these alterna-tive environments.

Because of the dominance of non-forest land uses along migration routes,the northward shift of some nativespecies to new habitats is likely to bedisrupted if the rate of climate change istoo rapid. For example, coniferencroachment, grazing, invasivespecies, and urban expansion are cur-rently displacing sagebrush and aspencommunities. The effects of climate

change on the rate and magnitude ofdisturbance (forest damage anddestruction associated with fires,storms, droughts, and pest outbreaks)will be important factors in determiningwhether transitions from one foresttype to another will be gradual orabrupt. If the rate and type of distur-bances in New England do not increase,for example, a smooth transition fromthe present maple, beech, and birchtree species to oak and hickory mayoccur. Where the frequency or inten-sity of disturbances increases, however,transitions are very likely to occur morerapidly. As these changes occur, inva-sive (weedy) species that disperse rap-idly are likely to find opportunities innewly forming ecological communities.As a result, the species composition ofthese communities will likely differ sig-nificantly in some areas from thoseoccupying similar habitats today.

Changes in the composition ofecosystems may, in turn, have impor-tant effects on wildlife. For example, tothe extent that climate change and ahigher CO2 concentration increase for-est productivity, this might result inreduced overall land disturbance andimproved water quality, tending to helpwildlife, at least in some areas. How-ever, changes in composition can alsoaffect predator–prey relationships, pesttypes and populations, the potential fornon-native species, links in the chain ofmigratory habitats, the health of key-stone species, and other factors. Giventhese many possibilities, much remainsto be examined in projecting influencesof climate change on wildlife.

Socioeconomic ImpactsNorth America is the world’s leading

producer and consumer of wood prod-ucts. U.S. forests provide for substantialexports of hardwood lumber, woodchips, logs, and some types of paper.Coming the other way, the UnitedStates imports, for example, about 35percent of its softwood lumber andmore than half of its newsprint fromCanada.

The U.S. market for wood productswill be highly dependent upon the

future area in forests, the species com-position of forests, future supplies ofwood, technological changes in pro-duction and use, the availability of suchsubstitutes as steel and vinyl, nationaland international demands for woodproducts, and competitiveness amongmajor trading partners. Analyses indi-cate that, for a range of climate scenar-ios, forest productivity gains are verylikely to increase timber inventoriesover the next 100 years (NFAG 2001).Under these scenarios, the increasedwood supply leads to reductions in logprices, helping consumers, butdecreasing producers’ profits. The pro-jected net effect on the economic wel-fare of participants in timber marketsincreases by about 1 percent abovecurrent values.

Analyses conducted for the forestsector assessment indicate that land usewill likely shift between forestry andagriculture as these economic sectorsadjust to climate-induced changes inproduction. U.S. hardwood and soft-wood production is projected to gener-ally increase, although the projectionsindicate that softwood output will onlyincrease under moderate warming. Tim-ber output is also projected to increasemore in the South than in the North,and saw-timber volume is projected toincrease more than pulpwood volume.

Patterns and seasons of outdoor, forest-oriented recreation are likely tobe modified by the projected changes inclimate. For example, changes in forest-oriented recreation, as measured byaggregate days of activities and totaleconomic value, are likely to be affectedand are likely to vary by type of recre-ation and location. In some areas, highertemperatures are likely to shift typicalsummer recreation activities, such ashiking, northward or to higher eleva-tions and into other seasons. In winter,downhill skiing opportunities are verylikely to shift geographically because offewer cold days and reduced snowpackin many existing ski areas. Therefore,costs to maintain skiing opportunitiesare likely to rise, especially for the moresouthern areas. Effects on fishing arealso likely to vary. For example, warmer


waters are likely to increase fish produc-tion and opportunities to fish for somewarm-water species, but decrease habi-tat and opportunities to fish for cold-water species.

Possible Adaptation Strategies to Protect Forests

Even though forests are likely to beaffected by the projected changes inclimate, the motivation for adaptationstrategies is likely to be most stronglyinfluenced by the level of U.S. eco-nomic activity. This level, in turn, isintertwined with the rate of populationgrowth, changes in taste, and generalpreferences, including society’s per-ceptions about these changes. Marketforces have proven to be powerfulwhen it comes to decisions involvingland use and forestry and, as such, willstrongly influence adaptation on pri-vate lands. For forests valued for theircurrent biodiversity, society and landmanagers will have to decide whethermore intense management is necessaryand appropriate for maintaining plantand animal species that may beaffected by climate change and otherfactors.

If new technologies and markets arerecognized in a timely manner, timberproducers could adjust and adapt toclimate change under plausible climatescenarios. One possible adaptationmeasure could be to salvage dead anddying timber and to replant speciesadapted to the changed climate condi-tions. The extent and pattern of U.S.timber harvesting and prices will alsobe influenced by the global changes inforest productivity and prices of over-seas products.

Potential climate-induced changesin forests must also be put into thecontext of other human-induced pres-sures, which will undoubtedly changesignificantly over future decades.While the potential for rapid changesin natural disturbances could challengecurrent management strategies, thesechanges will occur simultaneously withhuman activities, such as agriculturaland urban encroachment on forests,multiple uses of forests, and air pollu-

tion. Given these many interacting fac-tors, climate-induced changes shouldbe manageable if planning is proactive.

Potential Interactions with Water Resources

Water is a central resource support-ing human activities and ecosystems,and adaptive management of thisresource has been an essential aspect ofsocietal development. Increases inglobal temperatures during the 20thcentury have been accompanied bymore precipitation in the middle andhigh latitudes in many regions of NorthAmerica. For example, U.S. precipita-tion increased by 5–10 percent, pre-dominantly from the spring through theautumn. Much of this increase resultedfrom a rise in locally heavy and veryheavy precipitation events, which hasled to the observed increases in low tomoderate stream flow that have beencharacteristic of the warm season acrossmost of the contiguous United States.

Local to global aspects of the hydro-logic cycle, which determine the avail-ability of water resources, are likely tobe altered in important ways by climatechange (NWAG 2000). Because higherconcentrations of CO2 and other green-house gases tend to warm the surface,all models project that the global totalsof both evaporation and precipitationwill continue to increase, with increasesparticularly likely in middle and highlatitudes.

The regional patterns of the pro-jected changes in precipitation remainuncertain, however, although there aresome indications that changes in atmos-pheric circulation brought on by suchfactors as increasing Pacific Ocean tem-peratures may bring more precipitationto the Southwest and more winter pre-cipitation to the West. Continuingtrends first evident during the 20th cen-tury, model simulations project thatincreases in precipitation are likely tobe most evident in the most intenserainfall categories typical of variousregions. To the extent such increasesoccur during the warm season whenstream flows are typically low to mod-erate, they could augment available

water resources. If increases in precipi-tation occur during high stream flow orsaturated soil conditions, the resultssuggest a greater potential for floodingin susceptible areas where additionalcontrol measures are not taken, espe-cially because under these conditionsthe relative increase in runoff is gener-ally observed to be greater than the rel-ative increase in precipitation.

Effects on Available Water Supplies

Water is a critical national resource,providing services to society for refresh-ment, irrigation of crops, nourishment ofecosystems, creation of hydroelectricpower, industrial processing, and more.Many U.S. rivers and streams do nothave enough water to satisfy existingwater rights and claims. Changing publicvalues about preserving in-stream flows,protecting endangered species, and set-tling Indian water rights claims havemade competition for water suppliesincreasingly intense. Depending on howwater managers are able to take adaptivemeasures, the potential impacts of cli-mate change could include increasedcompetition for water supplies, stresseson water quality in areas where flows arediminished, adverse impacts on ground-water quantity and quality, an increasedpossibility of flooding in the winter andearly spring, a reduced possibility offlooding later in the spring, and morewater shortages in the summer. In someareas, however, an increase in precipita-tion could outweigh these factors andincrease available supplies.

Significant changes in average tem-perature, precipitation, and soil moistureresulting from climate change are alsolikely to affect water demand in mostsectors. For example, demand for waterassociated with electric power genera-tion is projected to increase due to theincreasing demand for air conditioningwith higher summer temperatures. Climate change is also likely to reducewater levels in the Great Lakes and sum-mertime river levels in the central UnitedStates, thereby adversely affecting navi-gation, general water supplies, and pop-ulations of aquatic species.


Effects on Water QualityIncreases in heavy precipitation

events are likely to flush more contam-inants and sediments into lakes andrivers, degrading water quality. Whereuptake of agricultural chemicals andother nonpoint sources could be exa-cerbated, steps to limit water pollutionare likely to be needed. In someregions, however, higher average flowswill likely dilute pollutants and, thus,improve water quality. In coastalregions where river flows are reduced,increased salinity could also becomemore of a problem. Flooding can alsocause overloading of storm-water andwastewater systems, and can damagewater and sewage treatment facilities,mine tailing impoundments, and land-fills, thereby increasing the risks of con-tamination and toxicity.

Because the warmer temperatures willlead to increased evaporation, soil mois-ture is likely to be reduced during thewarm season. Although this effect islikely be alleviated somewhat byincreased efficiency in water use andreduced demand by native plants forwater, the drying is likely to create agreater susceptibility to fire and thenloss of the vegetation that helps to control erosion and sediment flows. Inagricultural areas, the CO2-inducedimprovement of water-use efficiency bycrops is likely to decrease demands forwater, particularly for irrigation water. Inaddition, in some regions, increasing no-till or reduced-till agriculture is likely toimprove the water-holding capacity ofsoils, regardless of whether climatechanges, thereby reducing the suscepti-bility of agricultural lands to erosionfrom intensified heavy rains (NAAG2002, NWAG 2000).

Effects on Snowpack Rising temperatures are very likely to

affect snowfall and increase snowmeltconditions in much of the western andnorthern portions of the country thatdepend on winter snowpack for runoff.This is particularly important becausesnowpack provides a natural reservoirfor water storage in mountainous areas,gradually releasing its water in spring

and even summer under current climateconditions.

Model simulations project that snow-pack in western mountain regions islikely to decrease as U.S. climate warms(Figure 6-9). These reductions are pro-jected, despite an overall increase in pre-cipitation, because (1) a larger fraction ofprecipitation will fall as rain, rather thansnow; and (2) the snowpack is likely todevelop later and melt earlier. Theresulting changes in the amount and tim-ing of runoff are very likely to have sig-nificant implications in some basins forwater management, flood protection,power production, water quality, and theavailability of water resources for irriga-tion, hydropower, communities, indus-try, and the sustainability of naturalhabitats and species.

Effects on Ground-Water Quantity and Quality

Several U.S. regions, includingparts of California and the GreatPlains, are dependent on dwindlingground-water supplies. Althoughground-water supplies are less suscep-tible to short-term climate variabilitythan surface-water supplies, they aremore affected by long-term trends.Ground water serves as the base flowfor many streams and rivers. Especiallyin areas where springtime snow coveris reduced and where higher summertemperatures increase evaporation anduse of ground water for irrigation,ground-water levels are very likely tofall, thus reducing seasonal streamflows. River and stream temperaturesfluctuate more rapidly with reducedvolumes of water, affecting fresh-waterand estuarine habitats. Small streamsthat are heavily influenced by groundwater are more likely to have reducedflows and changes in seasonality offlows, which in turn is likely to damageexisting wetland habitats.

Pumping ground water at a faster ratethan it can be recharged is already amajor concern, especially in parts of thecountry where other water resources arelimited. In the Great Plains, for example,model projections indicate that droughtis likely to be more frequent and intense,

which will create additional stressesbecause ground-water levels are alreadydropping in parts of important aquifers,such as the Ogallala.

The quality of ground water is beingdiminished by a variety of factors,including chemical contamination. Salt-water intrusion is another key ground-water quality concern, particularly incoastal areas where changes in fresh-water flows and increases in sea level willboth occur. As ground-water pumpingincreases to serve municipal demandalong the coast and less recharge occurs,coastal ground-water aquifers areincreasingly being affected by sea-waterintrusion. Because the ground-waterresource has been compromised bymany factors, managers are increasinglylooking to surface-water supplies, whichare more sensitive to climate change andvariability.

Effects on Floods, Droughts, andHeavy Precipitation Events

Projected changes in the amount,timing, and distribution of rainfall andsnowfall are likely to lead to changes inthe amount and timing of high and lowwater flows—although the relation-ships of changes in precipitation rate tochanges in flood frequency and inten-sity are uncertain, especially due touncertainties in the timing and persist-ence of rainfall events and river levelsand capacities. Because changes in cli-mate extremes are more likely thanchanges in climate averages to affectthe magnitude of damages and raise theneed for adaptive measures at theregional level, changes in the timing ofprecipitation events, as well as increasesin the intensity of precipitation events,are likely to become increasinglyimportant considerations.

Climate change is likely to affect thefrequency and amplitude of high streamflows, with major implications for infrastructure and emergency manage-ment in areas vulnerable to flooding.Although projections of the number ofhurricanes that may develop remainuncertain, model simulations indicatethat, in a warmer climate, hurricanesthat do develop are likely to have


higher wind speeds and produce morerainfall. As a result, they are likely tocause more damage, unless more exten-sive (and therefore more costly) adap-tive measures are taken, includingreducing the increasing exposure ofproperty to such extreme events. His-torical records indicate that improvedwarning has been a major factor inreducing the annual number of deathsdue to storms, and that the primarycause of the increasing property dam-age in recent decades has been theincrease in at-risk structures, such aswidespread construction of vacationhomes on barrier islands.

Despite the overall increase in pre-cipitation and past trends indicating anincrease in low to moderate streamflow, model simulations suggest thatincreased air temperatures and moreintense evaporation are likely to causemany interior portions of the countryto experience more frequent and longerdry conditions. To the extent that thefrequency and intensity of these condi-tions lead to an increase in droughts,some areas are likely to experiencewide-ranging impacts on agriculture,water-based transportation, and ecosys-tems, although the effects on vegeta-tion (including crops and forests) arelikely to be mitigated under some con-ditions by increased efficiency in wateruse due to higher CO2 levels.

Water-driven Effects on Ecosystems

Species live in the larger context ofecosystems and have differing environ-mental needs. In some ecosystems,existing stresses could be reduced ifincreases in soil moisture or the inci-dence of freezing conditions arereduced. Other ecosystems, includingsome for which extreme conditions arecritical, are likely to be most affectedby changes in the frequency and intensity of flood, drought, or fireevents. For example, model projectionsindicate that changes in temperature,moisture availability, and the waterdemand from vegetation are likely tolead to significant changes in someecosystems in the coming decades

Source: Redrawn from McCabe and Wolock 1999, as presented in NAST 2000.

F IGURE 6-9 Pro jec ted Reduct ions in Western Snowpack Resu l t ing f rom Potent ia lChanges in C l imate

Climate model scenarios for the 21st century project significant decreases from the1961–1990 baseline in the average April 1 snowpack for four mountainous areas in thewestern United States. Scenarios from the Canadian model, which simulates warmingtoward the upper end of IPCC projections, and from the Hadley model, which simulateswarming near the middle of IPCC projections, provide similar results. Such a steep reduc-tion in the April 1 snowpack would significantly shift the time of peak runoff and reduceaverage river flows in spring and summer.

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(NAST 2000). As specific examples, thenatural ecosystems of the Arctic, GreatLakes, Great Basin, and Southeast, andthe prairie potholes of the Great Plainsappear highly vulnerable to the pro-jected changes in climate (see Figures6-6 and 6-8).

The effects of changes in water tem-peratures are also important. For exam-ple, rising water temperatures are likelyto force out some cold-water fishspecies (such as salmon and trout) thatare already near the threshold of theirviable habitat, while opening up addi-tional areas for warm-water species.Increasing temperatures are also likelyto decrease dissolved oxygen in water,degrade the health of ecosystems,reduce ice cover, and alter the mixingand stratification of water in lakes—allof which are key to maintaining opti-mal habitat and suitable nutrient levels.In addition, warmer lake waters com-bining with excess nutrients from agri-cultural fertilizers (washed into lakes byheavy rains) would be likely to createalgal blooms on the lake surfaces, fur-ther depleting some lake ecosystems oflife-sustaining oxygen.

Potential Adaptation Options toEnsure Adequate Water Resources

In contrast to the vulnerability of nat-ural ecosystems, humans have exhibiteda significant ability to adapt to the availability of different amounts ofwater. There are many types of waterbasins across the country, and manyapproaches are already in use to ensurecareful management of water resources.For example, more than 80,000 damsand reservoirs and millions of miles ofcanals, pipes, and tunnels have beendeveloped to store and transport water.Some types of approaches that studieshave indicated might prove useful arehighlighted on this page.

Strategies for adapting to climatechange and other stresses include chang-ing the operation of dams and reservoirs,re-evaluating basic engineering assump-tions used in facility construction, andbuilding new infrastructure (although for a variety of reasons, large dams areno longer generally viewed as a cost-

effective or environmentally acceptablesolution to water supply problems).Other potentially available optionsinclude conserving water; changingwater pricing; using reclaimed waste-water; using water transfers; and devel-oping markets for water, which can leadto increased prices that discouragewasteful practices.

Existing or new infrastructure canalso be used to dampen the impacts ofclimate-induced influences on flowregimes and aquatic ecosystems of manyof our nation’s rivers. While significantadaptation is possible, its cost could be reduced if the probable effects of climate change are factored in beforemaking major long-term investments inrepairing, maintaining, expanding, andoperating existing water supply andmanagement infrastructure.

Because of the uncertainties associ-ated with the magnitude and direction ofchanges in precipitation and runoff dueto climate change, more flexible institu-tional arrangements may be needed toensure optimal availability of water assupplies and demand change. Althoughsocial, equity, and environmental con-siderations must be addressed, marketsolutions offer the potential for resolvingsupply problems in some parts of thecountry. However, because water rightssystems vary from state to state and evenlocally, water managers will need to takethe lead in selecting the most appropri-ate adaptive responses.

Because the United States shareswater resources with Canada and Mex-ico, it participates in a number of insti-tutions designed to address commonwater issues. These institutions, whichinclude the U.S.–Canada Great LakesCommission and joint commissions andagreements covering the Colorado andRio Grande rivers, could provide theframework for designing adaptivemeasures for responding to the effectsof climate change. For example, theU.S.–Canada Great Lakes Commissionhas already conducted studies to evalu-ate options for dealing with the poten-tial for increased evaporation, shorterduration of lake ice, and other climatechanges that are projected to affect the

Great Lakes–St. Lawrence River basin.Close coordination will be needed toefficiently manage the levels of thesecrucial water resources to ensure ade-quate water supplies for communitiesand irrigation, high water quality,needed hydroelectric power, highenough levels for recreation and

Following are some potential adaptationoptions for water management in

response to climate change and otherstresses:• Improve capacity for moving water

within and between water-use sectors(including agriculture to urban).

• Use pricing and market mechanismsproactively to decrease waste.

• Incorporate potential changes indemand and supply in long-term plan-ning and infrastructure design.

• Create incentives to move people andstructures away from flood plains.

• Identify ways to sustainably managesupplies, including ground water, sur-face water, and effluent.

• Restore and maintain watersheds toreduce sediment loads and nutrients inrunoff, limit flooding, and lower watertemperature.

• Encourage the development of institu-tions to confer property rights towater. This would be intended toencourage conservation, recycling,and reuse of water by all users, as wellas to provide incentives for researchand development of such conservationtechnologies.

• Reduce agricultural demand for waterby focusing research on developmentof crops and farming practices forminimizing water use, for example, viaprecision agricultural techniques thatclosely monitor soil moisture.

• Reuse municipal wastewater, improvemanagement of urban storm-waterrunoff, and promote collection of rainwater for local use.

• Increase the use of forecasting toolsfor water management. Some weatherpatterns, such as those resulting fromEl Niño, can now be predicted, allow-ing for more efficient management ofwater resources.

• Enhance monitoring efforts to improvedata collection for weather, climate,and hydrologic modeling to aid under-standing of water-related impacts andmanagement strategies.

Source: Adapted from NWAG 2000.

Potent ia l Adaptat ion Opt ions fo r Water Management


shipping, low enough levels to protectcommunities and shorelines fromflooding and wave-induced erosion,and more.

Potential Interactions with Coastal Areas and Marine Resources

The United States has over 95,000miles of coastline and over 3.4 millionsquare miles of ocean within its territo-rial waters. These areas provide a widerange of goods and services to the U.S.economy. Approximately 53 percent ofthe U.S. population lives on the 17 per-cent of land in counties that are adja-cent to or relatively near the coast.Over recent decades, populations inthese coastal counties have been grow-ing more rapidly than elsewhere in thecountry. As a result of this populationgrowth and increased wealth, demandson coastal and marine resources forboth leisure activities and economicbenefits are rapidly intensifying, whileat the same time exposure to coastalhazards is increasing.

Coastal and marine environmentsare intrinsically linked to the prevailingclimate in many ways. Heat given offby the oceans warms the land duringthe winter, and ocean waters help tokeep coastal regions cooler during thesummer. Moisture evaporated from theoceans is the ultimate source of precip-itation, and the runoff of precipitationcarries nutrients, pollutants, and othermaterials from the land to the ocean.Sea level exerts a major influence onthe coastal zone, shaping barrierislands and pushing salt water up estu-aries and into aquifers. For example,cycles of beach and cliff erosion alongthe Pacific Coast have been linked tothe natural sequence of El Niño eventsthat alter storm tracks and temporarilyraise average sea levels by severalinches in this region (NCAG 2000).During the 1982–83 and 1997–98 ElNiño events, erosion damage was wide-spread along the Pacific coastline.

Climate change will affect interac-tions among conditions on the land andsea and in the atmosphere. Warming islikely to alter coastal weather and could

affect the intensity, frequency, andextent of severe storms. Melting of gla-ciers and ice sheets and thermal expan-sion of ocean waters will cause sea levelto rise, which is likely to intensify ero-sion and endanger coastal structures.Rising sea level and higher tempera-tures are also likely to affect the ecol-ogy of estuaries and coastal wetlands.Higher temperatures coupled withincreasing CO2 concentrations arelikely to severely stress coral reefs, andthe changing temperature patterns arelikely to cause fisheries to relocate andalter fish migration patterns. Whilequantifying these consequences is diffi-cult, indications of the types of out-comes that are possible have emergedfrom U.S. assessments (NCAG 2000).

Effects on Sea LevelGlobal sea level rose by 10–20 cm

(about 4–8 inches) during the 20thcentury, which was significantly morethan the rate of rise that was typicalover the last few thousand years. Evenin the absence of a change in Atlanticstorminess, the deeper inundation thathas resulted from recent storms hasexacerbated flooding and has led todamage to fixed coastal structures fromstorms that were previously inconse-quential.

Looking to the future, climate mod-els project that global warming willincrease sea level by 9–88 cm (4–35inches) during the 21st century, withmid-range values more likely than thevery high or very low estimates (IPCC2001d). Because of the long time con-stants involved in ocean warming andglacier and ice sheet melting, furthersea level rise is likely for several cen-turies, even after achieving significantlimitations in emissions of CO2 andother greenhouse gases. However,these global changes are only one fac-tor in what determines sea level changeat any particular coastal location. Forexample, along the Mid-Atlantic coast,where land levels are subsiding, relativesea level rise will be somewhat greater;conversely, in New England, whereland levels are rising, relative sea levelrise will be somewhat less.

Not surprisingly, an increased rate ofglobal sea level rise is likely to have themost dramatic impacts in regions wheresubsidence and erosion problemsalready exist. Estuaries, wetlands, andshorelines along the Atlantic and Gulfcoasts are especially vulnerable.Impacts on fixed structures will inten-sify, even in the absence of an increasein storminess. However, because theslope of these areas is so gentle, even asmall rise in sea level can produce alarge inland shift of the shoreline. Therise will be particularly important if thefrequency or intensity of storm surgesor hurricanes increases.

Increases in the frequency or inten-sity of El Niño events would also likelyexacerbate the impacts of long-term sealevel rise. Coastal erosion increases thethreats to coastal development, trans-portation infrastructure, tourism, fresh-water aquifers, fisheries (many of whichare already stressed by human activi-ties), and coastal ecosystems. Coastalcities and towns, especially those instorm-prone regions, such as the South-east, are particularly vulnerable. Inten-sive residential and commercialdevelopment in these regions is placingmore and more lives and property atrisk (Figure 6-10).

Effects on EstuariesClimate change and sea level rise

could present significant threats to valu-able, productive coastal ecosystems. Forexample, estuaries filter and purify waterand provide critical nursery and habitatfunctions for many commercially impor-tant fish and shellfish populations.Because the temperature increase is pro-jected to be greater in the winter than inthe summer, a narrowing of the annualwater temperature range of many estuar-ies is likely. This, in turn, is likely tocause a shift in species’ ranges and toincrease the vulnerability of some estuar-ies to invasive species (NCAG 2000).

Changes in runoff are also likely toadversely affect estuaries. Unless newagricultural technologies allow reduceduse of fertilizers, higher rates of runoffare likely to deliver greater amounts of nutrients such as nitrogen and


FIGURE 6-10 Pro jec ted Rates o f Annual E ros ion a long U.S. Shore l ines

Source: U.S. Geological Survey Coastal Geology Program, as presented in NAST 2000.

Relatively Stable

Severely Eroding

Moderately Eroding

The U.S. coastal areas that are most vulnerable to future increases in sea level are thosewith low relief and those that are already experiencing rapid erosion rates, such as theSoutheast and Gulf Coast.

phosphorus to estuaries, while simulta-neously increasing the stratificationbetween fresh-water runoff and marinewaters. Such conditions would be likelyto increase the potential for algalblooms that deplete the water of oxy-gen. These conditions would alsoincrease stresses on sea grasses, fish,shellfish, and other organisms living inlakes, streams, and oceans (NCAG2000, and regional assessment reportslisted at http://www.usgcrp.gov). Inaddition, decreased runoff is likely toreduce flushing, decrease the size ofestuarine nursery zones, and increasethe range of estuarine habitat suscepti-ble to predators and pathogens ofshellfish.

Effects on WetlandsCoastal wetlands (marshes and

mangroves) are highly productiveecosystems, particularly because theyare strongly linked to the productivityof fisheries. Dramatic losses of coastalwetlands have occurred along the GulfCoast due to subsidence, alterations inflow and sediment load caused bydams and levees, dredge and fill activ-ities, and sea level rise. Louisiana alonehas been losing land at rates of about68–104 square kilometers (24–40square miles) per year for the last 40years, accounting for as much as 80

percent of the total U.S. coastal wet-land loss.

In general, coastal wetlands will sur-vive if soil buildup equals the rate of rel-ative sea level rise or if they are able tomigrate inland (although this migrationnecessarily displaces other ecosystemsor land uses). However, if soil accumu-lation does not keep pace with sea levelrise, or if bluffs, coastal development,or shoreline protective structures (suchas dikes, sea walls, and jetties) blockwetland migration, wetlands may beexcessively inundated and, thus, lost.The projected increase in the currentrate of sea level rise is very likely toexacerbate the nationwide rate of lossof existing coastal wetlands, althoughthe extent of impacts will vary amongregions, and some impacts may bemoderated by the inland formation ofnew wetlands.

Effects on Coral ReefsThe demise or continued deteriora-

tion of reefs could have profound impli-cations for the United States. Coralreefs play a major role in the environ-ment and economies of Florida andHawaii as well as in most U.S. territo-ries in the Caribbean and Pacific. Theysupport fisheries, recreation, andtourism and protect coastal areas. Inaddition, coral reefs are one of the

largest global storehouses of marinebiodiversity, sheltering one-quarter ofall marine life and containing extensiveuntapped genetic resources.

The last few years have seen unprece-dented declines in the health of coralreefs. The 1998 El Niño was associatedwith record sea-surface temperaturesand associated coral bleaching (whichoccurs when coral expel the algae thatlive within them and that are necessaryto their survival). In some regions, asmuch as 70 percent of the coral mayhave died in a single season. There hasalso been an upsurge in the variety, inci-dence, and virulence of coral diseases inrecent years, with major die-offs inFlorida and much of the Caribbeanregion (NCAG 2000).

Other factors that are likely to becontributing to the decline of coralreefs include increased sediment depo-sition, sewage and agricultural runoff,excessive harvesting of fish, and dam-age from ships and tourists. In additionto the potential influences of furtherglobal warming, increasing atmosphericCO2 concentrations are likely todecrease the calcification rates of thereef-building corals, resulting in weakerskeletons, reduced growth rates, andincreased vulnerability to wave-induceddamage. Model results suggest that theseeffects would likely be most severe at thecurrent margins of coral reef distribu-tion, meaning that it is unlikely coralreefs will be able to spread northward toreach cooler waters. While steps can betaken to reduce the impacts of sometypes of stress on coral reefs (e.g., bycreating Marine Protected Areas, ascalled for in Executive Order 13158, andconstructing artificial reefs to providehabitat for threatened species), damageto coral reefs from climate change andthe increasing CO2 concentration maybe moderated to some extent only bysignificantly reducing other stresses.

Effects on Marine FisheriesBased on studies summarized in the

coastal sector assessment, recreationaland commercial fishing has contributedapproximately $40 billion a year to theU.S. economy, with total marine


landings averaging about 4.5 millionmetric tons over the last decade. Cli-mate change is very likely to substan-tially alter the distribution andabundance of major fish stocks, manyof which are a shared internationalresource.

Along the Pacific Coast, impacts tofisheries related to the El Niño–South-ern Oscillation illustrate how climatedirectly affects marine fisheries on shorttime scales. For example, elevated sea-surface temperatures associated withthe 1997–98 El Niño had a tremendousimpact on the distribution and abun-dance of market squid. Although Cali-fornia’s largest fishery by volume, squidlandings fell to less than 1,000 metrictons in the 1997–98 season, down froma record-breaking 110,000 metric tonsin the 1996–97 season. Many otherunusual events occurred during thissame El Niño as a result of elevated sea-surface temperatures. Examples includewidespread deaths of California sea lionpups, catches of warm-water marlin inthe usually frigid waters off Washing-ton State, and poor salmon returns inBristol Bay, Alaska.

The changes in fish stocks resultingfrom climate change are also likely tohave important implications formarine populations and ecosystems.Changes over the long term that willaffect all nations are likely to includepoleward shifts in distribution ofmarine populations, and changes inthe timing, locations, and, perhaps,viability of migration paths and nest-ing and feeding areas for marine mam-mals and other species.

With changing ocean temperaturesand conditions, shifts in the distribu-tion of commercially important speciesare likely, affecting U.S. and interna-tional fisheries. For example, modelprojections suggest that several speciesof Pacific salmon are likely to havereduced distribution and productivity,while species that thrive in warmerwaters, such as Pacific sardine andAtlantic menhaden, are likely to showan increased distribution. Presumingthat the rate of climate change is grad-ual, the many efforts being made to bet-

ter manage the world’s fisheries mightpromote adaptation to climate change,along with helping to relieve the manyother pressures on these resources.

Potential Adaptation Options for Coastal Regions

Because climate variability is cur-rently a dominant factor in shapingcoastal and marine systems, projectingthe specific effects of climate changeover the next few decades and evaluatingthe potential effectiveness of possibleresponse options is particularly challeng-ing. Effects will surely vary greatlyamong the diverse coastal regions of thenation. Human-induced disturbancesalso influence coastal and marine sys-tems, often reducing the ability of sys-tems to adapt, so that systems that mightordinarily be capable of responding tovariability and change are less able to doso. In this context, climate change islikely to add to the cumulative impact ofboth natural and human-caused stresseson ecological systems and resources. Asa result, strategies for adapting to thepotential consequences of long-term cli-mate change in the overall context ofcoastal development and managementare only beginning to be considered(NCAG 2000).

However, as further plans are madefor development of land in the coastalzone, it is especially urgent for govern-ing bodies at all levels to begin to consider the potential changes in the coastal climate and sea level. Forexample, the U.S. Geological Survey is expanding its gathering and assemblyof relevant coastal information, and the U.S. Environmental ProtectionAgency’s Sea Level Rise project is dedicated to motivating adaptation torising sea level. This project hasassessed the probability and has identified and mapped vulnerable low-elevation coastal zones. In addition,cost-effective strategies and land-useplanning approaches involving land-ward migration of wetlands, leveebuilding, incorporation of sea level risein beach conservation plans, engineeredlandward retreats, and sea walls have allbeen developed.

Several states have already includedsea level rise in their planning, andsome have already implemented adap-tation activities. For example, in NewJersey, where relative sea level is risingapproximately one inch (2.5 cm) everysix years, $15 million is now set asideeach year for shore protection, and thestate discourages construction thatwould later require sea walls. In addi-tion, Maine, Rhode Island, South Carolina, and Massachusetts haveimplemented various forms of “rollingeasement” policies to ensure that wet-lands and beaches can migrate inland assea level rises, and that coastallandowners and conservation agenciescan purchase the required easements.Other states have modified regulationson, for example, beach preservation,land reclamation, and inward migrationof wetlands and beaches. Wider consid-eration of potential consequences isespecially important, however, becausesome regulatory programs continue topermit structures that may block theinland shift of wetlands and beaches,and in some locations shoreline move-ment is precluded due to the highdegree of coastal development.

To safeguard people and better man-age resources along the coast, NOAAprovides weather forecasts andremotely sensed environmental data tofederal, state, and local governments,coastal resource managers and scien-tists, and the public. As part of its man-date and responsibilities to administerthe National Flood Insurance Program,the Federal Emergency ManagementAgency (FEMA) prepares Flood Insur-ance Rate Maps that identify and delin-eate areas subject to severe (1 percentannual chance) floods. FEMA also mapscoastal flood hazard areas as a separateflood hazard category in recognition ofthe additional risk associated with waveaction. In addition, FEMA is workingwith many coastal cities to encouragesteps to reduce their vulnerability tostorms and floods, including purchasingvulnerable properties.

University and state programs arealso underway across the country. Thisis particularly important because most


coastal planning in the United States isthe responsibility of state and localgovernments, with the federal govern-ment interacting with these effortsthrough the development of coastalzone management plans.

Potential Interactions with Human Health

Although the overall susceptibilityof Americans to environmental healthconcerns dropped dramatically duringthe 20th century, certain health out-comes are still recognized to be associ-ated with the prevailing environmentalconditions. These adverse outcomesinclude illnesses and deaths associatedwith temperature extremes; storms andother heavy precipitation events; airpollution; water contamination; anddiseases carried by mosquitoes, ticks,and rodents. As a result of the potentialconsequences of these stresses actingindividually or in combination, it ispossible that projected climate changewill have measurable beneficial andadverse impacts on health (see NHAG2000, 2001).

Adaptation offers the potential toreduce the vulnerability of the U.S.population to adverse health out-comes—including possible outcomes ofprojected climate change—primarily byensuring strong public health systems,improving their responsiveness tochanging weather and climate condi-tions, and expanding attention given tovulnerable subpopulations. Althoughthe costs, benefits, and availability ofresources for such adaptation must befound, and further research into keyknowledge gaps on the relationshipsbetween climate/weather and health isneeded, to the extent that the U.S. pop-ulation can keep from putting itself atgreater risk by where it lives and what itdoes, the potential impacts of climatechange on human health can likely beaddressed as a component of efforts toaddress current vulnerabilities.

Projections of the extent and direc-tion of potential impacts of climatevariability and change on health areextremely difficult to make with confi-dence because of the many confound-

ing and poorly understood factors asso-ciated with potential health outcomes.These factors include the sensitivity ofhuman health to aspects of weather andclimate, differing vulnerability of variousdemographic and geographic segmentsof the population, the internationalmovement of disease vectors, and howeffectively prospective problems can bedealt with. For example, uncertaintiesremain about how climate and associ-ated environmental conditions maychange. Even in the absence of improv-ing medical care and treatment, whilesome positive health outcomes—notably, reduced cold-weather mortal-ity—are possible, the balance betweenincreased risk of heat-related illnessesand death and changes in winter ill-nesses and death cannot yet be confi-dently assessed. In addition touncertainties about health outcomes, itis very difficult to anticipate whatfuture adaptive measures (e.g., vaccines,improved use of weather forecasting tofurther reduce exposure to severe con-ditions) might be taken to reduce therisks of adverse health outcomes.

Effects on Temperature-RelatedIllnesses and Deaths

Episodes of extreme heat cause moredeaths in the United States than anyother category of deaths associatedwith extreme weather. In one of themost severe examples of such an event,the number of deaths rose by 85 per-cent during a five-day heat wave in1995 in which maximum temperaturesin Chicago, Illinois, ranged from 34 to40°C (93 to 104°F) and minimum tem-peratures were nearly as high. At least700 excess deaths (deaths in that popu-lation beyond those expected for thatperiod) were recorded, most of whichwere directly attributable to heat.

For particular years, studies in cer-tain urban areas show a strong associa-tion between increases in mortality andincreases in heat, measured by maxi-mum or minimum daily temperatureand by heat index (a measure of tem-perature and humidity). Over longerperiods, determination of trends isoften difficult due to the episodic

nature of such events and the presenceof complicating health conditions, aswell as because many areas are takingsteps to reduce exposure to extremeheat. Recognizing these complications,no nationwide trend in deaths directlyattributed to extreme heat is evidentover the past two decades, even thoughsome warming has occurred.

Based on available studies, heat strokeand other health effects associated withexposure to extreme and prolonged heatappear to be related to environmentaltemperatures above those to which thepopulation is accustomed. Thus, theregions expected to be most sensitive toprojected increases in severity and fre-quency of heat waves are likely to bethose in which extremely high tempera-tures occur only irregularly. Within heat-sensitive regions, experience indicatesthat populations in urban areas are mostvulnerable to adverse heat-related healthoutcomes. Daily average heat indicesand heat-related mortality rates arehigher in these urban core areas than insurrounding areas, because urban areasremain warmer throughout the nightcompared to outlying suburban and ruralareas. The absence of nighttime relieffrom heat for urban residents has beenidentified as a factor in excessive heat-related deaths. The elderly, young chil-dren, the poor, and people who arebedridden, who are on certain medica-tions, or who have certain underlyingmedical conditions are at particular risk.

Plausible climate scenarios projectsignificant increases in average summertemperatures, leading to new recordhighs. Model results also indicate thatthe frequency and severity of heatwaves would be very likely to increasealong with the increase in average tem-peratures. The size of U.S. cities andthe proportion of U.S. residents livingin them are also projected to increasethrough the 21st century. Because citiestend to retain daytime heat and so arewarmer than surrounding areas, climatechange is very likely to lead to anincrease in the population potentially atrisk from heat events. While the poten-tial risk may increase, heat-related ill-nesses and deaths are largely preventable


through behavioral adaptations, includ-ing use of air conditioning, increasedfluid intake, and community warningand support systems. The degree towhich these adaptations can be evenmore broadly made available andadopted than in the 20th century, espe-cially for sensitive populations, willdetermine if the long-term trendtoward fewer deaths from extreme heatcan be maintained.

Death rates not only vary with sum-mertime temperature, but also show aseasonal dependence, with more deathsin winter than in summer. This relation-ship suggests that the relatively largeincreases in average winter temperaturecould reduce deaths in winter months.However, the relationship betweenwinter weather and mortality is not asclear as for summertime extremes.While there should be fewer deathsfrom shoveling snow and slipping onice, many winter deaths are due to res-piratory infections, such as influenza,and it is not clear how influenza trans-mission would be affected by higherwinter temperatures. As a result, the neteffect on winter mortality from milderwinters remains uncertain.

Influences on Health EffectsRelated to Extreme Weather Events

Injury and death also result fromnatural disasters, such as floods andhurricanes. Such outcomes can resultboth from direct bodily harm and fromsecondary influences, such as thosemediated by changes in ecological sys-tems (such as bacterial and fungal pro-liferation) and in public healthinfrastructures (such as reduced avail-ability of safe drinking water).

Projections of climate change forthe 21st century suggest a continuationof the 20th-century trend towardincreasing intensity of heavy precipita-tion events, including precipitationduring hurricanes. Such events, in addi-tion to the potential consequenceslisted above, pose an increased risk offloods and associated health impacts.However, much can be done to preparefor powerful storms and heavy precipi-tation events, both through community

design and through warning systems.As a result of such efforts, the loss oflife and the relative amounts of damagehave been decreasing. For the future,therefore, the net health impacts ofextreme weather events hinge on con-tinuing efforts to reduce societal vul-nerabilities. For example, FEMA’s SafeCommunities program is promotingimplementation of stronger buildingcodes and improved warning systems,as well as enhancing the recoverycapacities of the natural environmentand the local population, which are alsobeing addressed through disaster assis-tance programs.

Influences on Health EffectsRelated to Air Pollution

Current exposures to air pollutionexceed health-based standards in manyparts of the country. Health assess-ments indicate that ground-level ozonecan exacerbate respiratory diseases andcause short-term reductions in lungfunction. Such studies also indicate thatexposure to particulate matter canaggravate existing respiratory and car-diovascular diseases, alter the body’sdefense systems against foreign materi-als, damage lung tissue, lead to prema-ture death, and possibly contribute tocancer. Health effects of exposure tocarbon monoxide, sulfur dioxide, andnitrogen dioxide have also been relatedto reduced work capacity, aggravationof existing cardiovascular diseases,effects on breathing, respiratory ill-nesses, lung irritation, and alterations inthe lung’s defense systems.

Projected changes in climate wouldbe likely to affect air quality in severalways, some of which are likely to bedealt with by ongoing changes in tech-nology, and some of which can be dealtwith, if necessary, through changes inregulations. For example, changes inthe weather that affect regional pollu-tion emissions and concentrations canbe dealt with by controlling sources ofemissions. However, adaptation will beneeded in response to changes in natu-ral sources of air pollution that resultfrom changes in weather. Analysesshow that hotter, sunnier days tend to

increase the formation of ground-levelozone, other conditions being thesame. This creates a risk of higher con-centrations of ground-level ozone inthe future, especially because highertemperatures are frequently accompa-nied by stagnating circulation patterns.However, more specific projections ofexposure to air pollutants cannot bemade with confidence without moreaccurate projections of changes in localand regional weather and projections ofthe amounts and locations of futureemissions, which will in turn be affectedby the implementation and success ofair pollution control policies designedto ensure air quality. Also, more exten-sive health-warning systems could helpto reduce exposures, decreasing anypotential adverse consequences.

In addition to affecting exposure toair pollutants, there is some chance thatclimate change will play a role in exposure to airborne allergens. Forexample, it is possible that climatechange will alter pollen production insome plants and change the geographicdistribution of plant species. Conse-quently, there is some chance that cli-mate change will affect the timing orduration of seasonal allergies. Theimpact of pollen and of pollen changeson the occurrence and severity ofasthma, the most common chronic dis-ease among children, is currently veryuncertain.

Effects on Water- and Food-borne Diseases

In the United States, the incidenceof and deaths due to waterborne dis-eases declined dramatically during the20th century. While much less frequentor lethal nowadays, exposure to water-borne disease can still result from drink-ing contaminated water, eating seafoodfrom contaminated water, eating freshproduce irrigated or processed withcontaminated water, and participatingin such activities as fishing or swimmingin contaminated water. Water-bornepathogens of current concern includeviruses, bacteria (such as Vibrio vulnificus,a naturally occurring estuarine bac-terium responsible for a high


FIGURE 6-11 Reported Cases of DengueFever : 1980–1999

In 1922, there were an estimated 500,000cases of dengue fever in Texas. The mos-quitoes that transmit this viral diseaseremain abundant. The striking contrast in incidence in Texas over the last twodecades, and in three Mexican states thatborder Texas, illustrates the importance offactors other than climate in the incidenceof vector-borne diseases.

Sources: National Institute of Health, Mexico, TexasDepartment of Health, U.S. Public Health Service, andunpublished data analyzed by the National HealthAssessment Group and presented in NAST 2001.

Mexico

3 MexicanBorder States:62,514 Cases

Texas:64 Cases

percentage of the deaths associatedwith shellfish consumption), and proto-zoa (such as Cryptosporidium, associatedwith gastrointestinal illnesses).

Because changes in precipitation,temperature, humidity, salinity, andwind have a measurable effect on waterquality, future changes in climate havethe potential to increase exposure towater-borne pathogens. In 1993, forexample, Cryptosporidium contaminatedthe Milwaukee, Wisconsin, drinking-water supply. As a result, 400,000 peo-ple became ill. Of the 54 individualswho died, most had compromisedimmune systems because of HIV infec-tion or other illness. A contributing factor in the contamination, in additionto treatment system malfunctions, washeavy rainfall and runoff that resulted in a decline in the quality of raw surface water arriving at the Milwaukeedrinking-water plants.

In another example, during thestrong El Niño winter of 1997–98,heavy precipitation and runoff greatlyelevated the counts of fecal bacteriaand infectious viruses in Florida’scoastal waters. In addition, toxic redtides proliferate as sea-water tempera-tures increase. Reports of marine-related illnesses have risen over the pasttwo and a half decades along the EastCoast, in correlation with El Niñoevents. Therefore, climate changes pro-jected to occur in the next severaldecades—in particular, the likelyincrease in heavy precipitationevents—raise the risk of contaminationevents.

Effects on Insect-, Tick-, andRodent-borne Diseases

Malaria, yellow fever, dengue fever,and other diseases transmitted betweenhumans by blood-feeding insects, ticks,and mites were once common in theUnited States. The incidence of manyof these diseases has been significantlyreduced, mainly because of changes inland use, agricultural methods, residen-tial patterns, human behavior, vectorcontrol, and public health systems.However, diseases that may be trans-mitted to humans from wild animals

continue to circulate in nature in manyparts of the country. Humans maybecome infected with the pathogensthat cause these diseases through trans-mission by insects or ticks (such asLyme disease, which is tick-borne) orby direct contact with the host animalsor their body fluids (such as han-taviruses, which are carried by numer-ous rodent species and transmitted tohumans through contact with rodenturine, droppings, and saliva). Theorganisms that directly transmit thesediseases are known as vectors.

The ecology and transmissiondynamics of vector-borne infections arecomplex, and the factors that influencetransmission are unique for eachpathogen. Most vector-borne diseasesexhibit a distinct seasonal pattern,which clearly suggests that they areweather-sensitive. Rainfall, tempera-ture, and other weather variables affectboth vectors and the pathogens theytransmit in many ways. For example,epidemics of malaria are associatedwith rainy periods in some parts of theworld, but with drought in others.Higher temperatures may increase orreduce vector survival rate, dependingon each specific vector, its behavior,ecology, and many other factors. Insome cases, specific weather patternsover several seasons appear to be asso-ciated with increased transmissionrates. For example, in the Midwest, out-breaks of St. Louis encephalitis (a viralinfection of birds that can also infectand cause disease in humans) appear tobe associated with the sequence ofwarm, wet winters, cold springs, andhot, dry summers. Although the poten-tial for such diseases seems likely toincrease, both the U.S. NationalAssessment (NHAG 2000, 2001) and aspecial report prepared by the NationalResearch Council (NRC 2001b) agreethat significant outbreaks of these dis-eases as a result of climate change areunlikely because of U.S. health andcommunity standards and systems.However, even with actions to limitbreeding habitats of mosquitoes andother disease vectors and to carefullymonitor for infectious diseases, the

continued occurrence of local, isolatedincidences of such diseases probablycannot be fully eliminated.

Although the United States has beenable to reduce the incidence of such climatically related diseases as dengueand malaria, these diseases continue to extract a heavy toll elsewhere (Figure 6-11). Accordingly, the U.S.government and other governmentaland nongovernmental organizations areactively supporting efforts to reducethe incidence and impacts of such dis-eases. For instance, U.S. agencies andphilanthropies are in the forefront ofmalaria research, including the searchfor vaccines and genome sequencing ofthe anopheles mosquitoes and themalaria parasite Plasmodium falciparum.Efforts such as these should help toreduce global vulnerability to malariaand other vector-borne diseases, andneed to be considered in global adapta-tion strategies.

The results from this work will servethe world in the event that human-induced climate change, through what-ever mechanism, increases the potentialfor malaria. This work will also be


beneficial for U.S. residents because ournation cannot be isolated from diseasesoccurring elsewhere in the world. Ofsignificant importance, the potential fordisease vectors to spread into the UnitedStates via travel and trade is likely toincrease just as the natural, cold-winterconditions that have helped to protectU.S. residents are moderating.

Potential Adaptation Options to Ensure Public Health

The future vulnerability of the U.S.population to the health impacts of cli-mate change will largely depend onmaintaining—if not enhancing—thenation’s capacity to adapt to potentialadverse changes through legislative,administrative, institutional, technolog-ical, educational, and research-relatedmeasures. Examples include basicresearch into climate-sensitive diseases,building codes and zoning to preventstorm or flood damage, severe weatherwarning systems to allow evacuation,improved disease surveillance and prevention programs, improved sanita-tion systems, education of health pro-fessionals and the public, and researchaddressing key knowledge gaps in climate–health relationships.

Many of these adaptive responsesare desirable from a public health per-spective, irrespective of climate change.For example, reducing air pollutionobviously has both short- and long-term health benefits. Improving warn-ing systems for extreme weather eventsand eliminating existing combinedsewer and storm-water drainage sys-tems are other measures that can ame-liorate some of the potential adverseimpacts of current climate extremes andof the possible impacts of climatechange. Improved disease surveillance,prevention systems, and other publichealth infrastructure at the state andlocal levels are already needed. Becauseof this, we expect awareness of thepotential health consequences of cli-mate change to allow adaptation toproceed in the normal course of socialand economic development.

Potential Impacts in Various U.S. Regions

While some appreciation can begained about the potential nationalconsequences of climate change bylooking at sectors such as the six con-sidered above, the United States is avery large and diverse nation. There areboth important commonalities andimportant differences in the climate-related issues and in the potential eco-nomic and environmental consequencesfaced by different regions across thecountry. Therefore, there are many dif-ferent manifestations of a changing cli-mate in terms of vulnerability andimpacts, and the potential for adapta-tion. For example, while all coastalregions are at risk, the magnitude of thevulnerabilities and the types of adapta-tion necessary will depend on particularcoastal conditions and development.Water is a key issue in virtually allregions, but the specific changes andimpacts in the West, in the Great Lakes,and in the Southeast will differ.

With this variability in mind, 20regional workshops that broughttogether researchers, stakeholders, andcommunity, state, and national leaderswere conducted to help identify keyissues facing each region and to beginidentifying potential adaptation strate-gies. These workshops were followedby the initiation of 16 regionally basedassessment studies, some of which arealready completed and others of whichare nearing completion. Each of theregional studies has examined thepotential consequences that wouldresult from the climate model scenariosused in the national level analysis (the first finding in the Key NationalFindings on page 89), and from modelsimulations of how such climatechanges would affect the types and dis-tributions of ecosystems. The followingpage provides highlights of what hasbeen learned about the regional mosaicof consequences from these studies. Amuch more comprehensive presenta-tion of the results is included in the National Assessment regional reports(see http://www.usgcrp.gov).

In summarizing potential conse-quences for the United States, it isimportant to recognize that the U.S.government represents not only the 50states, but also has trust responsibilityfor a number of Caribbean and Pacificislands and for the homelands of NativeAmericans. In particular, the U.S. gov-ernment has responsibilities of varioustypes for Puerto Rico, the American Vir-gin Islands, American Samoa, the Com-monwealth of the Northern MarianaIslands, Guam, and more than 565 tribaland Alaska Native governments that arerecognized as “domestic dependentnations.”

For the island areas, the potentialconsequences are likely to be quite simi-lar to those experienced by nearby U.S.states. With regard to Native Americans,treaties, executive orders, tribal legisla-tion, acts of Congress, and decisions ofthe federal courts determine the rela-tionships between the tribes and the fed-eral government. These agreementscover a range of issues that will beimportant in facing the potential conse-quences of climate change, including useand maintenance of land and waterresources. Although the diversity of landareas and tribal perspectives and situa-tions makes generalizations difficult, anumber of key issues have been identi-fied for closer study concerning how cli-mate variability and change will affectNative populations and their communi-ties. These issues include tourism andcommunity development; human healthand extreme events; rights to and avail-ability of water and other naturalresources; subsistence economies andcultural resources; and cultural sites,wildlife, and natural resources. Closerexamination of the potential conse-quences for tribes in the Southwest is thetopic of one of the regional assessmentsnow underway.

FEDERAL RESEARCH ACTIVITIES

The types and nature of impacts ofclimate change that are projected toaffect the United States make clear thatclimate change is likely to become an


The following key vulnerability and consequence issues were identified across the set ofregions considered in the U.S. National Assessment. Additional details may be found in

the regional reports indexed at http://www.usgcrp.gov.

Northeast, Southeast, and Midwest—Rising temperatures are likely to increase the heatindex dramatically in summer. Warmer winters are likely to reduce cold-related stresses.Both types of changes are likely to affect health and comfort.

Appalachians—Warmer and moister air is likely to lead to more intense rainfall events inmountainous areas, increasing the potential for flash floods.

Great Lakes—Lake levels are likely to decline due to increased warm-season evaporation,leading to reduced water supply and degraded water quality. Lower lake levels are alsolikely to increase shipping costs, although a longer shipping season is likely. Shorelinedamage due to high water levels is likely to decrease, but reduced wintertime ice cover islikely to lead to higher waves and greater shoreline erosion.

Southeast—Under warmer, wetter scenarios, the range of southern tree species is likelyto expand. Under hotter, drier scenarios, it is likely that grasslands and savannas will even-tually displace southeastern forests in many areas, with the transformation likely acceler-ated by increased occurrence of large fires.

Southeast Atlantic Coast, Puerto Rico, and the Virgin Islands—Rising sea level and high-er storm surges are likely to cause loss of many coastal ecosystems that now provide animportant buffer for coastal development against the impacts of storms. Currently andnewly exposed communities are more likely to suffer damage from the increasing intensi-ty of storms.

Midwest/Great Plains—A rising CO2 concentration is likely to offset the effects of risingtemperatures on forests and agriculture for several decades, increasing productivity andthereby reducing commodity prices for the public. To the extent that overall production isnot increased, higher crop and forest productivity is likely to lead to less land being farmedand logged, which may promote recovery of some natural environments.

Great Plains—Prairie potholes, which provide important habitat for ducks and othermigratory waterfowl, are likely to become much drier in a warmer climate.

Southwest—With an increase in precipitation, the desert ecosystems native to this regionare likely to be replaced in many areas by grasslands and shrublands, increasing both fireand agricultural potential.

Northern and Mountain Regions—It is very likely that warm-weather recreational oppor-tunities like hiking will expand, while cold-weather activities like skiing will contract.

Mountain West—Higher winter temperatures are very likely to reduce late winter snow-pack. This is likely to cause peak runoff to be lower, which is likely to reduce the potentialfor spring floods associated with snowmelt. As the peak flow shifts to earlier in the spring,summer runoff is likely to be reduced, which is likely to require modifications in water man-agement to provide for flood control, power production, fish runs, cities, and irrigation.

Northwest—Increasing river and stream temperatures are very likely to further stressmigrating fish, complicating current restoration efforts.

Alaska—Sharp winter and springtime temperature increases are very likely to cause con-tinued melting of sea ice and thawing of permafrost, further disrupting ecosystems, infra-structure, and communities. A longer warm season could also increase opportunities forshipping, commerce, and tourism.

Hawaii and Pacific Trust Territories—More intense El Niño and La Niña events are possi-ble and would be likely to create extreme fluctuations in water resources for island citi-zens and the tourists who sustain local economies.

increasingly important factor in thefuture management of our land andwater resources. To better prepare forcoming changes, it is important toenhance the basis of understandingthrough research and to start to considerthe potential risks that may be createdby these impacts in the making of short-and long-term decisions in such areas asplanning for infrastructure, land use, andother natural resource management. Topromote these steps, the U.S. govern-ment sponsors a wide range of relatedactivities reaching across federal agen-cies and on to the states, communities,and the general public.

Interagency Research Subcommittees

At the federal level, climate changeand, even more generally, global envi-ronmental change and sustainability aretopics that have ties to many agenciesacross the U.S. government. To ensurecoordination, the U.S. Congress passedthe Global Change Research Act of1990 (Public Law 101-606). This lawprovides for the interagency coordina-tion of global change activities, includ-ing research on how the climate is likelyto change and on the potential conse-quences for the environment and soci-ety. Responsibility is assigned to theExecutive Office of the President and isimplemented under the guidance of theOffice of Science and Technology Pol-icy (OSTP). To implement this coordi-nation, OSTP has established severalinteragency subcommittees. The U.S.Global Change Research Program(USGCRP) provides a framework forcoordination of research to reduceuncertainties about climate change andpotential impacts on climate, ecosys-tems, natural resources, and society (seeChapter 8). A number of the activities ofthe other subcommittees are also relatedto the issues of vulnerability and adapta-tion to global climate change:• Natural Disaster Reduction—This sub-

committee promotes interagencyefforts to assemble and analyze dataand information about the occurrenceand vulnerability of the United Statesto a wide range of weather- and

Key Regiona l Vulnerab i l i ty and Consequence Issues


climate-related events. Through itsparticipating agencies, the subcom-mittee is also promoting efforts bycommunities, universities, and othersto increase their preparation for, andresilience to, natural disasters. In thatclimate change may alter the inten-sity, frequency, duration and locationof such disasters, enhancing resilienceand flexibility will assist in copingwith climate change.

• Air Quality—This subcommittee pro-motes interagency efforts to docu-ment and investigate the factorsaffecting air quality on scales fromregional and subcontinental to inter-continental and global, focusing par-ticularly on tropospheric ozone andparticulate matter, both of whichcontribute to climate change as wellas being affected by it.

• Ecological Systems—This subcommitteepromotes interagency efforts toassemble information about ecologi-cal systems and services and theircoupling to society and environmen-tal change. It is sponsoring assess-ments that document the currentstate of the nation’s ecosystems, andthat provide scenarios of future con-ditions under various managementand policy options, providing a base-line for the National Assessment stud-ies concerning how ecosystems arelikely to change over the long term.

Individual Agency Research Activities

In addition to their interagencyactivities, many of the USGCRP agen-cies have various responsibilities relat-ing to the potential consequences ofclimate change and of consideration ofresponses and means for coping withand adapting to climate change.

U.S. Department of AgricultureResearch sponsored by the U.S.

Department of Agriculture (USDA)focuses on understanding terrestrial sys-tems and the effects of global change(including water balance, atmosphericdeposition, vegetative quality, and UV-Bradiation) on food, fiber, and forestryproduction in agricultural, forest, and

range ecosystems. USDA research alsoaddresses how resilient managed agricul-tural, rangeland, and forest ecosystemsare to climate change and what adapta-tion strategies will be needed to adjust toa changing climate. Programs includelong-term studies addressing the struc-ture, function, and management of forestand grassland ecosystems; research inapplied sciences, including soils, climate,food and fiber crops, pest management,forests and wildlife, and social sciences;implementation of ecosystem manage-ment on the national forests and grass-lands; and human interaction withnatural resources.

For example, U.S. Forest Serviceresearch has established a national planof forest sustainability to continue toprovide water, recreation, timber, andclean air in a changing environment.Two goals of this program are to improvestrategies for sustaining forest healthunder multiple environmental stressesand to develop projections of future for-est water quality and yield in light ofpotential changes in climate.

Similarly, research at the U.S. Agri-cultural Research Service (ARS) looks todetermine the impacts of increasedatmospheric CO2 levels, rising tempera-tures, and water availability on crops andtheir interactions with other biologicalcomponents of agricultural ecosystems.ARS also conducts research on charac-terizing and measuring changes inweather and the water cycles at local andregional scales, and determining how tomanage agricultural production systemsfacing such changes.

National Oceanic and Atmospheric Administration

The National Oceanic and Atmos-pheric Administration (NOAA) supportsin situ and remote sensing and monitor-ing, research, and assessment to improvethe accuracy of forecasts of weather andintense storms, and projections of cli-mate change; to improve the scientificbasis for federal, state, and local manage-ment of the coastal and marine environ-ment and its natural resources; and toensure a safe and productive marinetransportation system. In addition to

direct responsibilities for managingNational Marine Sanctuaries and forprotecting threatened, endangered, andtrust resources, NOAA works with statesto implement their coastal zone manage-ment plans and with regional councils toensure sustained productivity of marinefisheries. Climate change and variabilityinfluence all areas of NOAA’s responsi-bilities, both through direct effects andthrough intensification of other stresses,such as pollution, invasive species, andland and resource use.

U.S. Department of Health and Human Services

Through the National Institutes ofHealth, the Department of Health andHuman Services sponsors research on awide variety of health-related issuesranging from research on treatments forexisting and emerging diseases to studiesof risks from exposures to environmentalstresses. For example, the National Insti-tute of Environmental Health Sciences(NIEHS) conducts research on theeffects of exposure to environmentalagents on human health. The core pro-grams of the NIEHS provide data andunderstanding for risk assessments dueto changes in human vulnerability andexposures. Climate change raises issuesof susceptibility to disease and needs forensuring public health services. Changesin crop production techniques canincrease human exposures to toxicagents and to disease vectors.

U.S. Department of the InteriorThe U.S. Department of the Interior

(DOI) is the largest manager of landand the associated biological and othernatural resources within the UnitedStates. Its land management agencies,which include the Bureau of Land Man-agement, the U.S. Fish and WildlifeService, and the National Park Service,cumulatively manage over 180 millionhectares (445 million acres) or 20 percent of the nation’s land area for avariety of purposes, including preserva-tion, tourism and recreation, timber harvesting, migratory birds, fish,wildlife, and a multiplicity of otherfunctions and uses.


DOI’s Bureau of Reclamation is thelargest supplier and manager of water inthe 17 western states, delivering waterto over 30 million people for agricul-tural, municipal, industrial, and domes-tic uses. The Bureau also generates overa billion dollars worth of hydroelectricpower and is responsible for multi-purpose projects encompassing floodcontrol, recreation, irrigation, fish, andwildlife. Management of land, water,and other natural resources is of neces-sity an exercise in adaptive manage-ment (IPCC 1991).

Research related to climate changeconducted by DOI’s U.S. Geological Sur-vey includes efforts to identify whichparts of the natural and human-controlledlandscapes, ecosystems, and coastlinesare at the highest risk under potentialchanges in climate and climate variability,water availability, and different land andresource management practices.

U.S. Department of Transportation The U.S. Department of Transporta-

tion has recognized that many of thenation's transportation facilities andoperations, which are now generallyexposed to weather extremes, are alsolikely to be affected as the climatechanges. Among a long list of potentialimpacts, sea level rise is likely to affectmany port facilities and coastal airports;higher peak stream flows are likely toaffect bridges and roadways, whereaslower summertime levels of rivers andthe Great Lakes are likely to inhibitbarge and ship traffic; and higher peaktemperatures and more intense storms

are likely to adversely affect pavementsand freight movement. An assessmentof the potential significance of changesfor the U.S. transportation system andof guidelines for improving resilience isbeing organized.

U.S. Environmental Protection Agency

The U.S. Environmental ProtectionAgency (EPA) works closely with otherfederal agencies, state and local govern-ments, and Native American tribes todevelop and enforce regulations underexisting environmental laws, such asthe Clean Air Act, the Clean WaterAct, and the Safe Drinking Water Act.In line with EPA’s mission to protecthuman health and safeguard the naturalenvironment, EPA’s Global ChangeResearch Program is assessing the con-sequences of global change for humanhealth, aquatic ecosystem health, airquality, and water quality. Recognizingthe need for “place-based” information,these assessments will focus on impactsat appropriate geographic scales (e.g.,regional, watershed). In addition, EPAis supporting three integrated regionalassessments in the Mid-Atlantic, GreatLakes, and Gulf Coast regions. Finally,in support of these assessments, EPAlaboratories and centers conductresearch through intramural and extra-mural programs.

OTHER RESEARCH ACTIVITIESIn addition to federal activities, a

number of local, state, and regional activ-ities are underway. Many of these activi-

ties have developed from the variousregional assessments sponsored by theUSGCRP or with the encouragement ofvarious federal agencies. In addition, theUSGCRP and federal agencies have beenexpanding their education and outreachactivities to the public and private sec-tors, as described in Chapter 9.

Recognizing our shared environmentand the resources it provides, it is impor-tant that the nations of the world worktogether in planning and coordinatingtheir steps to adapt to the changing cli-mate projected for coming decades. Aspart of this effort, the United States hasbeen co-chair of Working Group II ofthe Intergovernmental Panel on ClimateChange, which is focused on impacts,adaptation, and vulnerability. For theIPCC’s Fourth Assessment Report, theUnited States will co-chair IPCC Work-ing Group I on Climate Science.

The United States is also a leader inorganizing the Arctic Climate ImpactAssessment (ACIA), which is being car-ried out under the auspices of the eight-nation Arctic Council to “evaluate andsynthesize knowledge on climate vari-ability, climate change, and increasedultraviolet radiation and their conse-quences…. The ACIA will examine pos-sible future impacts on the environmentand its living resources, on human health,and on buildings, roads and other infra-structure” (see http://www.acia.uaf.edu/).These and other assessments need tocontinue to be pursued in order to ensurethe most accurate information possiblefor preparing for the changing climate.

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