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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATIONVOL. 35, NO.6 AMERICAN WATERRESOURCES ASSOCIATION DECEMBER 1999

RESPONSE OF NORTH AMERICAN FRESHWATER LAKESTO SIMULATED FUTURE CLIMATES'

S. W. Host etler and E. E. Small2

ABSTRACT: We apply a physically based lake model to assess theresponse of North American lakes to future climate conditions asportrayed by the transient trace-gas simulations conducted withthe Max Planck Institute (ECHAM4) and the Canadian ClimateCenter (CGCM1) atmosphere-ocean general circulation models(A/OGCMs). To quantify spatial patterns of lake responses (temper-ature, mixing, ice cover, evaporation) we ran the lake model for the-oretical lakes of specified area, depth, and transparency over auniformly spaced (50 km) grid. The simulations were conducted fortwo 10-year periods that represent present climatic conditions andthose around the time of CO2 doubling. Although the climate modeloutput produces simulated lake responses that differ in specificregional details, there is broad agreement with regard to the direc-tion and area of change. In particular, lake temperatures are gener-ally warmer in the future as a result of warmer climatic conditionsand a substantial loss (> 100 days/yr) of winter ice cover. Simulatedsummer lake temperatures are higher than 3OC over the Midwestand south, suggesting the potential for future disturbance of exist-ing aquatic ecosystems. Overall increases in lake evaporation com-bine with disparate changes in AIOGCM precipitation to producefuture changes in net moisture (precipitation minus evaporation)that are of less fidelity than those of lake temperature.(KEY TERMS: climate change; freshwater lakes; aquatic ecosys-tem; lake modeling.)

INTRODUCTION

Because lakes and wetlands respond directly to cli-mate, quantifying their sensitivity to possible climatechange and variability will provide information cru-cial to the assessment of water resources in the future(IPCC, 1996a; Covich et al., 1997; Grimm et al., 1997;Hauer et al., 1997; Magnuson et al., 1997; Melak etal., 1997; Mulholland et al., 1997; Rouse et al., 1997;Schindler, 1997; Clair, 1998). Most aquatic processesand the evaporative component of the water balanceof lakes and wetlands are controlled by the annual

cycle of water temperature. The annual temperaturecycle, which is characterized by the evolution of inter-nal temperature structure, mixing, and seasonal icecover, is governed climatically by the exchange ofmoisture, heat, and momentum within the planetaryboundary layer, and physically by properties of lakewater (e.g., transparency or trophic state, chemicaldensity) and the hypsometry (e.g., depth-area rela-tion, surface area-to-depth ratio) of the lake basin.

The sensitivity of lakes to climate change can beassessed using physically based models that simulateseasonal thermal evolution in response to atmospher-ic forcing (e.g., Croley, 1990; McCormick, 1990; Changet al., 1992; Stefan et al., 1993, 1998; Hondozo andStefan, 1993; Vavrus et al., 1996; Hostetler, 1995;Hostetler and Giorgi, 1995; Thompson et al., 1998;Fang et al., 1999). Many modeling studies havefocused either on a specific lake or on a number oflakes in a particular region of interest. Here, we applya lake model to simulate continental scale patterns oflake response to climate change as characterized bytwo general circulation models. Our modeling isaccomplished by applying a physically based lakemodel at each grid point of an equally spaced rectan-gular grid that covers most of North America. We sim-ulate hypothetical lakes for which we derivegeographically consistent climate forcing from climatemodels and specify realistic physical characteristics(e.g., depth and turbidity). Our approach yields spa-tially uniform simulations of lake temperature, thedepth and duration of mixing, the duration and thick-ness of ice cover, and net moisture (precipitationminus evaporation), thereby allowing us to quantifyregional to continental-scale lake responses to climat-ic change. The approach is of particular advantage

1Paper No. 99108 of the Journal of the American Water Resources Association. Discussions are open until August 1, 2000.2Respectively, Research Hydrologist, U.S. Geological Survey, 200 SW 35th St., Corvallis, Oregon 97333; and Assistant Professor, Depart-

ment of Earth and Environmental Science, New Mexico Tech, Soccoro, New Mexico 87801 (E-MaillHostetler: [email protected]).

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near coastlines, over mountainous terrain, and inareas of nearly balanced net moisture (precipitationequals evaporation) where steep climatic gradientscause accompanying gradients steep of lake respons-es.

MODEL DESCRIPTION AND METHODS

The lake model is a one-dimensional, energy-bal-ance model that simulates the vertical structure oftemperature and stratification, seasonal lake ice, andevaporation (Hostetler and Bartlein, 1990; Hostetler,1991; Small et al., 1999). Turbulent or wind-drivenmixing of the water is parameterized by eddy diffu-sion (Henderson-Sellers, 1985), and internal convec-tive mixing is simulated as a density-driven process.The one-dimensional heat transport equation for themodel is written as:

JT 1 a-- — _ {A(Z)[km + K(z,t)]il

az J

+ .__!:__ (aA(z)) / azA(z) pc

where T is temperature, t is time, A(z) is the area ofthe lake at depth z, km is molecular diffusion, K(z,t) iseddy diffusion, p is the density of the water, and isthe specific heat of water. The heat source term Drepresents downwelling solar radiation as a functionof water depth and the transparency of the water inaccordance with Beer's law:

= (1 -f3) (1 - a) 4e112

where f3 is the fraction of incoming global (solar) radi-ation absorbed at the surface, ci is the short-wave albedo of the lake surface, and 11 is the lightextinction coefficient, which is a function of turbiditycaused by suspended particles such as clay and phyto-plankton. Highly turbid (trophic) lakes (11> 1.0) gen-erally exhibit greater surface heating and less heatingat depth than transparent or oligotrophic lakes (r <0.3) in which radiation is transmitted deeper in thewater column.

The boundary condition for Equation (1) is the sur-face energy balance which is written as:

[km +K(z,t)12 =(i-az

+ (1— alw )4i — lu — ie —(3)

where cti is the longwave albedo of the water surface,4lw is downwelling atmospheric radiation, 41u is backradiation, and qie and q are the latent and sensibleheat fluxes, respectively.

A compatible energy balance submodel, modifiedafter Patterson and Hamblin (1988), is used to simu-late the formation and ablation of lake ice. The lakemodel has recently been updated to improve the icemodel and other components (Small et al., 1999). Themodel has been applied in a variety of climate-lakestudies (e.g., Hostetler and Bartlein, 1990; Hostetlerand Benson, 1990; Vassiljev et al., 1994; Hostetler andGiorgi, 1993, 1995; Boqiang and Qun, 1998; Thomp-son et al., 1998).

For this study, we used a 92-by-135 rectangulargrid with even grid spacing of 50 km that covers mostof North America (hereafter the lake model grid). Alake model was run at each grid point for a hypotheti-cal lake for the duration of the simulations (nominallyten years), thus providing geographic coverage ofmost lakes and wetlands in North America at rela-tively high spatial and topographic resolution.Because the model was run over all non-ocean pointsof the lake grid, lake responses were simulated atgridpoints in areas such as the desert southwest andthe Great Basin where evaporation greatly exceedsprecipitation and few or no natural lakes actuallyexist. However, some lakes that are supplied runofffrom moisture derived at higher elevations (e.g., Pyra-mid Lake, Nevada) and reservoirs (e.g., those on theColorado River system) do exist in arid regions, andour simulations are representative of such water bod-(2)

ies.Output from two transient climate simulations con-

ducted with coupled ocean-atmosphere models(A/OGCMs) by the Max Planck Institute (URLhttp://www.drkz.de/forschung/reports.html) (theECHAM4JOPYC3, hereafter ECHAM4) and the Cana-dian Climate Center (URL http://www.cccma.bc.ec.gc.ca/mdels/cgcml.html) (hereafter the CGCM1) wasused to derive climate inputs for the lake model. TheECHAM4 and CGCM1 transient climate simulations,which have been used widely for climate impactsassessments, were conducted for the Intergovernmen-tal Panel on Climate Change (IPCC) climate assess-ment following the IS92A protocol for increasing tracegases (CO2, CH4, N20, and hydrofluorocarbons) in themodels by 1 percent/year beginning in (simulated)1990 (IPCC, 1992, 1994, 1996b). The direct radiative

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Response of North American Freshwater Lakes to Simulated Future Climates

effects of sulfate aerosol loading are included inCGCM1, but not in the ECHAM4.

Lake responses and their sensitivity to warmer cli-mate conditions are evaluated by two continuous ten-year simulations conducted with the A/OGCM output:one for the present climate defined here as the cli-mate model years of 1970 through 1979 (hereafter thecontrol simulation) and one for the climate modelyears of 2070 through 2079 spanning the approximateyear (2072) of CO2 doubling (hereafter the 2xCO2 sim-ulation). Daily averages of screen height (2-m) valuesfor air temperature, wind speed, and atmospheric spe-cific humidity and net surface short-wave and atmo-spheric (longwave) radiation that are used to drivethe lake model were derived from the monthly valuesof A/OGCM output obtained from the Data Distribu-tion Center that is maintained by the IPCC (URLhttp://ipcc-ddc.cru.uea.ac.uk/dkrz/dkrz_ifldex.html)(IPCC-DDC). We used bi-linear interpolation to dis-tribute the monthly values of climate model output onthe lake model grid, and daily values at each lake gridpoint were computed by linear interpolation betweenbracketing months while the model was running.

Air temperatures from the coarser grid and thuslower topographic resolution A/OGCMs were adjustedto reflect the higher resolution topography of the lakemodel grid by applying a linear lapse-rate correction:

Ta Tgcm - 0.0065Sz

where Ta is the 2-m air temperature at the lake gridpoints, Tgcm is the air temperature from the AJOGCM,z (m) is the difference in elevation between the tar-get grid point on the lake grid and the mean elevationof the topography of the AJOGCM grid boxes used inthe interpolation, and 0.0065CC m-1 is a nominal freeatmospheric lapse rate for the mid-latitudes.

The 10-meter winds from the ECHAM4 outputwere scaled to a height of 2 meters using the relation:

U2 = 0.794u10,

where u2 and u10 are the 2- and 10-meter winds,respectively. The factor 0.794 is derived from a stan-dard (logarithmic) equation that is used to scale windspeeds under neutral atmospheric conditions as afunction of height and surface the roughness of thewater (e.g., Brutsaert, 1982).

Specific humidity q for the ECHAM4 is unavailableat the IPPC-DDC site. We computed it from availabledew-point temperature values (Sutton, 1953):

= 0.622eq

psurf—0.378e

where e = 101325 exp(13.3185-1.9760i2-0.64453-0.12990L4), L = 1.0(373.15/Tdew) (e.g., Brutsaert,1982), and psurf is surface pressure (—101325 Pa).

Longwave (atmospheric) radiation is not reportedfor either model at the IPCC-DDC site, so we derivedit from air vapor pressure e and air temperature Tausing the empirical method of Idso and Jackson(1969):

4)1w = 0.97EaTa4 (7)

where atmospheric emissivity e 1.24(e/Ta)"7 and ais the Stephan Boltzmann constant (5.67x 10-8 W rn-2

For the control period (1970-1979), the amplitudeof the seasonal cycle of the solar radiation simulatedby the ECHAM4 over North America is similar to thehistorical estimate reported in the Vegetation/Ecosys-tem Modeling and Analysis Project (VEMAP) data set(Kittel et al., 1997), but the magnitude of the values is—25 percent lower than those of the VEMAP data,particularly over high latitudes during summer. Thebias is not surprising because solar radiation inAGCMs is adjusted to produce realistic zonal aver-ages, so that simulated values may over- or underesti-mate observations over any particular region. Forinput to the lake model, the ECHAM4 solar radiationvalues were scaled by a factor of 1.25 to obtain values

(4) in better agreement with the VEMAP values.In the CGCM1 control simulation, monthly aver-

aged 2-rn wind speeds are unrealistically low (<1 msec-1) over much of western North America. As aresult, very low simulated lake evaporation rates(which are dependent on wind speed) caused unrealis-tically high lake temperatures in the lake model.Based on values of wind speed in the VEMAP dataset, monthly wind speeds from the CGCM1 wererestricted to be � 1.25 m sec'. As we discuss below,the modified radiation and wind-speed values producesimulated lake responses that are in good agreementwith observations. To produce self-consistent lakeresponses to climate change, we applied the adjust-ments to both the control and 2xCO2 GCM values.

We present three lake simulations: one each con-ducted with the ECHAM4 and CGCM1 output for arelatively small (surface area of 0.5 km2), shallow(4-m) lake of intermediate turbidity (il 0.6 in Equa-tion 2), and an additional simulation of a deeper(30-m) lake of relatively low turbidity (ii = 0.3 inEquation 2) conducted using only the CGCM1 output.A similar simulation of the 30-rn lake with theECHAM4 output is not reported because intramodelcomparison sufficiently illustrates depth-dependent

(6) responses. The specified lakes provide a broad rangeof conditions over which to evaluate the sensitivity to

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(5)

Hostetler and Small

climate change. At the 1-rn vertical resolution of thelake model, the 4-rn lake remains essentially free ofstratification, and, owing to its limited thermal iner-tia, the simulated annual temperature cycle is inphase with the climatic forcing, so the lake simula-tions are nominally representative of wetlands, albeitwithout the effect of emergent vegetation.

CLIMATOLOGY OF THE LAKE MODEL INPUTS

Although the lake model was run only over non-ocean grid points, for perspective on large-scale cli-mate patterns and their changes, we present theclimate inputs over the entire lake grid that includesboth land and ocean gridpoints (Figure 1). For theECHAM4, the mean annual air temperature over theland points is 8°C for control and 13°C for 2xCO2,compared with temperatures of 7°C for control and11°C for 2xCO2 for the CGCM1. The 2°C differencebetween the 2xCO2 experiments reflects model differ-ences and the cooling effect of sulfate aerosols in theCGCM1 simulation. Although the magnitude of theair temperature anomalies (2xCO2 minus control) aresimilar for the ECHAM4 and CGCM1, there are sub-stantial differences in their spatial patterns (Figure1).

The warmer atmosphere of the 2xCO2 simulationsincreases the moisture-holding capacity of the air and

leads to accompanying increases of 25 percent or morein atmospheric mixing ratios (Equation 6, Figure 1).The largest changes in mixing ratios correspond tothe largest temperature increases. Changes in futurewind speeds, attributed to atmospheric warming, andreductions or amplifications of regional temperaturegradients are relatively small and range from a maxi-mum decrease of —10 percent to increases of> 3 per-cent (Figure 1).

Mean-annual values of future incident solar radia-tion increase by —5 percent over a large area of themid-latitudes in the ECHAM4 (Figure 1), probably asa result of change in cloud cover and surface albedoassociated with a loss of snow cover. Future solarradiation is largely unchanged in the CGCM1 simula-tion, although there are limited areas of reduced radi-ation that suggest some effect of increased cloudinessor the scattering by sulfate aerosol loading, or both.Future longwave radiation values are higher in bothmodels than their respective control values. Increasesranging up to 20 percent are coincident with areas ofwarmer air temperatures and increased atmosphericmixing ratios.

In contrast to the CGCM1 simulation, future annu-al precipitation rates in the ECHAM4 simulationexhibit a fairly large range of change (Figure 1). Drierconditions that prevail over the deserts of southernCalifornia and the Southwest diminish to the northover the Great Basin and into the Pacific Northwest.Wetter conditions generally prevail east of the Rocky

Figure 1. Changes in Lake Model Inputs for the ECHAM4 Model (top row) and for the CGCM1 Model (bottom row) Expressed asDifferences [2xCO2 (2070-2079) Minus Control (1970-1979)] for Air Temperature, and as Ratios [2xCO2 (207O-2079)Contml

(1970.1979)] for Other Variables. Vertical and horizontal axis labels indicate latitude and longitude, respectively.



Mountains, with maximum increases located over theMidwest and Southeast. In agreement with theECHAM4 simulation, the future climate of theCGCM1 displays increased precipitation east of theRocky Mountains and over the Midwest. In contrast,however, large increases in future precipitation aresimulated by the CGCM1 along the west coast, whiledecreases are simulated over the Southeast.

While there is broad agreement regarding thedirection and the general location of the simulatedfuture changes in some of the lake inputs, disagree-ments between the model results are apparent at theregional scale. Some of the differences in the spatialdistribution of the climate anomalies are attributableto differences in the topographic resolution of theAIOGCMs (2.8° latitude by 2.8° longitude in theECHAM4 versus 3.75° latitude by 3.75° longitude inthe CGCM1). In addition, substantial interannual tointerdecadal variability exists in precipitation pat-

terns and rates simulated by A/OGCMs, so a giventen-year period may differ from other ten-year peri-ods.

CONTROL LAKE MODEL RESULTS

For the control period (1970-1979), the geographicpatterns of lake surface temperature (including bothopen and ice-covered periods) simulated with theECHAM4 and CGCM1 model output are similar (Fig-ure 2) and reflect the topographic and latitudinaleffects on climate patterns. For the shallow lake, how-ever, the ECHAM4 output yields mean-annual watertemperatures of about 0°C as far south as about 40°Nlatitude, and temperatures well below 0°C over partsof the Cordilleran and higher latitudes. These surfacetemperatures are colder than both the CGCM1 values

Figure 2. Ten-Year Averages of Selected Lake Responses as Simulated with the Control (1970-1979) Input Data Sets from theECHAM4 and CGCM1. Surface temperature and ice thickness are expressed as annual averages. Ice cover and full-depth

mixing (to the bottom) are expressed in the total number of days annually. The top and middle rows are responses for a4-rn deep lake and the bottom row are responses for a 30-m deep lake. Net moisture (P-E) is precipitation minus evaporation.


Hostetler and Small

and observed values. The cold bias in the ECHAM4 isalso evident in the simulated thickness and durationof winter ice cover. At 45°N over the Great Plains andMidwest (the approximate latitude of the GreatLakes), the annual duration of simulated ice coverexceeds 200 days, substantially longer than observedvalues that range between 120 to 150 days (Robertsonet al., 1992; Vavrus et al., 1996; Stefan et al., 1998).Essentially permanent lake ice is simulated over highlatitudes and portions of the northern Rocky Moun-tains. The CGCM1 output, on the other hand, pro-duces a more realistic geographic pattern of iceproperties on the shallow lake, with an average dura-tion of about 140 days at 45°N in the Midwest, andmaximum durations of around 275 days over thehigher elevations and latitudes.

In general, the responses of the deep lake to theCGCM1 forcing are similar to those for the shallowlake. There are small differences in simulated surfacetemperature, ice thickness, and ice duration thatreflect the effects of thermal inertia and mixing in thedeeper lake. Relative to the shallow lake, the greaterthermal inertia of the deep lake tends to reduce theamplitude of the seasonal cycle of surface tempera-ture and delay freezing in late fall and early winter.

In both the ECHAM4 and CGCM1 simulations,wind-driven turbulence mixes the 4-rn lake to the bot-tom more-or-less continuously during ice-free periodsin temperate regions and throughout the year alongthe coasts and in the south. Differences in the numberof simulated days of full-depth mixing between themodels are explained primarily by differences in theduration of simulated ice cover.

In general, fewer days of full-depth mixing are sim-ulated for the 30-m deep lake with the CGCM1 out-put. The latitudinal gradient in mixing apparent inthe 4-m lake simulation is much less well defined forthe deeper lake. In ice-free areas along the coasts andin the south, the 30-m deep lakes mix for a total ofaround four months of the year. In more continentalareas where winter ice cover occurs, the duration ofmixing ranges from a month or less up to two to threemonths. The general reduction in the duration of full-depth mixing in the deep lake illustrates how theshallow turbulent mixing that was sufficient to mixthe shallow lake effectively maintains a stable ther-mocline in the deeper lake. In the stratified lake, con-vection plays a more dominant role in producingseasonal overturn, particularly in areas of winter icecover. Modeled convection is caused by subsurfacedensity (temperature) instabilities that are set up bythe penetration of solar radiation (r = 0.3 in Equation2) and the loss of heat in the fall and gain of heat inthe spring while the temperature of the water columnis around 4°C (neutral stability). In contrast, over thewarmer areas of the southern Great Plains and Texas,

the deep lake mixes for over six months of the year,indicating a lack of stable stratification in the gener-ally warmer lake water.

The patterns of lake net moisture (precipitationminus evaporation, P-E) produced by the ECHAM4and CGCM1 output are similar in the control simula-tions (Figure 2), indicating agreement in simulatedlake evaporation rates (Figure 3). The most negativevalues of P-E are simulated in both models over thewestern deserts and southwest regions where reser-voirs, ephemeral lakes, and lakes that derive waterfrom higher elevation catchrnents exist. Negative val-ues also occur over the the southern Great Plains andparts of the Caribbean islands and Puerto Rico. Val-ues that are less negative, or nearly balanced (P = E),extend northward through the Midwest and eastwardto the Southeast. The north-to-south line runningthrough the Midwest and Great Plains that approxi-mately divides positive P-E values to the east fromnegative P-E values to the west (prairie-forest border)is well captured by both models. Positive P-E valuesoccur in the high-precipitation and low evaporationareas of the Pacific Northwest, along the east coast,and at higher latitudes.

2xCO2 LAKE MODEL RESPONSES

For both the ECHAM4 and CGCM1 output, mean-annual temperatures for the lake surface increasefrom about 2°C to more than 7°C in the 2xCO2 simula-tions (Figure 4). The largest temperature changes arelocated over the Rocky Mountains and the Midwestwhere there is a loss of seasonal lake ice in the 2xCO2simulations. Warming of greater than 3°C occurs inareas where the lakes were ice free in the control sim-ulation. No increases in the duration of ice cover weresimulated. The warmer temperatures in areas thatwere ice free in the control simulation are attributedto generally warmer air temperatures and higher lev-els of longwave radiation throughout the year. Inareas where simulated ice cover occurs in the control,warming is attributed in part to higher summer airtemperatures and higher levels of longwave radiation,and in part to a reduction (or complete loss) of icecover, which allows to warming to commence earlierin spring and persist later in autumn.

Summer lake temperatures increase substantiallyover much of the domain in both 2xCO2 simulations.August temperatures simulated with the CGCM1 out-put generally range from 30°C to 35°C and higherover most of the domain south of —45°N (Figure 5).Surface temperatures of —25CC in the 2xCO2 simula-tion occur as far north as Hudson's Bay and over alarge area of the Rocky Mountains. Average summer



Annual Lake Evaporation (mm/yr)

Figure 3. Annual Evaporation Rates for Present Derived fromPan Estimates and as Simulated by the Lake Model

Using the ECHAM4 and CGCM1 Control (1970-1979)Input Data Sets. The contour intervals for the panestimates are not evenly spaced; a contour interval

of 250 mm/yr is used for the maps of simulatedvalues. Pan-based estimates from the

U.S. Weather Bureau (1968).

temperatures (June through August) increase from2C to greater than 4C in the 2xCO2 simulation.Coincident with relatively low increases in air tem-perature (Figure 1), less summer warming is simulat-ed over the Pacific Northwest and Southeast. Coolertemperatures over the southern Great Plains areattributed to higher wind speeds in the 2xCO2 simu-lation (Figure 1), which enhance evaporation and thuscool lake surface temperatures.

The magnitude of lake warming simulated by boththe ECHAM4 and CGCM1 over the upper Midwestagrees with results obtained by Stefan et al. (1998),who used CGCM1 output to model temperatureresponses of Minnesota lakes. The warming simulat-ed here is also in general agreement over the westernU.S. with results of Hostetler and Giorgi (1995) andThompson et al. (1998) in which the lake model wasdriven by output from a regional climate model.

Widespread loss of winter ice cover in the RockyMountain region, the western mountains, throughoutthe Great Plains, and in the East is evident in boththe ECHAM4 and CGCM1 2xCO2 simulations (Figure4). Although there is a general decrease in the dura-tion and thickness of ice in both simulations, thereare areas where the ECHAM4 and CGCM1 producedifferent results. The differences are primarilyattributed to changes in the simulated snow coverover ice. In winter, snow cover insulates the ice fromextreme air temperatures and, in general, reduces thethickness of simulated ice relative to what it would bein the absence of snow. The area of maximum reduc-tion in the duration of ice (>100 days) is centered overthe Rocky Mountains and extends eastward over thehigh plains and into the Midwest. Relative to the con-trol, the boundary of ice-free conditions is shiftednorthward by 10 latitude or more over much of theMidwest and East.

Reductions in the duration of ice cover are accom-panied by substantial increases in the number of daysof mixing in the 4-rn lake (Figure 4). Smaller increas-es in mixing are simulated for the 30-rn lake becausesummer stratification that was present in the controlsimulation continues to inhibit mixing in the 2xCO2simulations. There is a tendency for no increase or adecrease in the number of days of mixing simulatedfor the deep lake over the South. The largest reduc-tions, corresponding to lower wind speeds in the2xCO2 simulation, occur over the Southeast and Flori-da where reduced evaporation during summer andwind driven mixing during winter combine to increasethe stability of a warmer water column.

Changes in mixing and internal heating associatedwith climate are illustrated in the simulated interan-nual evolution of temperature with depth in an ice-free lake (located at 30N latitude and 105Wlongitude; Figure 6) and a dimictic lake (seasonal lake

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Hostetler and Small

Figure 4. Changes in Responses as Simulated with the ECHAM4 and CGCM1 Input Data Sets Expressedas Differences Between the 2xCO2 (2070-2079) and Control (1970- 1979) Annual Average Values.The top and middle rows are for the 4-rn deep lake and the bottom row is for the 30-rn deep lake.

Figure 5. Ten-Year Average Lake Surface Temperatures Simulated with the CGCM1 Input Data Sets. Augustcontrol (A); August 2xCO2 (B); and summer (June through August) differences (2xCO2 minus control) (C).

ice, located at 41°N latitude and 88W longitude; Fig-ure 7). Over the last nine years of the control simula-tion, the ice-free lake displays a regular seasonalcycle of heating. Limited stratification occurs onlyduring the fourth summer. In the 2xCO2 simulation,

the lake is 2-4CC warmer than the control during allseasons (Figure 6). During some periods, changes inthe seasonal onset or cessation of mixing cause largerchanges in temperature (� 6CC) at various depths.Warming near the bottom during the spring-through-



Figure 6. Simulated Evolution of Depth-Temperature Structure for a 30-rn Deep Lake Over the Last Nine Years of SimulationConducted with the CGCM1 Output. The lake is located at 30'N latitude and 105'W longitude on the model grid.

Control (A), 2xCO2 (B) and anomalies (control minus 2xCO2) (C). Time axes are labeled for January, April,July, and October (J, A, J. 0). The upper color scale corresponds to the control and

2xCO2 plots, and the lower scale corresponds to the anomalies. Units of 'C.

summer period of the fourth, seventh, and eighthyears suggests inhibited deep mixing, but the patternof anomalies is not persistent. Inhibited deep mixingwould likely be more important in deeper lakes(—50-100 m) that mix less frequently, and may lead toa complete loss or intermittent occurrence of seasonaloverturn (Hostetler and Giorgi, 1995; Thompson etal., 1998).

Substantially more change and variability in bothtemperature and mixing is simulated for the dimicticlake (Figure 7). Winter ice cover that is present in thecontrol simulation is absent in the 2xCO2 simulation,and the lake warms throughout the water column by� 4°C in all seasons. As in the control simulation,under 2xCO2 climatic conditions the lake stratifiesduring most summers, but at higher temperatures.The occurrence of smaller warm or cold anomalies(� 2'C), suggests a tendency for stratification atwarmer temperatures to limit the duration of mixingin some years.

Combined with changes in precipitation, broadincreases in lake evaporation in the 2xCO2 simula-tions (Figures 3 and 8) cause widespread changes insimulated net moisture. In the ECHAM4 simulation,drier conditions occur over much of the West andGreat Plains, whereas wetter conditions occur overthe Southeast, Midwest, and Caribbean. The CGCM1produces similar drying over the Great Plains, butconditions wetter than those of the ECHAM4 simula-tion are produced over California and the PacificNorthwest. In opposition to the ECHAM4 results,somewhat drier conditions are simulated by theCGCM1 output over the East and the Southeast.Without a complete water-balance model thataccounts for stream- and groundwater flow, exactdetails of lake-level changes cannot be quantified;however, the general pattern of the P-E anomaliessuggests the overall direction of change that could beexpected.


Hostetler and Small

Figure 7. Simulated Evolution of Depth-Temperature Structure for a 30-rn Deep Lake Over the Last Nine Years ofSimulation Conducted with the CGCM1 Output. The lake is located at 41'N latitude and 88'W longitude on the

model grid. Control (A), 2xCO2 (B) and anomalies (control minus 2xCO2) (C). Black bars on the control figureindicate the duration of winter ice cover, which displays substantial interannual variability. Time axes are

labeled for Januarç April, July, and October (J, A, J, 0). The upper color scale corresponds to thecontrol and 2xCO2 plots, and the lower scale corresponds to the anomalies. Units ofC.

DISCUSSION

Although in detail the lake responses to climatewarming differ in the ECHAM4 and CGCM1 simula-tions, several broad areas of agreement emerge. Bothsimulations exhibit a general warming of lakes of—3CC in ice-free areas and 5C or more over the RockyMountains, the Great Plains, and higher latitudes.Greater warming of the latter regions is attributed tothe loss or reduction of winter ice cover. Substantialsummer warming is simulated in response to longerheating seasons, warmer air temperatures, andincreased longwave radiation in the 2xCO2 climates.Changes in the duration of mixing in the shallow lakeare broadly similar in both simulations. There is lessagreement between simulated changes in P-E, pri-marily due to differences in precipitation rates pro-duced by the different AJOGCMs (Figure 1).

In our modeling study, we used the present andfuture climates produced by two AIOGCMs. Manyother A/OGCMs have been used to simulate future cli-mate change (IPCC, 1996a). Given the relatively widerange of simulated present and future climate pro-duced by the various climate models (IPPC, 1996a),we expect that in detail we would obtain a similarrange of lake responses that in aggregate would yieldthe same level of broad agreement we obtained withthe ECHAM4 and CGCM1.

The lake responses presented here were obtainedwith a lake model that was applied in what is termeda "stand-alone mode." In such an application, outputfrom a climate model is used to drive the lake model,but there is no feedback between the lake and theatmosphere. Both positive and negative feedbacksbetween a lake and the atmosphere govern the overallresponse and sensitivity of a lake to climate. Becausethe lake cannot modify the atmospheric inputs, and



because the climate inputs were obtained from atmo-spheric models in which lakes are unresolved, thesimulated responses obtained in our stand-aloneapplication maximize the sensitivity of lakes to cli-mate change. Nonetheless, our results provide physi-cally consistent estimates of the sensitivity of lakes tofuture climate states. We are addressing the issue ofinteractive feedback by coupling the lake model witha high-resolution atmospheric model (e.g., Hostetleret al., 1993, 1994; Bates et al., 1993, 1995; Small etal., 1999).

Lake Evaporation 2070-79 (mm/yr)

Figure 8. Annual Evaporation Rates as Simulated for 2xCO2Conditions (2070 -2079) Using the ECHAM4 and CGCM1

Input Data Sets. Contour interval of 250 mm/yr.

Our focus here has been on assessing changes inmean lake thermal properties in response to changesin the mean climate state as portrayed in the 2xCO2simulations. It is likely that climatic variability willplay an equal or greater role in determining future

conditions in lakes. Extended and more frequent peri-ods of climatic extremes, for example, could amplify oroffset changes in the mean state. Although it is aninherent property of some lakes and wetlands toattenuate the effects of short-term climatic variability,if the amplitude and duration of such variability islarge, the effect on the ecology of lakes and wetlandsmay override responses to change in average condi-tions (e.g., Katz and Brown, 1992). Assessing theeffects of climate variability on lakes and wetlandswarrants further study.

The effect of average warming on the aquatic ecolo-gy of a given lake would be determined by many fac-tors including geographic location, physiographicsetting, original trophic state, and water chemistry.Some basic qualitative responses to overall warmerlake conditions can be inferred by comparing ourmodel results with more site-specific studies, but acomplete analysis of the effects of climate-drivenchanges in lake thermal structure on the aquatichabitats of specific lakes or wetlands requires addi-tional modeling of parameters such as dissolved oxy-gen, biologic productivity, water chemistry, and thecomplete water balance.

Over the southern and south-central regions of thedomain, August water temperatures increase from—28°C to 32°C or higher. Combined with sustainedincreases of 3-4°C throughout summer, such warmingwould likely present a substantial stress on fish, par-ticularly if thermal refugia at depth were reduced oreliminated (Figures 6 and 7). Where productivity isnot limited by high water temperatures, warmer con-ditions may lead to more eutrophic states. Warmerwater could affect dissolved oxygen levels, especiallyif warmer conditions were accompanied by increasedproductivity, leading to increased oxygen demandsfor aerobic decomposition and possibly anoxic condi-tions at depth (Lam et al., 1987; McCormick, 1990;Schertzer and Sawchuck, 1990; Jewell, 1992; Fang etal., 1999). Warmer conditions may also lead todegraded water quality (McCormick, 1990), increasedsummer anoxia (Stefan et al., 1993; Stefan and Fang,1994), and loss of productivity in boreal lakes(Schindler et al., 1996).

Loss of winter ice cover and associated warmingmay be beneficial to fish populations where productiv-ity and growth presently are limited by the durationof open water periods and cold summer temperatures(e.g., Magnuson et al., 1990; Hostetler and Giorgi;1995; Porter et al., 1996; Stefan et al., 1998; Fang etal., 1999). Productivity may be negatively affected,however, if longer growing periods for fish are notaccompanied by increased food availability (McDonaldet al., 1996). A reduction in the duration of ice mayalso be beneficial to northern lakes in which winteranoxia under ice causes fish kills (Fang et al., 1999).

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Ecological communities that rely on physical process-es that currently occur under ice before springbreakup could be disrupted by a substantial loss oflake ice (Kilham et al., 1996).

Reduced P-E values in the 2xCO2 simulation indi-cate the potential for lower water levels in lake basinsand wetlands, whereas increased P-E would probablyraise the level of most lakes and wetlands. Decline inlake depth, coupled with associated lower flows fromsurface and groundwater sources, may exacerbateincreases in temperature and related problems withloss of thermal refugia and decreased levels of dis-solved oxygen during summer and winter. In addition,increased evaporative losses coupled with decreasedfreshwater inputs may alter lake chemistry, causingfreshwater lakes to become more saline, and perhapslead to meromixis (Romero and Melak, 1996). A trendto drier conditions in one region may accompany atrend to wetter conditions in a neighboring region;thus migratory waterfowl that rely on wetlands fornesting grounds may be able to adapt to drier condi-tions by finding adequate habitat elsewhere.

ACKNOWLEDGMENTS

We thank A. C. Mix, D. L. Zahnle, and an anonymous reviewerfor helpful cemments and suggestions. This research was supportedby the U.S. Geological Survey (SWH).

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