How Humans First How Humans First Altered the Global Altered the Global Climate (William Climate (William

Ruddiman)Ruddiman)

Presenter: Chris SimmonsPresenter: Chris Simmons

November 13, 2006November 13, 2006

The Main IdeaThe Main Idea► COCO22 (Carbon Dioxide) and CH (Carbon Dioxide) and CH44 (Methane) have (Methane) have

varied according to regular cycles for the past varied according to regular cycles for the past several hundred years, according to ice core several hundred years, according to ice core evidenceevidence

► About 10,000 years ago, this 400,000 year-long About 10,000 years ago, this 400,000 year-long cycle ended and both gases gradually began cycle ended and both gases gradually began increasing when they should have been increasing when they should have been decreasingdecreasing

► This cycle breakdown also corresponds to the This cycle breakdown also corresponds to the time period of the advent of time period of the advent of horticulture/pastoralism/agriculture as a human horticulture/pastoralism/agriculture as a human subsistence strategysubsistence strategy

► Ruddiman argues that the changes in GHG Ruddiman argues that the changes in GHG cycles is directly correlated to the change in cycles is directly correlated to the change in human cultural developmenthuman cultural development

Natural Methane and CONatural Methane and CO22 cycles cycles

►As we have seen in class, the Earth’s As we have seen in class, the Earth’s major orbital/axial changes (and major orbital/axial changes (and related solar forcing changes) occur on related solar forcing changes) occur on 22,000, 41,000, and 100,000 year 22,000, 41,000, and 100,000 year cycles (precession, obliquity, and cycles (precession, obliquity, and eccentricity respectively)eccentricity respectively)

►3 Km long ice cores in Vostok (with air 3 Km long ice cores in Vostok (with air bubbles bubbles preserved in each ice bubbles bubbles preserved in each ice layer) record atmospheric composition layer) record atmospheric composition and show cyclical GHG changesand show cyclical GHG changes

The Methane CycleThe Methane Cycle► Methane follows the Methane follows the

precession cycleprecession cycle► Most atmospheric methane Most atmospheric methane

is formed from the decay of is formed from the decay of wetland vegetation wetland vegetation (Methane is also known as (Methane is also known as ‘swamp gas’ because of this)‘swamp gas’ because of this)

► Most of the world’s Most of the world’s continental landmass and continental landmass and wetlands are in the Northern wetlands are in the Northern Hemisphere, so methane Hemisphere, so methane levels are closely connected levels are closely connected to solar radiation variation in to solar radiation variation in the Northern Hemispherethe Northern Hemisphere

The Methane CycleThe Methane Cycle►Strong sunshine in NH results in high Strong sunshine in NH results in high

methane outputmethane output More meltwater in high latitude continental More meltwater in high latitude continental

regionsregions Greater monsoon activity in Asia (greater Greater monsoon activity in Asia (greater

heating of land/ water, more evaporation heating of land/ water, more evaporation and precipitation possible)and precipitation possible)

►As the NH summer becomes aligned As the NH summer becomes aligned with the aphelion, wetland activity with the aphelion, wetland activity decreases and methane levels dropdecreases and methane levels drop

►Thus, methane levels generally bottom Thus, methane levels generally bottom out around now (NH summer is furthest out around now (NH summer is furthest away from the sun, SH summer closest away from the sun, SH summer closest to sun)to sun)

An Illustration of the Methane An Illustration of the Methane CycleCycle

The Recent Breakdown in the The Recent Breakdown in the Methane CycleMethane Cycle

Some Values for the Natural Some Values for the Natural Methane CycleMethane Cycle

►Maximum Methane—Interglacial period, NH Maximum Methane—Interglacial period, NH summers at the perihelion—700 ppbsummers at the perihelion—700 ppb

►Minimum Methane—450 ppb when NH Minimum Methane—450 ppb when NH summer reaches the aphelionsummer reaches the aphelion

► RECENT REVERSAL: We are now in the RECENT REVERSAL: We are now in the aphelion, and over the past 5,000 years aphelion, and over the past 5,000 years methane has been methane has been 250 ppb above250 ppb above the the expected valueexpected value

Carbon Dioxide CycleCarbon Dioxide Cycle► Similar type of predictable pattern as Similar type of predictable pattern as

Methane through most of the Vostok CoreMethane through most of the Vostok Core► The sum effects of precession, eccentricity, The sum effects of precession, eccentricity,

and obliquity produces predictable COand obliquity produces predictable CO22 variationsvariations

► peak at 275-300 ppm at very beginning of peak at 275-300 ppm at very beginning of each warm period, decline steadilyeach warm period, decline steadily

► minimum—245 ppmminimum—245 ppm

► RECENT REVERSAL: 8,000 years ago, began RECENT REVERSAL: 8,000 years ago, began increasing when it should have been increasing when it should have been decreasing, as much as decreasing, as much as 40 ppm above40 ppm above cyclical average by start of the industrial era.cyclical average by start of the industrial era.

An Illustration of the Carbon An Illustration of the Carbon Dioxide CycleDioxide Cycle

The Recent Breakdown in the The Recent Breakdown in the Carbon Dioxide CycleCarbon Dioxide Cycle

Properties of Greenhouse Gases Properties of Greenhouse Gases (Particularly Methane)(Particularly Methane)

►CHCH44 is a very important greenhouse is a very important greenhouse gas; a relatively small increase in gas; a relatively small increase in Methane can have a large impact on Methane can have a large impact on the longwave absorptivity of the the longwave absorptivity of the atmosphereatmosphere

►Demonstration using MODTRAN3, a Demonstration using MODTRAN3, a moderate resolution radiation transfer moderate resolution radiation transfer model, available at:model, available at:

http://geosci.uchicago.edu/~archer/http://geosci.uchicago.edu/~archer/cgimodels/radiation.htmlcgimodels/radiation.html

An Atmosphere of No COAn Atmosphere of No CO22 or or Methane (At 70 km, looking Methane (At 70 km, looking

down)down)

Ground T, K = 272.20 (fixed for average Midlatitude Winter)

Iout, W/m2

= 250.352

An Atmosphere of No COAn Atmosphere of No CO22 and and 10000ppm Methane (At 70 km, 10000ppm Methane (At 70 km,

looking down)looking down)

Ground T, K = 272.20 (fixed for average Midlatitude Winter)

Iout, W/m2 = 234.37

An Atmosphere of No COAn Atmosphere of No CO22 or or Methane (At 70 km, looking Methane (At 70 km, looking

down)down)

Ground T, K = 272.20 (fixed for average Midlatitude Winter)

Iout, W/m2

= 250.352

An Atmosphere of 10000ppm An Atmosphere of 10000ppm COCO22 and No Methane (At 70 km, and No Methane (At 70 km,

looking down)looking down)

Ground T, K = 272.20 (fixed for average Midlatitude Winter)

Iout, W / m2 = 218.23

Model Output ConclusionsModel Output Conclusions

► Carbon Dioxide absorbs near the wavelength Carbon Dioxide absorbs near the wavelength of maximum emission of the Earth (mid to of maximum emission of the Earth (mid to thermal IR), Methane absorbs at significantly thermal IR), Methane absorbs at significantly shorter, higher energy wavelengths (near IR)shorter, higher energy wavelengths (near IR)

► Extreme values of Carbon Dioxide (without Extreme values of Carbon Dioxide (without any Methane present) lead to greater warming any Methane present) lead to greater warming than the same extreme values of Methane than the same extreme values of Methane (without any Carbon Dioxide present)—(without any Carbon Dioxide present)—Methane ‘saturates’ sooner due to less Methane ‘saturates’ sooner due to less radiation emitted at higher near-IR radiation emitted at higher near-IR wavelengthswavelengths

► However, let’s look at a more realistic scenario However, let’s look at a more realistic scenario with less extreme variation…..with less extreme variation…..

Today’s Atmosphere of 330ppm COToday’s Atmosphere of 330ppm CO22 and 1.75ppm Methane (At 70 km, and 1.75ppm Methane (At 70 km,

looking down)looking down)

Ground T, K = 272.20 (fixed for average Midlatitude Winter)

Iout, W/m2 = 228.247

An increase of 0.25ppm of An increase of 0.25ppm of Methane (COMethane (CO22 kept constant) kept constant)

Ground T, K = 272.20 (fixed for average Midlatitude Winter)

Iout, W/m2 = 228.184So we have an increase of +0.063 W/m2 flux at the surface

An increase of 0.25ppm COAn increase of 0.25ppm CO22 (Methane kept constant)(Methane kept constant)

Ground T, K = 272.20 (fixed for average Midlatitude Winter)

Iout, W / m2 = 228.247

So we have an increase of <+0.0005 W/m2 at the surface (several orders or magnitude less than with the same increase in Methane)

Things to Learn from this Things to Learn from this Demonstration: The Properties Demonstration: The Properties

of Methaneof Methane►Given reasonable initial GHG levels, a small Given reasonable initial GHG levels, a small

increase in CHincrease in CH44 has a much larger impact on has a much larger impact on global temperature than the same small global temperature than the same small increase in COincrease in CO22

►Methane absorbs at shorter, higher energy Methane absorbs at shorter, higher energy wavelengths than COwavelengths than CO22 and absorbs across a and absorbs across a longer spectrum of wavelengths than COlonger spectrum of wavelengths than CO22

► So, smaller impacts (pre-industrial human So, smaller impacts (pre-industrial human activities) on methane levels can drive activities) on methane levels can drive significant climate change, hence the author’s significant climate change, hence the author’s and others’ focus on human-related methane and others’ focus on human-related methane increasesincreases

► Post-industrial: Methane has increased over Post-industrial: Methane has increased over 1ppm since 1750. 1ppm since 1750.

Changes in Human Cultural Changes in Human Cultural DevelopmentDevelopment

►Before 10,000 years ago, humans lived Before 10,000 years ago, humans lived primarily in foraging culturesprimarily in foraging cultures Also called ‘hunting and gathering’Also called ‘hunting and gathering’ After the end of the last ice age, humans After the end of the last ice age, humans

began cultivating crops and keeping livestockbegan cultivating crops and keeping livestock this required deforestation, fire burns, etc, this required deforestation, fire burns, etc,

which allowed for increasing COwhich allowed for increasing CO22 and CH and CH44

Horticulture, Pastoralism, and Agriculture Horticulture, Pastoralism, and Agriculture widespread across much of the world by widespread across much of the world by 5,000 years ago5,000 years ago

Allowed for increased population density Allowed for increased population density (more food than before with foraging cultures)(more food than before with foraging cultures)

Pre-industrial Humans’ Pre-industrial Humans’ Influence on Methane LevelsInfluence on Methane Levels

► Rice PaddiesRice Paddies – Humans create – Humans create their own wetlands by trapping their own wetlands by trapping rainwater and runoffrainwater and runoff particularly in SE Asiaparticularly in SE Asia increases methane Levels (author increases methane Levels (author

stresses this is the reason for stresses this is the reason for reversal 5,000 years ago)reversal 5,000 years ago)

► Livestock Livestock – Cows and other – Cows and other animals release methane in animals release methane in feces and belches, and human feces and belches, and human waste as well adds to waste as well adds to methane(yes, and it does make methane(yes, and it does make a difference!). a difference!). Greater population associated with Greater population associated with

agricultural subsistence means agricultural subsistence means more waste and livestock.more waste and livestock.

Pre-industrial Humans’ Influence Pre-industrial Humans’ Influence on Methane Levelson Methane Levels

►Agricultural and Agricultural and Deforestation FiresDeforestation Fires – perhaps the – perhaps the primary source of primary source of methane increasesmethane increases

NIWA Study: NIWA Study: http://www.niwascience.co.nz/puhttp://www.niwascience.co.nz/pubs/mr/archive/2005-09-09-1bs/mr/archive/2005-09-09-1

Agricultural BurnsAgricultural Burns► The NIWA group analyzed isotopic signatures of The NIWA group analyzed isotopic signatures of

Carbon in methane found in the Vostok ice coreCarbon in methane found in the Vostok ice core Can track how methane was produced by its carbon Can track how methane was produced by its carbon

isotope ratio (rice paddies vs. vegetative fires).isotope ratio (rice paddies vs. vegetative fires). fires produce more Carbon-12, which becomes the fires produce more Carbon-12, which becomes the

carbon element of methane moleculescarbon element of methane molecules Low Carbon-13/Carbon-12 ratios in ice cores reveal that Low Carbon-13/Carbon-12 ratios in ice cores reveal that

much of the atmospheric methane of the past few much of the atmospheric methane of the past few thousand years has been connected to burns.thousand years has been connected to burns.

thus, more grassfires/wildfires than one would expect thus, more grassfires/wildfires than one would expect producing higher methane levels (anthropogenic)producing higher methane levels (anthropogenic)

Native Americans and other groups used burning as a Native Americans and other groups used burning as a primary means or replenishing the land, agricultural primary means or replenishing the land, agricultural burns still practiced today around the world.burns still practiced today around the world.

Increase in Carbon DioxideIncrease in Carbon Dioxide►DeforestationDeforestation for agriculture removes trees for agriculture removes trees

that provide important carbon dioxide sinksthat provide important carbon dioxide sinks we know from tools used to cut down trees that we know from tools used to cut down trees that

this process began 8,000 years ago in China, this process began 8,000 years ago in China, EuropeEurope

silts and clays eroding from previously-forested silts and clays eroding from previously-forested hillsides can also provide clues to deforestation hillsides can also provide clues to deforestation ratesrates

Much of the deforestation in N Europe took place Much of the deforestation in N Europe took place during Charlemagne’s time, Domesday book from during Charlemagne’s time, Domesday book from 1100s indicates ~90% of England deforested at 1100s indicates ~90% of England deforested at that time.that time.

Cautionary Note: most important deforestation Cautionary Note: most important deforestation effects will be in tropical regions that hold the effects will be in tropical regions that hold the most extensive forests (lack of data in this region)most extensive forests (lack of data in this region)

An Ice Age Prevented?An Ice Age Prevented?►+250 ppb Methane, +40 ppm Carbon +250 ppb Methane, +40 ppm Carbon

Dioxide from expected levels. What are Dioxide from expected levels. What are the consequences of these increases?the consequences of these increases? average warming of 0.8°C (with all models average warming of 0.8°C (with all models

sensitivities)sensitivities) Author’s individual climate model run: 1.5-Author’s individual climate model run: 1.5-

2°C cooling, last ice age was only 5-6°C 2°C cooling, last ice age was only 5-6°C cooler than todaycooler than today

put 1.5°C cooler temps in model, significant put 1.5°C cooler temps in model, significant glaciers start forming in Canadaglaciers start forming in Canada

ice age might have been only a few thousand ice age might have been only a few thousand years away without this anthropogenic, years away without this anthropogenic, agriculture-related warming, so we’ve agriculture-related warming, so we’ve essentially delayed the next ice ageessentially delayed the next ice age

Pandemics and GHG LevelsPandemics and GHG Levels

►Pandemics reduce agricultural activity Pandemics reduce agricultural activity and aerial coverage and aerial coverage allow trees/vegetation to grow back (in 50 allow trees/vegetation to grow back (in 50

yrs)yrs) leads to decreasing COleads to decreasing CO22

►Major Pandemics: Justinian’s Plague, Major Pandemics: Justinian’s Plague, Bubonic Plague Outbreaks, the Native Bubonic Plague Outbreaks, the Native American pandemics after discoveryAmerican pandemics after discovery

Pandemics and GHG Levels: A Pandemics and GHG Levels: A Critical View Critical View

►Pandemics do not always cause significant Pandemics do not always cause significant changes in agricultural activitieschanges in agricultural activities Immediately after the Black Death, some Immediately after the Black Death, some

villages were abandoned, but most land was villages were abandoned, but most land was recultivated within a few years, and the recultivated within a few years, and the population rebounded within 100 yearspopulation rebounded within 100 years

Urban areas most affected by the Black DeathUrban areas most affected by the Black Death Europe has a very small land area and its Europe has a very small land area and its

forests’ COforests’ CO22 sink is small on a global scale sink is small on a global scale

Pandemics and GHG Levels: A Pandemics and GHG Levels: A Critical ViewCritical View

► The North American Pandemic might have had The North American Pandemic might have had a greater impact on global methane increases a greater impact on global methane increases than COthan CO22 decreases decreases Native Americans burnt vast areas (Great Plains), Native Americans burnt vast areas (Great Plains),

which produced C-12 rich methanewhich produced C-12 rich methane Pandemic wiped out 90% of native population, Pandemic wiped out 90% of native population,

European immigration halted cultural European immigration halted cultural farming/burning practices in many parts of the farming/burning practices in many parts of the AmericasAmericas

Lead to methane decreases (NIWA)Lead to methane decreases (NIWA) Possible COPossible CO22 decrease associated with less decrease associated with less

horticultural slash-burning agriculture in the horticultural slash-burning agriculture in the rainforests of Central Americarainforests of Central America

Other Thoughts?Other Thoughts?

►Foraging humans likely set fires as Foraging humans likely set fires as well, so why is this trend only seen in well, so why is this trend only seen in recent data from the past 10,000 recent data from the past 10,000 years? Why not from before the last years? Why not from before the last ice age as well? Were fires really ice age as well? Were fires really increased because of agriculture?increased because of agriculture?