How Humans First Altered the Global Climate (William Ruddiman)

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How Humans First Altered the Global Climate
(William Ruddiman)

• Presenter Chris Simmons
• November 13, 2006

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The Main Idea

• CO2 (Carbon Dioxide) and CH4 (Methane) have
  varied according to regular cycles for the past
  several hundred years, according to ice core
  evidence
• About 10,000 years ago, this 400,000 year-long
  cycle ended and both gases gradually began
  increasing when they should have been decreasing
• This cycle breakdown also corresponds to the time
  period of the advent of horticulture/pastoralism/a
  griculture as a human subsistence strategy
• Ruddiman argues that the changes in GHG cycles is
  directly correlated to the change in human
  cultural development

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Natural Methane and CO2 cycles

• As we have seen in class, the Earths major
  orbital/axial changes (and related solar forcing
  changes) occur on 22,000, 41,000, and 100,000
  year cycles (precession, obliquity, and
  eccentricity respectively)
• 3 Km long ice cores in Vostok (with air bubbles
  bubbles preserved in each ice layer) record
  atmospheric composition and show cyclical GHG
  changes

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The Methane Cycle

• Methane follows the precession cycle
• Most atmospheric methane is formed from the decay
  of wetland vegetation (Methane is also known as
  swamp gas because of this)
• Most of the worlds continental landmass and
  wetlands are in the Northern Hemisphere, so
  methane levels are closely connected to solar
  radiation variation in the Northern Hemisphere

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The Methane Cycle

• Strong sunshine in NH results in high methane
  output
• More meltwater in high latitude continental
  regions
• Greater monsoon activity in Asia (greater heating
  of land/ water, more evaporation and
  precipitation possible)
• As the NH summer becomes aligned with the
  aphelion, wetland activity decreases and methane
  levels drop
• Thus, methane levels generally bottom out around
  now (NH summer is furthest away from the sun, SH
  summer closest to sun)

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An Illustration of the Methane Cycle
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The Recent Breakdown in the Methane Cycle
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Some Values for the Natural Methane Cycle

• Maximum MethaneInterglacial period, NH summers
  at the perihelion700 ppb
• Minimum Methane450 ppb when NH summer reaches
  the aphelion
• RECENT REVERSAL We are now in the aphelion, and
  over the past 5,000 years methane has been 250
  ppb above the expected value

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Carbon Dioxide Cycle

• Similar type of predictable pattern as Methane
  through most of the Vostok Core
• The sum effects of precession, eccentricity, and
  obliquity produces predictable CO2 variations
• peak at 275-300 ppm at very beginning of each
  warm period, decline steadily
• minimum245 ppm
• RECENT REVERSAL 8,000 years ago, began
  increasing when it should have been decreasing,
  as much as 40 ppm above cyclical average by start
  of the industrial era.

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An Illustration of the Carbon Dioxide Cycle
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The Recent Breakdown in the Carbon Dioxide Cycle
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Properties of Greenhouse Gases (Particularly
Methane)

• CH4 is a very important greenhouse gas a
  relatively small increase in Methane can have a
  large impact on the longwave absorptivity of the
  atmosphere
• Demonstration using MODTRAN3, a moderate
  resolution radiation transfer model, available
  at
• http//geosci.uchicago.edu/archer/cgimodels/radia
  tion.html

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An Atmosphere of No CO2 or Methane (At 70 km,
looking down)
Ground T, K 272.20 (fixed for average
Midlatitude Winter) Iout, W/m2 250.352
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An Atmosphere of No CO2 and 10000ppm Methane (At
70 km, looking down)
Ground T, K 272.20 (fixed for average
Midlatitude Winter) Iout, W/m2 234.37
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An Atmosphere of No CO2 or Methane (At 70 km,
looking down)
Ground T, K 272.20 (fixed for average
Midlatitude Winter) Iout, W/m2 250.352
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An Atmosphere of 10000ppm CO2 and No Methane (At
70 km, looking down)
Ground T, K 272.20 (fixed for average
Midlatitude Winter) Iout, W / m2 218.23
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Model Output Conclusions

• Carbon Dioxide absorbs near the wavelength of
  maximum emission of the Earth (mid to thermal
  IR), Methane absorbs at significantly shorter,
  higher energy wavelengths (near IR)
• Extreme values of Carbon Dioxide (without any
  Methane present) lead to greater warming than the
  same extreme values of Methane (without any
  Carbon Dioxide present)Methane saturates
  sooner due to less radiation emitted at higher
  near-IR wavelengths
• However, lets look at a more realistic scenario
  with less extreme variation..

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Todays Atmosphere of 330ppm CO2 and 1.75ppm
Methane (At 70 km, looking down)
Ground T, K 272.20 (fixed for average
Midlatitude Winter) Iout, W/m2 228.247
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An increase of 0.25ppm of Methane (CO2 kept
constant)
Ground T, K 272.20 (fixed for average
Midlatitude Winter) Iout, W/m2 228.184
So we have an increase of 0.063 W/m2 flux at the
surface
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An increase of 0.25ppm CO2 (Methane kept constant)
Ground T, K 272.20 (fixed for average
Midlatitude Winter) Iout, W / m2 228.247
So we have an increase of lt0.0005 W/m2 at the
surface (several orders or magnitude less than
with the same increase in Methane)
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Things to Learn from this Demonstration The
Properties of Methane

• Given reasonable initial GHG levels, a small
  increase in CH4 has a much larger impact on
  global temperature than the same small increase
  in CO2
• Methane absorbs at shorter, higher energy
  wavelengths than CO2 and absorbs across a longer
  spectrum of wavelengths than CO2
• So, smaller impacts (pre-industrial human
  activities) on methane levels can drive
  significant climate change, hence the authors
  and others focus on human-related methane
  increases
• Post-industrial Methane has increased over 1ppm
  since 1750.

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Changes in Human Cultural Development

• Before 10,000 years ago, humans lived primarily
  in foraging cultures
• Also called hunting and gathering
• After the end of the last ice age, humans began
  cultivating crops and keeping livestock
• this required deforestation, fire burns, etc,
  which allowed for increasing CO2 and CH4
• Horticulture, Pastoralism, and Agriculture
  widespread across much of the world by 5,000
  years ago
• Allowed for increased population density (more
  food than before with foraging cultures)

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Pre-industrial Humans Influence on Methane Levels

• Rice Paddies Humans create their own wetlands
  by trapping rainwater and runoff
• particularly in SE Asia
• increases methane Levels (author stresses this is
  the reason for reversal 5,000 years ago)
• Livestock Cows and other animals release
  methane in feces and belches, and human waste as
  well adds to methane(yes, and it does make a
  difference!).
• Greater population associated with agricultural
  subsistence means more waste and livestock.

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Pre-industrial Humans Influence on Methane Levels

• Agricultural and Deforestation Fires perhaps
  the primary source of methane increases
• NIWA Study http//www.niwascience.co.nz/pubs/mr/
  archive/2005-09-09-1

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Agricultural Burns

• The NIWA group analyzed isotopic signatures of
  Carbon in methane found in the Vostok ice core
• Can track how methane was produced by its carbon
  isotope ratio (rice paddies vs. vegetative
  fires).
• fires produce more Carbon-12, which becomes the
  carbon element of methane molecules
• Low Carbon-13/Carbon-12 ratios in ice cores
  reveal that much of the atmospheric methane of
  the past few thousand years has been connected to
  burns.
• thus, more grassfires/wildfires than one would
  expect producing higher methane levels
  (anthropogenic)
• Native Americans and other groups used burning as
  a primary means or replenishing the land,
  agricultural burns still practiced today around
  the world.

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Increase in Carbon Dioxide

• Deforestation for agriculture removes trees that
  provide important carbon dioxide sinks
• we know from tools used to cut down trees that
  this process began 8,000 years ago in China,
  Europe
• silts and clays eroding from previously-forested
  hillsides can also provide clues to deforestation
  rates
• Much of the deforestation in N Europe took place
  during Charlemagnes time, Domesday book from
  1100s indicates 90 of England deforested at
  that time.
• Cautionary Note most important deforestation
  effects will be in tropical regions that hold the
  most extensive forests (lack of data in this
  region)

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An Ice Age Prevented?

• 250 ppb Methane, 40 ppm Carbon Dioxide from
  expected levels. What are the consequences of
  these increases?
• average warming of 0.8C (with all models
  sensitivities)
• Authors individual climate model run 1.5-2C
  cooling, last ice age was only 5-6C cooler than
  today
• put 1.5C cooler temps in model, significant
  glaciers start forming in Canada
• ice age might have been only a few thousand years
  away without this anthropogenic,
  agriculture-related warming, so weve essentially
  delayed the next ice age

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Pandemics and GHG Levels

• Pandemics reduce agricultural activity and aerial
  coverage
• allow trees/vegetation to grow back (in 50 yrs)
• leads to decreasing CO2
• Major Pandemics Justinians Plague, Bubonic
  Plague Outbreaks, the Native American pandemics
  after discovery

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Pandemics and GHG Levels A Critical View

• Pandemics do not always cause significant changes
  in agricultural activities
• Immediately after the Black Death, some villages
  were abandoned, but most land was recultivated
  within a few years, and the population rebounded
  within 100 years
• Urban areas most affected by the Black Death
• Europe has a very small land area and its
  forests CO2 sink is small on a global scale

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Pandemics and GHG Levels A Critical View

• The North American Pandemic might have had a
  greater impact on global methane increases than
  CO2 decreases
• Native Americans burnt vast areas (Great Plains),
  which produced C-12 rich methane
• Pandemic wiped out 90 of native population,
  European immigration halted cultural
  farming/burning practices in many parts of the
  Americas
• Lead to methane decreases (NIWA)
• Possible CO2 decrease associated with less
  horticultural slash-burning agriculture in the
  rainforests of Central America

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Other Thoughts?

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