How Humans First Altered the Global Climate (William Ruddiman) Presenter: Chris Simmons November 13, 2006

The Main Idea ► CO 2 (Carbon Dioxide) and CH 4 (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/agriculture as a human subsistence strategy ► Ruddiman argues that the changes in GHG cycles is directly correlated to the change in human cultural development

Natural Methane and CO 2 cycles ► As we have seen in class, the Earth’s 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

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

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)

An Illustration of the Methane Cycle

The Recent Breakdown in the Methane Cycle

Some Values for the Natural Methane Cycle ► Maximum Methane—Interglacial period, NH summers at the perihelion—700 ppb ► Minimum Methane—450 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

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 CO 2 variations ► peak at ppm at very beginning of each warm period, decline steadily ► minimum—245 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.

An Illustration of the Carbon Dioxide Cycle

The Recent Breakdown in the Carbon Dioxide Cycle

Properties of Greenhouse Gases (Particularly Methane) ► CH 4 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: /radiation.html

An Atmosphere of No CO 2 or Methane (At 70 km, looking down) Ground T, K = (fixed for average Midlatitude Winter) Iout, W/m 2 =

An Atmosphere of No CO 2 and 10000ppm Methane (At 70 km, looking down) Ground T, K = (fixed for average Midlatitude Winter) Iout, W/m 2 =

An Atmosphere of No CO 2 or Methane (At 70 km, looking down) Ground T, K = (fixed for average Midlatitude Winter) Iout, W/m 2 =

An Atmosphere of 10000ppm CO 2 and No Methane (At 70 km, looking down) Ground T, K = (fixed for average Midlatitude Winter) Iout, W / m2 =

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

Today’s Atmosphere of 330ppm CO 2 and 1.75ppm Methane (At 70 km, looking down) Ground T, K = (fixed for average Midlatitude Winter) Iout, W/m 2 =

An increase of 0.25ppm of Methane (CO 2 kept constant) Ground T, K = (fixed for average Midlatitud e Winter) Iout, W/m 2 = So we have an increase of W/m 2 flux at the surface

An increase of 0.25ppm CO 2 (Methane kept constant) Ground T, K = (fixed for average Midlatitude Winter) Iout, W / m 2 = So we have an increase of < W/m 2 at the surface (several orders or magnitude less than with the same increase in Methane)

Things to Learn from this Demonstration: The Properties of Methane ► Given reasonable initial GHG levels, a small increase in CH 4 has a much larger impact on global temperature than the same small increase in CO 2 ► Methane absorbs at shorter, higher energy wavelengths than CO 2 and absorbs across a longer spectrum of wavelengths than CO 2 ► So, smaller impacts (pre-industrial human activities) on methane levels can drive significant climate change, hence the author’s and others’ focus on human-related methane increases ► Post-industrial: Methane has increased over 1ppm since 1750.

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 CO 2 and CH 4  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)

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.

Pre-industrial Humans’ Influence on Methane Levels ► Agricultural and Deforestation Fires – perhaps the primary source of methane increases NIWA Study: r/archive/ NIWA Study: r/archive/

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.

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 Charlemagne’s 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)

An Ice Age Prevented? ► +250 ppb Methane, +40 ppm Carbon Dioxide from expected levels. What are the consequences of these increases?  average warming of 0.8°C (with all models sensitivities)  Author’s individual climate model run: 1.5-2°C cooling, last ice age was only 5-6°C cooler than today  put 1.5°C 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 we’ve essentially delayed the next ice age

Pandemics and GHG Levels ► Pandemics reduce agricultural activity and aerial coverage  allow trees/vegetation to grow back (in 50 yrs)  leads to decreasing CO 2 ► Major Pandemics: Justinian’s Plague, Bubonic Plague Outbreaks, the Native American pandemics after discovery

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’ CO 2 sink is small on a global scale

Pandemics and GHG Levels: A Critical View ► The North American Pandemic might have had a greater impact on global methane increases than CO 2 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 CO 2 decrease associated with less horticultural slash-burning agriculture in the rainforests of Central America

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?