Curious Behavior of Methane as Earthly Greenhouse Gas

Methane (CH4), a compound found in great abundance throughout our solar system, including in the deep hot Earth, is one of the greenhouse gases supposedly formed by biological processes occurring in oxygen-poor environments. Once in the atmosphere, methane absorbs terrestrial infrared radiation that would otherwise escape to space. This trapping property can contribute to the warming of the atmosphere, which is why scientists count methane as a greenhouse gas. Other greenhouse gases include water vapor, carbon dioxide, nitrous oxide, and ozone, among many others. The Intergovernmental Panel on Climate Change (IPCC), a body formed by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), created a list of greenhouse gases available elsewhere. (1,2)

Methane is about 21 times more powerful at warming the atmosphere than carbon dioxide (CO2) by weight. Methane’s chemical lifetime in the atmosphere is about 12 years, compared with carbon dioxide’s 100+ years. Its potency and relatively short atmospheric lifetime have rendered it a candidate for mitigating the global warming hazard over the near-term. (3)

I. Historical Record of Methane in the Atmosphere

Since the early 1980s, several laboratories have measured the methane (CH4) concentration in industrial age and pre-industrial age ice cores in Greenland and Antarctica. Air bubbles trapped in ice sheets contain methane. (4) A compilation of the most recent studies covering the past 1000 years shows a dramatic increase in atmospheric methane, beginning around 1800 A.D. In fact, methane (like carbon dioxide and nitrous oxide, see graph below) is more abundant in the Earth’s atmosphere now than at any time during the past 400,000 years. (5)

Graph showing increase in methane, carbon dioxide and nitrous oxide in Earth’s atmosphere since 1000 A.D. Source: http://www.manicore.com/anglais/documentation_a/greenhouse/anthropic_graph5.gif; accessed June 5, 2007.	Graph showing atmospheric methane concentration since 1984, showing leveling off. Source: http://www.epa.gov/methane/scientific.html#atmospheric; accessed June 5, 2007.

The steep increase in methane initially has caused concern. Recently however, atmospheric methane has stopped increasing and has leveled off. Scientists anticipated this “happy development” because the rate of increase has been slowing for at least 25 years. (6) Despite the leveling off, the United Nations IPCC scenarios have not accounted for it in their doomsday predictions. Edward J. Dlugokencky, an atmospheric chemist with the National Oceanic and Atmospheric Administration (NOAA), maintains that the evolution of methane levels in the atmosphere reflects the attainment of a chemical equilibrium, meaning the attainment of a balance between methane production and destruction. In sum, he says, atmospheric methane ‘looks like a system approaching a steady state.’” (6) In spite of the apparent leveling off, the methane increase over the last 100 years, if the ice core measurements are reliable and valid, has been impressive indeed.

II. Sources of Methane in the Atmosphere

Khalil attributes the increase in methane levels in the Earth’s atmosphere to the evolution of human populations, which have added methane to the natural methane budget via a number of pathways. Khalil in 2000 estimated the global sources of atmospheric methane as follows (6):

A. How do Wetlands Produce Methane (~20% of total) ?

Methane gas emitted from wetlands, including peatlands (bogs), fens, forested swamps, marshes, permafrost, and rice paddies, have “generally been assumed to:

1. emanate from the sediment or soil surface through the water column,
2. bubble or diffuse to the surface in a process called ebullitive flux and then exit to the atmosphere, or
3. pass through the vascular system of emergent plants,” according to one source. (7)

Numerous other researchers cite enormous quantities of methane (around 400 gigatons) in methane hydrates (ice-like structures, also called methane clathrates) in the cold northern latitude muds and in the muds on the ocean floor. (8) The late Thomas Gold, in his book titled The Deep Hot Biosphere, posited the existence of enormous amounts of non-biogenic petroleum hydrocarbons including methane in the deep hot Earth. These hydrocarbons provided the food source for the original life on earth as well as provide for a vast existing underground biosphere consisting of hypthermophilic (love heat), hyperbarophilic (love pressure) microbes. Gold asserted that the methane and other hydrocarbons came from relic, pre-planetary sources, not from the actions of extreme heat and pressure on buried “fossils”. (9)

The concern with the methane locked up in clathrates is that when the ice melts because of global warming, huge amounts of methane will move into the atmosphere. Measurements of methane in the atmosphere, which have so far remained leveled off in spite of global warming (see above) do not support this concern at the current time.

Map showing the peat regions of the world. Source: http://www.worldenergy.org/wec-geis/publications/reports/ser/peat/image50.gif; accessed June 5, 2007.	Rice field in Philippines. Source: http://www.terradaily.com/images/phillipines-rice-field-afp-bg.jpg; accessed June 5, 2007.

B. How do Rice Paddies Produce Methane? (~12%  of total)?

Mechanistically, the rice plant emits more than 90% of methane released from “rice soil” into the atmosphere. (10) “Well-developed intercellular air spaces in leaf blades, leaf sheaths, culm and roots of [the] rice plant provide an efficient gas-exchange medium between the atmosphere and the anaerobic [oxygen-poor] soil [oxygen poor because it is flooded with water to plant the rice]. CH4  dissolved in soil water surrounding the rice root diffuses into the cell wall-water of the root cells, gasifies in the root cortex and is transported in the gaseous state to the shoots via aerenchyma.” “Thus rice plants influence the methane dynamics in paddy soil by (1) providing substrate in the form of root exudates to methanogens [bacteria that eat methane] and thus enhance the production of CH4; (2) transporting CH4 from soil to atmosphere (conduit effect), and (3) creating aerobic microhabitat in rhizosphere, which is suitable for growth and multiplication of methanotrophic bacteria.” (10)

C. How do Animals Produce Methane (~16% of total)?

Around 95% of global animal enteric (gut) methane is from ruminants. A ruminant is any hooved animal that digests its food in two steps, first by eating the raw material and regurgitating a semi-digested form known as cud, then eating the cud, a process called ruminating. (11) Cattle, goats, sheep, llamas, giraffes, bison, buffalo, deer, wildebeest, and antelope are ruminants. Domestic ruminants contribute such a high percentage of methane emissions as a consequence of their large populations, body size and appetites, combined with the extensive degree of anaerobic microbial fermentation occurring in their guts. (12) The methane presumably is passed out their anuses or perhaps belched into the atmosphere.

D. How does Biomass Burning Produce Methane (~10% of total)?

Biomass burning is the burning of living and dead vegetation. It includes the human-initiated burning of vegetation for land clearing and land-use change (around 90% of total biomass burning) as well as natural, lightning-induced fires (10% of total). (13) Africa is the fire center of the earth with more biomass burned on an annual basis than anywhere else. (13) Methane is a byproduct of this burning.

III. Is Climate Change Occuring? How Do We Know?

The Committee on the Science of Climate Change, National Research Council, in 2001, issued a statement to answer this question, as follows: “Weather station records and ship-based observations indicate that global mean surface air temperature warmed between about 0.4 and 0.8°C (0.7 and 1.5°F) during the 20th century. Although the magnitude of warming varies locally, the warming trend is spatially widespread and is consistent with an array of other evidence…The ocean, which represents the largest reservoir of heat in the climate system, has warmed by about 0.05°C (0.09°F) averaged over the layer extending from the surface down to 10,000 feet, since the 1950s.

“The observed warming has not proceeded at a uniform rate. Virtually all the 20th century warming in global surface air temperature occurred between the early 1900s and the 1940s and during the past few decades. The troposphere warmed much more during the 1970s than during the two subsequent decades, whereas Earth’s surface warmed more during the past two decades than during the 1970s. The causes of these irregularities and the disparities in the timing are not completely understood. One striking change of the past 35 years is the cooling of the stratosphere at altitudes of ~13 miles, which has tended to be concentrated in the wintertime polar cap region.” (14)

IV. Are Greenhouse Gases Causing the Climate Change?

The National Research Council weighs in again: “The IPCC’s conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community on this issue. The stated degree of confidence in the IPCC assessment is higher today than it was 10, or even 5 years ago, but uncertainty remains because of (1) the level of natural variability inherent in the climate system on time scales of decades to centuries, (2) the questionable ability of models to accurately simulate natural variability on those long time scales, and (3) the degree of confidence that can be placed on reconstructions of global mean temperature over the past millennium based on proxy evidence. Despite the uncertainties, there is general agreement that the observed warming is real and particularly strong within the past 20 years. Whether it is consistent with the change that would be expected in response to human activities is dependent upon what assumptions one makes about the time history of atmospheric concentrations of the various forcing agents, particularly aerosols.” (14)

V. Summary

Methane is ubiquitous in our solar system, and on and in the Earth as a relic of pre-planetary sources. Its level in the Earth’s atmosphere has increased dramatically since around 1800 A.D., which is scaring people because of its capacity to behave like a greenhouse gas, intensifying global warming and melting the Earth’s ice caps (think of New Orleans underwater permanently). The majority of the methane produced is from anthropogenic (human activities), such as rice and animal agriculture. How does one decrease rice and animal production without adversely affecting the amount of food available for Earth’s human populations?

Notes:

1. IPCC list of Greenhouse Gases” at: http://en.wikipedia.org/wiki/IPCC_list_of_greenhouse_gases; accessed June 4, 2007.
2. Intergovernmental Panel on Climate Change website is at http://www.ipcc.ch/; accessed June 4, 2007. “Recognizing the problem of potential global climate change, the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) established the Intergovernmental Panel on Climate Change (IPCC) in 1988. It is open to all members of the UN and WMO. The role of the IPCC is to assess on a comprehensive, objective, open and transparent basis the scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential impacts and options for adaptation and mitigation. The IPCC does not carry out research nor does it monitor climate related data or other relevant parameters. It bases its assessment mainly on peer reviewed and published scientific/technical literature.”
3. U.S. Environmental Protection Agency: “Methane”. Available at: http://www.epa.gov/methane/scientific.html#atmospheric; accessed June 4, 2007. Carbon dioxide data from Committee on the Science of Climate Change, National Research Council:  Climate Change Science: An Analysis of Some Key Questions, p. 3. Available online at: http://books.nap.edu/openbook.php?record_id=10139&page=3; accessed June 4, 2007.
4. J. Chappellaz and D. Raynaud: “The Ice Core Record of Atmospheric Methane” In Atmospheric Methane: Its Role in the Global Environment. Mohammad Aslan Khan Khalil (Ed.), Springer-Verlag, 2000, pp. 9-10
5. Committee on the Science of Climate Change, National Research Council:  Climate Change Science: An Analysis of Some Key Questions, p. 2. Available online at: http://books.nap.edu/openbook.php?isbn=0309075742&page=2; accessed June 4, 2007.
6. David Schneider: “That Other Greenhouse Gas”. In Science Observer, American Scientist Online, available at: http://www.americanscientist.org/template/AssetDetail/assetid/54097#54103; accessed June 4, 2007.
7. Mohammad Aslan Khan Khalil (Ed.): Atmospheric Methane: Its Role in the Global Environment, Springer-Verlag, 2000, pp. 1-8.
8. William J. Mitsch, James G. Gosselink: Wetlands. 3rd Edition. Wiley, 2000, p.179.
9. John Atcheson: “Methane Burps: Ticking Time Bomb” in Baltimore Sun December 15, 2004. Available online at: http://www.energybulletin.net/3647.html; accessed June 4, 2007.
10. SEMP Biot #182: “Oil Doesn’t Come from Squashed Ferns and Fish??” (March 4, 2005), available at: http://www.semp.us/publications/biot_reader.php?BiotID=182; accessed June 4, 2007.
11. “Methane emission and rice agriculture” Scientific Correspondence in Current Science, Volume 81, Number 2, August 2001, pp. 345-346. Available at: http://www.ias.ac.in/currsci/aug252001/345.pdf
12. “Ruminant” at: http://en.wikipedia.org/wiki/Ruminant; accessed June 4, 2007.
13. Donald E. Johnson and Gerald M. Ward: “Estimates of animal methane emissions” in Environmental Monitoring and Assessment,  Springer Netherlands, Volume 42, Number  1-2/September, 1996. Abstract available online at:
    http://www.springerlink.com/content/h307k69711m5nh00/; accessed June 4, 2007.
14. NASA Fact Sheet: “Biomass Burning: A Hot Issue in Global Change”, Langley Research Center, Hampton, Virginia, FB-2001-02-56-LaRC. Available at: http://asd-www.larc.nasa.gov/biomass_burn/Factsheet.pdf; and http://asd-www.larc.nasa.gov/biomass_burn/biomass_burn.html; accessed June 4, 2007.
15. Committee on the Science of Climate Change, National Research Council:  Climate Change Science: An Analysis of Some Key Questions, Summary. Available online at: http://www.nap.edu/html/climatechange/summary.html; accessed June 4, 2007.