Methane & Climate Science

by Joe Melton, MSc

Methane (CH4) is the most abundant organic molecule in the Earth’s atmosphere and plays important roles in both the planet’s radiative energy budget and global atmospheric chemistry (Brasseur et al., 1999). It’s presence in the atmosphere was first noted in 1948 from features in the infrared absorption spectrum [Migeotte, 1948] and it is now routinely measured. CH4 is the third most important greenhouse gas after H2O vapor and carbon dioxide (CO2) and has a Global Warming Potential (GWP) 25 times that of CO2 on a 100 –year timescale (Forster et al., 2007).

The direct radiative forcing (warming) due to the CH4 concentration increase in the industrial era (i.e. after 1750 AD) is 0.48 W/m² (Forster et al., 2007). Increasing methane concentration also contributes a radiative forcing indirectly, through tropospheric interactions that influence ozone concentrations, increasing stratospheric water vapor (of which it is the main source), as well as providing a small additional source of CO2 (methane, in its destruction, is oxidized to CO2). If these indirect effects are taken into account, the radiative forcing due to anthropogenic (from human origin) methane increase is estimated at ~ 0.85 W/m² , as compared to 1.66 W/m²for CO2 (Forster et al., 2007).

Methane production processes include methanogenesis by microorganisms under anoxic (without oxygen) conditions, thermogenic (heat) and abiogenic (non-living) methane production occurring below the Earth’s surface, and recently discovered, and controversial, aerobic methane production by plant matter (Keppler et al. 2006). Methane destruction, in turn proceeds by the actions of oxidative methantrophic bacteria, both aerobically and anaerobically, and abiotic processes. It is the second most important sink (after carbon monoxide (CO)) of tropospheric hydroxyl radicals (·OH) (Brasseur et al., 1999). Tropospheric ·OH is the “cleaning agent” of the atmosphere and is responsible for the oxidation and removal of most organic compounds. Substantial fluctuations in CH4 concentration have the power to influence ·OH concentrations and thereby change the lifetimes of many other atmospheric trace species (Forster et al., 2007). The oxidation of CH4 by ·OH is also one of the main sources of CO and formaldehyde in the troposphere (Hobbs, 2000). Changes in CH4 concentration also influence tropospheric ozone production (Brasseur et al., 1999).

Methane records, along with paleo-temperature proxies, from both Antarctica (Petit, et al., 1999) and Greenland (Chappellaz, et al., 1993) reveal the close correlation between methane and millennial-scale warming and cooling. Indeed, CH4 more closely parallels the rapid variations of polar temperature records than any other measured gas (Chappellaz, et al., 1993). Humans have perturbed the atmospheric methane budget to a remarkable extent. Ice core records have shown that the CH4 concentration had remained between 350 and 800 parts per billion (ppb) for the past 650 thousand years (kyr) (Brook et al., 2000; Spahni et al., 2005); whereas presently global mean CH4 concentration is 1775 ppb (Forster et al., 2007).

The CH4 concentration rise due to anthropogenic activities (which includes rice agriculture, ruminants (for eg. cattle), biomass burning (forest and grass fires), coal mining, and landfills) is usually considered to have begun around 1750 AD, although it is also argued that humans significantly affected the global methane budget starting around 5 kyr before present (Ruddiman, 2003).

From the methane record, the CH4 concentration is seen to start to rise from ~700 ppb around 1750 AD and increased in an approximately exponential fashion until about the 1980s. In the 1990s, the rise slowed and CH4 concentration leveled off around the year 2000, which has been attributed to the stabilization of the cumulative CH4 emissions (Forster et al., 2007).

References by Joe Melton, MSc

Brasseur, G., Orlando, J., Tyndall, G. (Editors), 1999. Atmospheric Chemistry and Global Change. Topics in Environmental Chemistry. Oxford University Press, New York, 654 pp.

Brook, E.J., Harder, S., Severinghaus, J., Steig, E.J., Sucher, C.M., 2000. On the origin and timing of rapid changes in atmospheric methane during the last glacial period. Global Biogeochemical Cycles 14, 559-572.

Chappellaz, J., et al. (1993), The atmospheric CH4 increase since the Last Glacial Maximum (1) Source Estimates, Tellus, 45B, 228-241.

Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., Fahey, D.W., Haywood, J., Lean, J., Lowe, D.C., Myhre, G., Nganga, J., Prinn, R., Raga, G., M., S., Van Dorland, R., 2007. Changes in Atmospheric Constituents and in Radiative Forcing. In: S. Solomon et al. (Editors), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, U.K.

Hobbs, P., 2000. Introduction to Atmospheric Chemistry. Cambridge University Press, Cambridge, U.K., 262 pp.

Keppler, F., et al. (2006), Methane emissions from terrestrial plants under aerobic conditions, Nature, 439, 187-191.

Migeotte, M. V. (1948), Methane in earth's atmosphere, Journal of Astrophysics, 107, 400-403.

Petit, J. R., et al. (1999), Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica, Nature, 399, 429-436.

Ruddiman, W.F., 2003. The anthropogenic greenhouse era began thousands of years ago. Climatic Change 61, 261-293.

Spahni, R., Chappellaz, J., Stocker, T.F., Loulergue, L., Hausammann, G., Kawamura, K., Fluckiger, J., Schwander, J., Raynaud, D., Masson-Delmotte, V., Jouzel, J., 2005. Atmospheric methane and nitrous oxide of the late Pleistocene from Antarctic ice cores. Science 310, 1317-1321.

International Panel on Climate Change

"The global atmospheric concentration of CH4 has increased from a pre-industrial value of about 715 ppb to 1732 ppb in the early 1990s and was 1774 ppb in 2005. Growth rates have declined since the early 1990s, consistent with total emissions (sum of anthropogenic and natural sources) being nearly constant during this period."

International Panel on Climate Change, Climate Change 2007: Synthesis Report, p. 37