Chapter 15 - Global methane

Summary

The biosphere retains in its midst gases such as hydrogen and methane that otherwise would long ago have been lost from the Earth by cosmic processes. It is a souvenir of itself.

Lynn Margulis and Dorion Sagan: Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors (1986) (Summit Books: New York).

Issues

Methane (CH₄) is the second most important of the anthropogenic greenhouse gases in terms of its current contribution to radiative forcing. Mole for mole, the radiative forcing is 21 times that of CO₂, but the climatic impact from methane is smaller than that from CO₂ due to the lower concentrations which result from smaller sources and faster removal from the atmosphere.

Over the period of direct observations, the atmospheric concentration of CH₄ increased for several decades, but during the late 1980s and the 1990s the rate of growth dropped considerably. On longer time-scales, ice-core records show that the atmospheric concentration has more than doubled over the last 400 years. On still longer time-scales, concentrations of CH₄ are closely coupled to glacial/interglacial cycles. However, the current budget of atmospheric CH₄ remains rather uncertain (see Section 15.2 and Table 15.1 below).

Analysis of the behaviour of CH₄ is in many ways more complicated than that for CO₂ because the main process by which CH₄ is lost from the atmosphere is by oxidation through the OH radical in the free atmosphere, rather than by gas exchange at the earth's surface.