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.