Methane - CH4
Methane (CH4) is a very effective greenhouse gas. While its atmospheric concentration is much less than that of carbon dioxide, methane is 20 times more effective at trapping infrared radiation! The atmospheric residence time of methane is approximately 8 years. Residence time is the average time it takes for a molecule to be removed, so in this case for every molecule of methane that goes into the atmosphere it stays there for 8 years until it is removed by some process. The methane biogeochemical cycle is shown in Figure 8. The global processes and fluxes of methane are difficult to measure and thus the atmospheric sources and sinks are difficult to balance. It is estimated that up to 60% of the current methane flux from land to the atmosphere is from activities that are related to human society. Some of these activities include emissions from fermentation processes associated with livestock, from cultivated rice paddies, from fossil fuel and biomass burning, and from landfills. Methane concentrations have been increasing steadily for the past 200 years, although the rate of increase is declining. Over this time period, atmospheric methane concentrations have more than doubled (Figure 9).
Figure 8. The biogeochemical cycle of methane. Fluxes are in millions of tons of carbon per year, and the reservoir size of methane is in millions of tons of carbon.
Of future concern to the issue of global warming is the methane stored in cold environments such as peat bogs in tundra biomes and methane hydrates (frozen methane-ice compounds) found in permafrost regions and in sediments beneath the sea of continental margins. If the climate were to significantly warm, the methane tied up in these areas and forms could be released when those forms melt resulting in a positive feedback (a positive feedback is a process or mechanism that amplifies a change in a system) to global warming.
Figure 9. (Top plot) Global average atmospheric methane concentrations. The y-axis is atmospheric concentration of methane in parts per billion (ppb). The a-xis is the year of the measurement from 1984 to 2003. The red line is an average value line and the blue line is the actual measurement, which varies on yearly basis just like carbon dioxide. (Bottom plot) Methane growth rate which tells how much the concentration of methane increased year to year in the atmosphere. The y-axis is the yearly growth rate in parts per billion per year (ppb y-1) and the x-axis is the year from 1984 to 2003. For example, 1991 and 1998 had increases in atmospheric methane of about 15 parts per billions per year.