The Enigmatic Growth of Atmospheric Methane

Methane (CH₄) is a strong greenhouse gas and, after CO₂, the 2nd largest contributor to atmospheric radiative forcing. Global CH₄ emissions are divided between natural microbial production (mainly in wetland environments), anthropogenic emissions from fossil fuel production, agriculture and waste. Natural CH₄ sources are very likely to be sensitive to future changes in temperature and precipitation. Although some mitigation of anthropogenic emissions seems possible, population growth may ensure future increases in emissions, especially from agriculture and waste.

Global atmospheric observations of CH₄ show that after stabilizing in the late 1990s and early 2000s, atmospheric methane has resumed its rapid rise since about 2006. The cause of this trend is not currently well understood, but it is of importance because of the possibility that the increase is a response of natural sources to changing climate. High-quality, long-term observational records are essential for understanding the changing methane budget. Global observational network observations provide information about the global budget of CH₄ and its variability, as well as some information about the latitudinal distribution of emissions. Observations of CH₄ isotopes gives additional information about how emissions are partitioned between microbial and thermogenic sources. Use of atmospheric models and data assimilation techniques can result in more specific (though uncertain) spatial, temporal and process information. At the current time, global network observations seem to rule out large changes in Arctic emissions, but we still do not know for whether the observed atmospheric CH₄ increase is an indication of feedback between climate and natural emissions.