Changing concentration, lifetime and climate forcing of atmospheric methane

Previous studies on ice core analyses and recent in situ measurements have shown that CH4 has increased from about 0.75 to 1.73 mmol/mol during the past 150 years. Here, we review sources and sink estimates and we present global 3D model calculations, showing that the main features of the global CH4 distribution are well represented. The model has been used to derive the total CH4 emission source, being about 600 Tg yr−1. Based on published results of isotope measurements the total contribution of fossil fuel related CH4 emissions has been estimated to be about 110 Tg yr−1. However, the individual coal, natural gas and oil associated CH4 emissions can not be accurately quantified. In particular natural gas and oil associated emissions remain speculative. Since the total anthropogenic CH4 source is about 410 Tg yr−1 (~70% of the total source) and the mean recent atmospheric CH4 increase is ~20 Tg yr−1 an anthropogenic source reduction of 5% could stabilize the atmospheric CH4 level. We have calculated the indirect chemical effects of increasing CH4 on climate forcing on the basis of global 3D chemistry-transport and radiative transfer calculations. These indicate an enhancement of the direct radiative effect by about 30%, in agreement with previous work. The contribution of CH4 (direct and indirect effects) to climate forcing during the past 150 years is 0.57Wm−2 (direct 0.44Wm−2, indirect 0.13W m−2). This is about 35% of the climate forcing by CO2 (1.6Wm−2) and about 22% of the forcing by all long-lived greenhouse gases (2.6 Wm−2). Scenario calculations (IPCC-IS92a) indicate that the CH4 lifetime in the atmosphere increased by about 25–30%during the past 150 years to a current value of 7.9 years. Future lifetime changes are expected to be much smaller, about 6%, mostly due to the expected increase of tropospheric O3 (OH) in the tropics. The global mean concentration of CH4 may increase to about 2.55 mmol/mol, its lifetime is expected to increase to 8.4 years in the year 2050. Further, we have calculated a CH4 global warming potential (GWP) of 21 (kgCH4/kgCO2) over a time horizon of 100 years, in agreement with IPCC (1996). Scenario calculations indicate that the importance of the climate forcing by CH4 (including indirect effects) relative to that of CO2 will decrease in future; currently this is about 35%, while this is expected to decrease to about 15% in the year 2050.