Changing concentration, lifetime and climate forcing of atmospheric methane

Abstract

Previous studies on ice core analyses and recent in situ measurements have shown that CH₄ has increased from about 0.75–1.73 μmol/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 CH₄emissions 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 0.57W m^-2 (direct 0.44W m^-2, indirect 0.13 W m^-2). This is about 35% of the climate forcing by CO₂ (1.6W m^-2) and about 22% of the forcing by all long-lived greenhouse gases (2.6 W m^-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 μmol/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.