Previous studies on ice core analyses and recent in situ measurements have shown that CH 4 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 CH 4 distribution are well represented. The model has been used to derive the total CH 4 emission source, being about 600 Tg yr-1. Based on published results of isotope measurements the total contribution of fossil fuel related CH 4 emissions has been estimated to be about 110 Tg yr-1. However, the individual coal, natural gas and oil associated CH 4 emissions can not be accurately quantified. In particular natural gas and oil associated emissions remain speculative. Since the total anthropogenic CH 4 source is about 410 Tg yr-1 (~70% of the total source) and the mean recent atmospheric CH 4 increase is ~20 Tg yr-1 an anthropogenic source reduction of 5% could stabilize the atmospheric CH 4 level. We have calculated the indirect chemical effects of increasing CH 4 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 CH 4 (direct and indirect effects) to climate forcing during the past 150 years is 0.57 W m-2 (direct 0.44 W m-2, indirect 0.13 W m-2). This is about 35% of the climate forcing by CO 2 (1.6 W 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 CH 4 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 O 3 ( OH) in the tropics. The global mean concentration of CH 4 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 CH 4 global warming potential (GWP) of 21 (kgCH 4 /kgCO 2 ) over a time horizon of 100 years, in agreement with IPCC (1996). Scenario calculations indicate that the importance of the climate forcing by CH 4 (including indirect effects) relative to that of CO 2 will decrease in future; currently this is about 35%, while this is expected to decrease to about 15% in the year 2050. * Corresponding author. et al., 1990). The current global mean atmospheric ** The CH 4 ''lifetime'' (or turnover time) is defined as CH 4 level (year 1995) is about 1.73 mmol/mol (i.e., the total atmospheric CH 4 burden divided by the sum mmol CH 4 /mol air). During the last glacial maxof all loss processes; the ''adjustment'' time (or response imum (~18,000 years BP) the CH 4 level was time) quantifies the decay of an instantaneous CH 4 emission pulse (Subsection 4.4). about 0.35 mmol/mol; it increased to about Tellus 50B (1998), 2