Constraints on the global sources of methane and an analysis of recent budgets

Recent observations show that the concentration of methane before being affected by human activities was about 650 ppbv; now it is about 1690 ppbv and has increased at an average rate of 16.5 ppbv/yr over the last decade. The present lifetime of methane is 8—12 years. These facts produce constraints on the global budgets of methane and particularly on the anthropogenic fraction (ratio of anthropogenic to total emission rates). Taking into account the possibilities that the lifetime of methane has gotten longer over the last century because of a possible decline in the natural sinks and that emissions from natural sources have also changed, we show that the anthropogenic fraction should be between 40%—70%, and the total present emissions should be between 420—620 Tg/yr. Budgets that do not meet these conditions would be inconsistent with one or more of the observations mentioned above. When we analyzed 11 budgets of methane published over the last decade, we found that only 2 meet these conditions.