Balancing the global methane budget: constraints imposed by isotopes and anthropogenic emission inventories

Abstract

The doubling of atmospheric methane (CH4) during the twentieth century due largely to growth in anthropogenic emissions has made CH4 the second largest contributor behind carbon dioxide to anthropogenic forcing of climate change. However, the global CH4 budget and its decadal evolution remain poorly quantified despite re-evaluations that include the IPCC Fourth Assessment Report. Potentially, the aggregation of national anthropogenic emission inventories as reported to the United Nations Framework Convention on Climate Change could document the changing anthropogenic emission since 1990, as could other “bottom–up” inventories such as EDGAR. As an examination of the recent CH4 budget evolution, we compare two constructions of CH4 source history, one based on an aggregation of national emission inventories, the other version 4 of EDGAR, each in combination with alternative natural CH4 emissions, for consistency with observed atmospheric mixing ratio and carbon isotope content (δ13C(CH4)). We conclude that despite the utility of isotopic constraints on budget evolution, the level of uncertainty in sink strengths and their isotopic fractionation limits the confidence in constructing anthropogenic emission histories over recent decades.