Understanding the Recent Methane Budget

Anthropogenic sources are thought to account for roughly 2/3 of the global atmospheric methane budget, with natural sources making up the other 1/3. Emissions from wetlands are the largest contribution from natural sources while agriculture (rice and ruminants) and waste dominate anthropogenic emissions. Fugitive emissions from fossil fuel extraction are thought to make up about 20% of the global atmospheric methane budget. It is generally recognized that observed inter-annual variability in global network observations can be attributed to natural sources such as wetlands and biomass burning, while longer-term trends likely indicate changes in anthropogenic sources. Exceptions include an abrupt decrease in fossil fuel emissions in the early 1990s associated with political changes in the Former Soviet Union, and long-term trends in emissions from the Arctic due to a warming climate. The growth rate of global average atmospheric methane since the 1980s shows a steady decline until recent years when it started to increase again. Superimposed on these trends are episodes of higher growth rates. The cause of the recent increase is not currently well-understood, although climate-driven increases in wetland emissions likely played an important role, especially in the tropics. Recent increases in anthropogenic emissions, especially from rapidly expanding Asian economies cannot be ruled out. In addition, trends in the photochemical lifetime of methane must also be considered. In this paper we use both traditional data analysis of observations of methane and related species, and a state-of-the-art ensemble data assimilation system (CarbonTracker-CH4) to attribute methane variability and trends to anthropogenic and natural source processes. We pay particular attention to the Arctic, where some recent years have been the warmest on record, and to the tropics and the potential role of ENSO in driving variability of wetland emissions. Finally, we explore whether a signal in anthropogenic emissions is present in the atmospheric network observation, and whether it is present in flux estimates from the methane assimilation. We find that high latitude and tropical wetland emission anomalies are readily identified by the assimilation, however, changes in Asian anthropogenic emissions are still difficult to estimate using the assimilation because of the sparseness of the global network. This highlights the importance of increased surface observations as well as the use of space-based observations.