Understanding the Global Methane Budget: The Present and Future View Using Top-Down Approaches

Atmospheric methane (CH₄) contributes 0.5 W m^-2 to global radiative forcing, making it the second most important anthropogenic greenhouse gas after carbon dioxide.  Over half of global CH₄ emissions is related to human activities that range from food and energy production to waste disposal.  The largest natural source of atmospheric methane is microbial production in wetlands, and this source is difficult to quantify and potentially sensitive to changing climate.  Unlike CO₂, CH₄ has an atmospheric chemical sink that approximately balances emissions globally, and until 2006 when observed CH₄ abundance started to increase again, it seemed that atmospheric CH₄ had reached an equilibrium distribution.  The reason for the recent increase is not currently well understood.  In addition, there are other intriguing gaps in our understanding of the global atmospheric methane budget, such as why bottom-up estimates of anthropogenic emissions and their increases over recent decades do not appear to be consistent with atmospheric observations, and whether or not we are currently able to detect increases in emissions from high-latitude wetlands due to a rapidly warming Arctic climate. 

Global flux inversions may provide insights into these issues, and we consider results from a suite of 10 global methane inversions using multiple atmospheric transport models, both in-situ and remotely-sensed data, and prior emission models.  We find that there is agreement among the inversions that emissions are highest at low latitudes, especially where high population coincides with strong natural sources, however the details of how emissions are distributed with latitude vary widely among the inversions.  There also does not appear to be systematic differences between those inversions using space-based retrievals and those using only in-situ observations.  In the Arctic, there are no trends in emissions estimated by the inversions over at least the period 2000-2010, raising the possibility that the trends are very small and that the existing observational network is not sensitive enough to detect them.  In addition, the inversions do not agree well on inter-annual variability in emissions.  All of this implies that CH₄ inversions have a ways to go before they are capable of the providing information needed by society for making policy regarding emissions.  It has often been pointed out that the most critical limitation is sparse observational coverage, and while this is certainly true, use of continental observations that could better constrain the spatial distribution of emissions critically depends on the ability to accurately model transport.  Finally we describe development of the NOAA ESRL Earth System Analyzer, that will use a high-resolution global weather model and state of the art assimilation techniques to estimate the atmospheric methane budget.