Recent trends in global methane emissions inferred from 30-years of surface CH4 and δ13C-CH4 measurements

Abstract:

Methane (CH₄) is the second most important greenhouse gas after carbon dioxide. Quantitative understanding of the global CH₄ budget is important in order to predict future contributions of global warming and to develop effective strategies for its mitigation. After a decade-long period of decreasing growth rates, atmospheric CH₄ concentrations have risen in recent years raising concerns about its long-term stability. Despite much study there is no clear consensus about the causes of this increase. Some bottom-up inventories suggest that fugitive emissions from the fossil fuel industry may be increasing though there are considerable uncertainties in these estimates due in part to the variable nature of this source. To better understand the changing budget of atmospheric CH₄, we performed a time-dependent retrieval of CH₄ fluxes spanning nearly 30 years using global surface and isotopic CH₄ measurements. An earlier inversion from our group suggested that fugitive fossil fuel emissions increased from 2000-2008, contributing to the recent rise. Here we extend this study to year 2015 using the most recent NOAA Global Monitoring Division (GMD) CH₄ measurements and available δ¹³C -CH₄ data along with a higher resolution version of the chemical transport model GEOS-Chem (2^ox2.5^o). The higher resolution grid allowed us to extract more information from the observations due to the improved ability of the model to simulate CH₄ at each station and the decreased need to aggregate neighboring stations. This in turn increased the error reduction between the a priori and posterior emissions leading to more accurate estimates of the retrieved fluxes and improved understanding of the drivers underlying the current atmospheric record.