Using global isotopic data to constrain the role of shale gas production in recent increases in atmospheric methane

Abstract

The accelerated increase in global methane (CH₄) in the atmosphere, accompanied by a decrease in its ¹³C/¹²C isotopic ratio (δ¹³C_CH4) from −47.1‰ to −47.3‰ observed since 2008, has been attributed to increased emissions from wetlands and cattle, as well as from shale gas and shale oil developments. To date both explanations have relied on poorly constrained δ¹³C_CH4 source signatures. We use a dataset of δ¹³C_CH4 from >1600 produced shale gas samples from regions that account for >97% of global shale gas production to constrain the contribution of shale gas emissions to observed atmospheric increases in the global methane burden. We find that US shale gas extracted since 2008 has volume-weighted-average δ¹³C_CH4 of −39.6‰. The average δ¹³C_CH4 weighted by US basin-level measured emissions in 2015 was −41.8‰. Therefore, emission increases from shale gas would contribute to an opposite atmospheric δ¹³C_CH4 signal in the observed decrease since 2008 (while noting that the global isotopic trend is the net of all dynamic source and sink processes). This observation strongly suggests that changing emissions of other (isotopically-lighter) CH₄ source terms is dominating the increase in global CH₄ emissions. Although production of shale gas has increased rapidly since 2008, and CH₄ emissions associated with this increased production are expected to have increased overall in that timeframe, the simultaneously-observed increase in global atmospheric CH₄ is not dominated by emissions from shale gas and shale oil developments.