Inter-Annual Variability in Atmospheric Transport Complicates Estimation of US Methane Emissions Trends

US natural gas production increased by & SIM;43% between 2005 and 2015, but there is disagreement among existing studies on whether this growth led to increased methane emissions. We evaluate the likely contributions of atmospheric transport to an upward trend in atmospheric methane enhancements during 2007-2015, defined as the contribution of North American emissions to atmospheric observations across the US. We find that interannual variability (IAV) in transport yields an apparent upward trend in enhancements across much of the US during this time and can explain disagreements among existing studies over emissions trends. We further find that enhancements at satellite and in situ monitoring sites are 19% higher during El Nino than La Nina, possibly because air masses spend more time over North America on average during some years. The results show that accurate modeling of IAV in transport is a key prerequisite to quantifying emissions trends.

Automated summary

US natural gas production increased by 43% between 2005 and 2015, and there is disagreement among studies on whether this growth led to increased methane emissions. The study evaluates the contribution of atmospheric transport to an upward trend in atmospheric methane during 2007-2015, finding that interannual variability in transport explains disagreements among existing studies over emissions trends. The study also shows that enhancements in methane levels are higher during El Nino than La Nina, possibly due to air masses spending more time over North America during certain years.