Little evidence for significant increases in total U.S. methane emissions over the past decade

Recent studies show conflicting estimates of trends in methane (CH₄) emissions from oil and natural gas (ONG) operations in the U.S. We analyze atmospheric CH₄ measurements from 20 North American air sampling sites in the NOAA Global Greenhouse Gas Reference Network (GGGRN) and determined trends for 2006-2015. Using CH₄ vertical gradients (surface or PBL measurements minus mid-troposphere background) as an indicator of regional surface emissions, we find no significant emission increase at most GGGRN sites, but modest increases at three sites heavily influenced by ONG activities. The suggested increases in North American ONG CH₄ emissions (on average 3.4 ± 1.4 % yr^-1) are about one order of magnitude smaller than estimates from some previous studies and below our detection threshold for significant total emissions at the east coast sites that capture the outflows of the U.S. We also find that when enhancements of ethane (C₂H₆) or propane (C₃H₈) and temporally-averaged emission ratios are used to infer trends in ONG CH₄ emissions, the CH₄ trends are significantly overestimated compared with observations. Our analysis finds that the enhancement ratio of C₂H₆ or C₃H₈ versus CH₄ increases over time, and neglecting this trend induces a significant high bias in the estimated ONG CH₄ trend.