U.S. CH4 emissions from oil and gas production: Have recent large increases been detected?

International audienceRecent studies have proposed significant increases in CH 4 emissions possibly from oil and gas (O&G) production, especially for the U.S. where O&G production has reached historically high levels over the past decade. In this study, we show that an ensemble of time-dependent atmospheric inversions constrained by calibrated atmospheric observations of surface CH 4 mole fraction, with some including space-based retrievals of column average CH 4 mole fractions, suggests that North American CH 4 emissions have been flat over years spanning 2000 through 2012. Estimates of emission trends using zonal gradients of column average CH 4 calculated relative to an upstream background are not easy to make due to atmospheric variability, relative insensitivity of column average CH 4 to surface emissions at regional scales, and fast zonal synoptic transport. In addition, any trends in continental enhancements of column average CH 4 are sensitive to how the upstream background is chosen, and model simulations imply that short-term (4 years or less) trends in column average CH 4 horizontal gradients of up to 1.5 ppb/yr can occur just from interannual transport variability acting on a strong latitudinal CH 4 gradient. Finally, trends in spatial gradients calculated from space-based column average CH 4 can be significantly biased (>2-3 ppb/yr) due to the nonuniform and seasonally varying temporal coverage of satellite retrievals. Plain Language Summary In this paper we address recent claims of significant increases in methane emissions from U.S. oil and gas production. We find that such claims are inconsistent with observations by examining atmospheric inversions and observations from the NOAA aircraft monitoring program. Furthermore, we show how atmospheric variability, sampling biases, and choice of upwind background can lead to spurious trends in atmospheric column average methane when using both in situ and space-based retrievals