US CH4 Emissions from Oil and Gas Production: Have Recent Large Increases Been Detected?
Recent studies have proposed significant increases in CH4 emissions possibly from oil and gas (O&G) production, especially for the US where O&G production has reached historically high levels over the past decade [US EIA, 2016; Turner et al., 2016; Hausmann et al., 2015; Franco et al., 2016]. In this study, we show that an ensemble of time-dependent atmospheric inversions constrained by calibrated atmospheric observations of surface CH4 mole fraction, with some including space-based retrievals of column average CH4 mole fractions, suggests that North American CH4 emissions have been flat over years spanning 2000 through 2012. Estimates of emission trends using zonal gradients of column average CH4 calculated relative to an upstream background are not easy to make due to atmospheric variability, relative insensitivity of column average CH4 to surface emissions at regional scales, and fast zonal synoptic transport. In addition, any trends in continental enhancements of column average CH4 are sensitive to how the upstream background is chosen, and model simulations imply that short-term (4 years or less) trends in column average CH4 horizontal gradients of up to 1.5 ppb/yr can occur just from inter-annual transport variability acting on a strong latitudinal CH4 gradient. Finally, trends in spatial gradients calculated from space-based column average CH4 can be significantly biased (>2-3 ppb/yr) due to the non-uniform and seasonally varying temporal coverage of satellite retrievals.