The greenhouse gas, methane, budget has significant uncertainty for many sources, including natural geological emissions. A major uncertainty of geological methane emissions, including onshore and offshore hydrocarbon seepage from subsurface hydrocarbon reservoirs is the gas emissions' temporal variability. Current atmospheric methane budget models assume seepage is constant; nevertheless, available data and seepage conceptual models suggest gas seepage can vary considerably on timescales from second to century. The assumption of steady-seepage is used because long-term datasets to characterize these variabilities are lacking. A 30-year air quality dataset downwind of the Coal Oil Point seep field, offshore California found methane, CH4, concentrations downwind of the seep field increased from a 1995 minimum to a 2008 peak, decreasing exponentially afterward with a 10.2-year timescale (R2 = 0.91). Atmospheric emissions, EA, were derived by a time-resolved Gaussian plume inversion model of the concentration anomaly using observed winds and gridded sonar source location maps. EA increased from 27,200 to 161,000 m3 day-1 (corresponding to 6.5-38 Gg CH4 year-1 for 91% CH4 content) for 1995-2009, respectively, with 15% uncertainty, then decreased exponentially from 2009 to 2015 before rising above the trend. 2015 corresponded to the cessation of oil and gas production, which affects the western seep field. EA varied sinusoidally with a 26.3-year period (R2 = 0.89) that largely tracked the Pacific Decadal Oscillation (PDO), which is driven on these timescales by an 18.6-year earth-tidal cycle (27.9-year beat). A similar controlling factor may underlie both, specifically varying compressional stresses on migration pathways. This also suggests the seep atmospheric budget may exhibit multi-decadal trends.
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