Quantification of methane emissions from hotspots and during COVID-19 using a global atmospheric inversion

Concentrations of atmospheric methane (CH4), the second most important greenhouse gas, continue to grow. In recent years this growth rate has increased further (2020: +15.6gppb), the cause of which remains largely unknown. Here, we demonstrate a high-resolution (g1/480gkm), short-window (24gh) 4D-Var global inversion system based on the ECMWF Integrated Forecasting System (IFS) and newly available satellite observations. The largest national disagreement found between prior (5.3gTg per month) and posterior (5.0gTg per month) CH4 emissions is from China, mainly attributed to the energy sector. Emissions estimated from our global system are in good agreement with those of previous regional studies and point source-specific studies. Emission events (leaks or blowouts)g>g10gtgCH4gh-1 were detected, but without appropriate prior uncertainty information, were not well quantified. Our results suggest that global anthropogenic CH4 emissions for the first 6 months of 2020 were, on average, 470gGg per month (+1.6g%) higher than for 2019, mainly attributed to the energy and agricultural sectors. Regionally, the largest increases were seen from China (+220gGg per month, 4.3g%), with smaller increases from India (+50gGg per month, 1.5g%) and the USA (+40gGg per month, 2.2g%). When assuming a consistent year-on-year positive trend in emissions, results show that during the onset of the global slowdown (March-April 2020) energy sector CH4 emissions from China increased above expected levels; however, during later months (May-June 2020) emissions decreased below expected levels. Results for the first 6 months of 2019/20 suggest that the accumulated impact of the COVID-19 slowdown on CH4 emissions from March-June 2020 might be small relative to the long-term positive trend in emissions. Changes in OH concentration, not investigated here, may have contributed to the observed growth in 2020.