Abstract
Concentrations of atmospheric methane (CH4), the second most important greenhouse gas, continue to grow. In recent years this growth rate has increased further (2020: +14.7 ppb), the cause of which remains largely unknown. Here, we demonstrate a high-resolution (~80 km), short-window (24-hour) 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 (63.1 Tg yr−1) and posterior (59.8 Tg yr−1) CH4 emissions is from China, mainly attributed to the energy sector. Emissions estimated form our global system agree well with previous basin-wide regional studies and point source specific studies. Emission events (leaks/blowouts) >10 t hr−1 were detected, but without accurate prior uncertainty information, were not well quantified. Our results suggest that global anthropogenic CH4 emissions for 2020 were 5.7 Tg yr−1 (+1.6 %) higher than for 2019, mainly attributed to the energy and agricultural sectors. Regionally, the largest 2020 increases were seen from China (+2.6 Tg yr−1, 4.3 %), with smaller increases from India (+0.8 Tg yr−1, 2.2 %) and Indonesia (+0.3 Tg yr−1, 2.6 %). Results show the rise in emissions, and subsequent atmospheric growth, would have occurred with or without the COVID-19 slowdown. During the onset of the global slowdown (March–April, 2020) energy sector CH4 emissions from China increased; however, during later months (May–June, 2020) emissions decreased below expected pre-slowdown levels. The accumulated impact of the slowdown on CH4 emissions from March–June 2020 is found to be small. Changes in atmospheric chemistry, not investigated here, may have contributed to the observed growth in 2020. Future work aims to develop the global IFS inversion system and to extend the 4D-Var window-length using a hybrid ensemble-variational method.
Highlights
Atmospheric methane as a long-lived greenhouse gas (GHG) has contributed to ~23% of the additional radiative forcing 30 since 1750 (Etminan et al, 2016), second only to CO2
We demonstrate a high-resolution (~80km), short-window (24-hour) 4D-Var global inversion system based on the ECMWF Integrated Forecasting System (IFS) and newly available satellite observations
This study presents and evaluates a first version of the new capabilities introduced in the ECMWF Integrated Forecasting 65 System (IFS) to estimate emissions of greenhouse gases and atmospheric pollutants using satellite observations of their atmospheric concentrations
Summary
Atmospheric methane as a long-lived greenhouse gas (GHG) has contributed to ~23% of the additional radiative forcing 30 since 1750 (Etminan et al, 2016), second only to CO2. Near-surface concentrations have more than doubled since the preindustrial era, with the global average dry air mole fraction reaching 1891 ppb in 2020 (gml.noaa.gov, 2021). The reduction in global human activities, triggered by the COVID-19 pandemic, provided an opportunity to assess the impact of potential rapid climate mitigation strategies to reduce GHG emissions (Diffenbaugh et al, 2020). The change in energy and fuel demand is estimated to have reduced oil and gas CH4 emissions by 10 % for 2020 when compared to 2019 (IEA, 2021). The reduction in demand could have increased venting/flaring when extracting fossil fuels, leading to increased atmospheric concentrations.
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