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European Geosciences Union, Atmospheric Chemistry and Physics, 23(19), p. 14721-14740, 2019

DOI: 10.5194/acp-19-14721-2019

European Geosciences Union, Atmospheric Chemistry and Physics Discussions, p. 1-30, 2019

DOI: 10.5194/acp-2019-477

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An increase in methane emissions from tropical Africa between 2010 and 2016 inferred from satellite data

This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

<p><strong>Abstract.</strong> Emissions of methane (CH<sub>4</sub>) from tropical ecosystems, and how they respond to changes in climate, represent one of the biggest uncertainties associated with the global CH<sub>4</sub> budget. Historically, this has been due to the dearth of pan-tropical in situ measurements, which is particularly acute in Africa. By virtue of their superior spatial coverage, satellite observations of atmospheric CH<sub>4</sub> columns can help to narrow down some of the uncertainties in the tropical CH<sub>4</sub> emission budget. We use proxy column retrievals of atmospheric CH<sub>4</sub> (XCH<sub>4</sub>) from the Japanese Greenhouse gases Observing SATellite (GOSAT) and the nested version of the GEOS-Chem atmospheric chemistry and transport model (0.5&amp;thinsp;&amp;times;&amp;thinsp;0.625) to infer emissions from tropical Africa between 2010 and 2016. Proxy retrievals of XCH<sub>4</sub> are less sensitive to scattering due to clouds and aerosol than full physics retrievals but the method assumes that the global distribution of carbon dioxide (CO<sub>2</sub>) is known. We explore the sensitivity of inferred a posteriori emissions to this source of systematic error by using two different XCH<sub>4</sub> data products that are determined using different model CO<sub>2</sub> fields. We infer monthly emissions from GOSAT XCH<sub>4</sub> data using a hierarchical Bayesian framework, allowing us to report seasonal cycles and trends in annual mean values. We find mean tropical African emissions between 2010&amp;ndash;2016 range from 75 (72&amp;ndash;78)&amp;thinsp;Tg&amp;thinsp;yr<sup>&amp;minus;1</sup> to 80 (78&amp;ndash;83)&amp;thinsp;Tg&amp;thinsp;yr<sup>&amp;minus;1</sup>, dependent on the proxy XCH<sub>4</sub> data used, with larger differences in northern hemisphere Africa than southern hemisphere Africa. We find a robust positive linear trend in tropical African CH<sub>4</sub> emissions for our seven-year study period, with values of 1.5 (1.1&amp;ndash;1.9)&amp;thinsp;Tg&amp;thinsp;yr<sup>&amp;minus;1</sup> or 2.1 (1.7&amp;ndash;2.5)&amp;thinsp;Tg&amp;thinsp;yr<sup>&amp;minus;1</sup>, dependent on the CO<sub>2</sub> data product used in the proxy retrieval. A substantial portion of this increase is due to a short-term increase in emissions of 3&amp;thinsp;Tg&amp;thinsp;yr<sup>&amp;minus;1</sup> between 2011 and 2015 from the Sudd in South Sudan. Using satellite land surface temperature anomalies and altimetry data we find this increase in CH<sub>4</sub> emission is consistent with an increase in wetland extent due to increased inflow from the White Nile. We find a strong seasonality in emissions across northern hemisphere Africa, with the timing of the seasonal emissions peak coincident with the seasonal peak in ground water storage. In contrast, we find that a posteriori CH<sub>4</sub> emissions from the wetland area of the Congo basin are approximately constant throughout the year, consistent with less temporal variability in wetland extent, and significantly smaller than a priori estimates.</p>