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
Full text: Download
<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&thinsp;&times;&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&ndash;2016 range from 75 (72&ndash;78)&thinsp;Tg&thinsp;yr<sup>&minus;1</sup> to 80 (78&ndash;83)&thinsp;Tg&thinsp;yr<sup>&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&ndash;1.9)&thinsp;Tg&thinsp;yr<sup>&minus;1</sup> or 2.1 (1.7&ndash;2.5)&thinsp;Tg&thinsp;yr<sup>&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&thinsp;Tg&thinsp;yr<sup>&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>