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

Abstract

Abstract. Emissions of methane ( CH4 ) from tropical ecosystems, and how they\nrespond to changes in climate, represent one of the biggest\nuncertainties associated with the global CH4 budget. Historically,\nthis has been due to the dearth of pan-tropical in\xa0situ measurements,\nwhich is particularly acute in Africa. By virtue of their superior\nspatial coverage, satellite observations of atmospheric CH4 \ncolumns can help to narrow down some of the uncertainties in the\ntropical CH4 emission budget. We use proxy column retrievals of\natmospheric CH4 ( XCH4 ) from the Japanese Greenhouse gases\nObserving Satellite (GOSAT) and the nested version of the GEOS-Chem\natmospheric chemistry and transport model\n( 0.5 ∘ × 0.625 ∘ ) to infer emissions from tropical\nAfrica between 2010 and 2016. Proxy retrievals of XCH4 are less\nsensitive to scattering due to clouds and aerosol than full physics retrievals, but the method assumes\nthat the global distribution of carbon dioxide ( CO2 ) is known. We\nexplore the sensitivity of inferred a\xa0posteriori emissions to\nthis source of systematic error by using two different XCH4 data\nproducts that are determined using different model CO2 fields. We\ninfer monthly emissions from GOSAT XCH4 data using a hierarchical\nBayesian framework, allowing us to report seasonal cycles and trends\nin annual mean values. We find mean tropical African emissions between 2010 and 2016 range\nfrom 76 (74–78) to 80 (78–82)\u2009Tg\u2009yr −1 ,\ndepending on the proxy XCH4 data used, with larger differences in\nNorthern Hemisphere Africa than Southern Hemisphere Africa. We find a\nrobust positive linear trend in tropical African CH4 emissions for our 7-year study period, with values of 1.5\xa0(1.1–1.9)\u2009Tg\u2009yr −1 \nor 2.1\xa0(1.7–2.5)\u2009Tg\u2009yr −1 , depending on the CO2 data\nproduct used in the proxy retrieval. This linear emissions trend accounts for around a third of the global emissions growth rate during this period. A substantial portion of this increase is due to a short-term increase in emissions of 3\u2009Tg\u2009yr −1 between 2011 and 2015 from the Sudd in South Sudan. Using satellite land surface temperature anomalies and altimetry data, we find this increase in CH4 emissions is consistent with an increase in wetland extent due to increased inflow from the White Nile, although the data indicate that the Sudd was anomalously dry at the start of our inversion period. 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\xa0posteriori CH4 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\xa0priori estimates.