Abstract
We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane δ13CCH4 isotopic signatures were in the range −55 to −49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely −60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar δ13CCH4 signatures were measured over the Amazonian wetlands of NE Bolivia (around −59‰) and the overall δ13CCH4 signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was −59 ± 2‰. These results were more negative than expected. For African cattle, δ13CCH4 values were around −60 to −50‰. Isotopic ratios in methane emitted by tropical fires depended on the C3 : C4 ratio of the biomass fuel. In smoke from tropical C3 dry forest fires in Senegal, δ13CCH4 values were around −28‰. By contrast, African C4 tropical grass fire δ13CCH4 values were −16 to −12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had δ13CCH4 around −37 to −36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric δ13CCH4 values predicted by global atmospheric models are highly sensitive to the δ13CCH4 isotopic signatures applied to tropical wetland emissions. Field and aircraft campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. The work highlights the need to understand tropical greenhouse gas emissions in order to meet the goals of the UNFCCC Paris Agreement, and to help reduce air pollution over wide regions of Africa.This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.
Highlights
The objectives were to measure methane in air over major tropical sources, especially African wetlands, regional agriculture and biomass burning, to determine at regional scale the characteristic isotopic signatures of these methane sources, and thereby to help constrain regional methane source fluxes and their roles in global methane budget.There is strong evidence to suggest increasing tropical biological sources such as ruminants and wetlands are major drivers of methane’s recent growth [1,2,3,4]
Wetlands are one of the largest global sources of atmospheric methane, estimated to contribute up to approximately 35% of global methane emissions (e.g. [8,53]), with the latitudinal gradient in atmospheric methane mole fractions observed in the NOAA network indicating the bulk of these emissions are situated in tropical rather than high latitude regions
Atmospheric δ13CCH4 values predicted by global atmospheric models are sensitive to the δ13CCH4 isotopic signature applied to tropical wetland emissions
Summary
There is strong evidence to suggest increasing tropical biological sources such as ruminants and wetlands are major drivers of methane’s recent growth [1,2,3,4]. Major tropical methane sources such as wetlands and cattle emit methane isotopically depleted in 13C compared to the bulk global source [2,3,34]. Though isotopic source signatures are key inputs needed if isotopic modelling is to help impose better constraints on global methane budgets, there have been very few studies of the isotopic signatures of methane sources emitting into tropical air masses, especially over central Africa
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