Isotopic signatures of methane emissions from tropical fires, agriculture and wetlands: the MOYA and ZWAMPS flights

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 δ 13 C CH 4 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 δ 13 C CH 4 signatures were measured over the Amazonian wetlands of NE Bolivia (around −59‰) and the overall δ 13 C CH 4 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, δ 13 C CH 4 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, δ 13 C CH 4 values were around −28‰. By contrast, African C4 tropical grass fire δ 13 C CH 4 values were −16 to −12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had δ 13 C CH 4 around −37 to −36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric δ 13 C CH 4 values predicted by global atmospheric models are highly sensitive to the δ 13 C CH 4 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)'.

Very large fluxes of methane measured above Bolivian seasonal wetlands

“…The isotopic results are closely comparable to results from parallel southern outer tropical wetlands in Zambia ( 12 ) and to previously reported overflight measurements from Brazilian Amazonian wetlands that gave a δ 13 C CH4 source signature of −58.8‰ ( 13 ). Such sources, if increasing, would help drive the global methane burden to more negative δ 13 C CH4 emissions.…”

Atmospheric Methane: Comparison Between Methane's Record in 2006–2022 and During Glacial Terminations

“…This work suggests total emissions from wetlands in the Congo and Zambezi basins may be very large. Interestingly, the regional $${\delta }^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{H}4}$$ isotopic signatures of emissions of outer (single rainy season) tropical C4/C3 wetlands in Zambia and Bolivian Amazonia were very similar, around −59‰ to −60‰ (France et al., 2021; MOYA Team et al., 2021), presumably reflecting the similarity of the seasonal outer tropical C4/C3 vegetation mix.…”

“…(2022) estimated a mean global wetland $${\delta }^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{H}4}$$ signature of −61.3 ± 0.7‰. This reflects a mix between northern high‐latitude C3 plant wetlands (mean −67.8‰) and tropical C4‐rich wetlands (mean −56.7‰) (Dlugokencky et al., 2011; Ganesan et al., 2018; MOYA/ZWAMPS Team et al., 2021; Quay et al., 1988; Menoud et al., 2022; Sherwood et al., 2017). Note the means and precisions cited here are from partial and perhaps unrepresentative sampling: real world averages may be several ‰ different.…”

“…(2021) found $${\delta }^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{H}4}$$ −61 ± 4‰. Farmed ruminant emissions vary with diet, with a global average $${\delta }^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{H}4}$$ perhaps around −65‰ (e.g., J. Chang et al., 2019), but data are scarce from tropical ruminants where C4 fodder can dominate; for example, for Kenyan pastured cattle, $${\delta }^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{H}4}$$ is around −57‰ (MOYA/ZWAMPS Team et al., 2021). Fossil fuel emissions have $${\delta }^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{H}4}$$ globally roughly between −43‰ and −45‰ (Lan, Basu, et al., 2021).…”

Metán a légkörben: kockázatok és lehetőségek

“…A metánkoncentráció-mérések örvendetesen szaporodnak a világ fejlettebb országaiban. Sajnos, éppen a kritikus trópusi területek lefedetlenek, ezért ott az eseti expedíciós mérések mellett (Gondwe et al, 2021;Stell et al, 2021;Gauci et al, 2022;Nisbet et al, 2022a) mindenképpen folyamatos megfigyelésekre is szükség lenne. A kulcsfontosságú stabilizotóp-összetétel mérések ( 12 CH 4 , 13 CH 4 ) még meglehetősen ritkák a világban, de Magyarországon már folytak ilyen mérések (Varga et al, 2021).…”