The indirect global warming potential of methane oxidation in the IPCC's sixth assessment report

Greenhouse-gas emission (GHG) metric values for methane in the sixth assessment report by the Intergovernmental Panel on Climate Change (IPCC) include the indirect effect associated to the oxidation of methane to CO ₂. An analysis of the figures provided by the IPCC reveals they assume that in average 75% of methane is ultimately oxidized to CO ₂, while the remaining 25% is converted to intermediate degradation products, most notably formaldehyde, which are removed from the atmosphere via wet and dry deposition and treated by the IPCC as a potential carbon sink in terrestrial and aquatic ecosystems. In this short article, we present a critique of this assumption, based on existing knowledge about the environmental fate of methane’s degradation products. We conclude that any assumption other than full degradation of methane to CO ₂ in a rather short time frame is questionable, whereby the default CO ₂ yield from this oxidation, as far as GHG metrics are concerned, should be 100%. We re-calculate values for the global warming potential (GWP) metric in accordance with our findings, resulting in an increase in GWP100 from 29.8 and 27.0-30.40 and 27.65 kg CO ₂-equivalents/kg fossil and biogenic methane, respectively. Although we only present the implications in terms of GWP, our proposal is conceptually valid for other GHG metrics as well.