Multi-Species Constraints Resolve Methane Source Versus Sink Changes: Lessons From 2020

Satellite observations show that atmospheric methane (CH₄) increased by a record-breaking 19 ppb in 2020, but the causes are unclear because of uncertain impacts from COVID-related pollutant shifts on hydroxyl (OH) radical concentrations. Here we employ an ensemble of constraints to simultaneously quantify methane sources and sinks for 2020 in a manner consistent with space-based measurements of its oxidation products (formaldehyde, HCHO; carbon monoxide, CO) and with in situ measurements of methyl chloroform (MCF, an OH proxy). We find that resolving methane source and sink changes requires constraints from the CH₄-HCHO-CO cascade and from MCF: reliance on either alone causes overfitting. The optimized year-2020 global methane sources (557 [514–601] Tg/y) and sinks (505 [459–550] Tg/y) are at the low end of Global Carbon Project ensemble estimates. We further resolve the 2020 methane increase into two distinct periods. During March-May, inversions reveal that methane emissions and global-mean OH values are both lower than predicted; the latter likely reflects offsetting impacts from methane and other pollutants. From June-August, inversions identify methane emission underestimates that mainly reflect temperature-driven biogenic sources. Our study highlights the methane lifetime increase that can accompany reduced pollutant emissions, and demonstrates the necessity of multi-species, satellite-based observations for understanding and attributing future methane trends.