A Post‐Phaseout Retrospective Reassessment of the Global Methyl Bromide Budget

Methyl bromide is a stratospheric ozone‐depleting substance with both natural and anthropogenic sources. The global budget of methyl bromide has never been fully understood as evidenced by the significant budget gap between the bottom‐up source estimates and calculated atmospheric losses. Atmospheric methyl bromide levels have declined significantly since Phase‐out under the Montreal Protocol began in 1999, and the atmosphere appears to have reached a new steady state during the past five years. Here, we reassess the global methyl bromide budget utilizing the 25‐year record of atmospheric methyl bromide measurements from the National Oceanic and Atmospheric Administration Global Monitoring Laboratory global flask network and a zonal 6‐box coupled global ocean/atmosphere model. Model inversions were used to estimate the total emissions required to account for the observed atmospheric methyl bromide levels. From 1995 to 2019, global land‐based emissions (natural and anthropogenic) declined from about 120 to 85 Gg y−1 and net ocean emissions increased from −5 to +5 Gg y−1. There remains an imbalance between the bottom‐up estimates of terrestrial sources and the inversion result. Based on the timing, magnitude, and spatial distribution of the imbalance we partition it into (a) a persistent or time invariant source located primarily in the tropics, and (b) a smaller time‐varying component that scales with the anthropogenic source during phase‐out. We hypothesize that the persistent source is likely natural and the time variant component is an artifact resulting from a slight underestimation of anthropogenic emissions.