Reactive halogens increase the global methane lifetime and radiative forcing in the 21st century.

Abstract

CH₄ is the most abundant reactive greenhouse gas and a complete understanding of its atmospheric fate is needed to formulate mitigation policies. Current chemistry-climate models tend to underestimate the lifetime of CH₄, suggesting uncertainties in its sources and sinks. Reactive halogens substantially perturb the budget of tropospheric OH, the main CH₄ loss. However, such an effect of atmospheric halogens is not considered in existing climate projections of CH₄ burden and radiative forcing. Here, we demonstrate that reactive halogen chemistry increases the global CH₄ lifetime by 6-9% during the 21st century. This effect arises from significant halogen-mediated decrease, mainly by iodine and bromine, in OH-driven CH₄ loss that surpasses the direct Cl-induced CH₄ sink. This increase in CH₄ lifetime helps to reduce the gap between models and observations and results in a greater burden and radiative forcing during this century. The increase in CH₄ burden due to halogens (up to 700 Tg or 8% by 2100) is equivalent to the observed atmospheric CH₄ growth during the last three to four decades. Notably, the halogen-driven enhancement in CH₄ radiative forcing is 0.05 W/m² at present and is projected to increase in the future (0.06 W/m² by 2100); such enhancement equals ~10% of present-day CH₄ radiative forcing and one-third of N₂O radiative forcing, the third-largest well-mixed greenhouse gas. Both direct (Cl-driven) and indirect (via OH) impacts of halogens should be included in future CH₄ projections.