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

CH 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 CH, suggesting uncertainties in its sources and sinks. Reactive halogens substantially perturb the budget of tropospheric OH, the main CH loss. However, such an effect of atmospheric halogens is not considered in existing climate projections of CH burden and radiative forcing. Here, we demonstrate that reactive halogen chemistry increases the global CH lifetime by 6-9% during the 21st century. This effect arises from significant halogen-mediated decrease, mainly by iodine and bromine, in OH-driven CH loss that surpasses the direct Cl-induced CH sink. This increase in CH 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 CH burden due to halogens (up to 700 Tg or 8% by 2100) is equivalent to the observed atmospheric CH growth during the last three to four decades. Notably, the halogen-driven enhancement in CH radiative forcing is 0.05 W/m at present and is projected to increase in the future (0.06 W/m by 2100); such enhancement equals ~10% of present-day CH radiative forcing and one-third of NO 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 CH projections.