Qinyi Li, Rafael P. Fernandez, Ryan Hossaini, Fernando Iglesias-Suarez, Carlos A. Cuevas, Eric C. Apel, Douglas E. Kinnison, Jean-François Lamarque & Alfonso Saiz-Lopez.
Nature Communications13:2768, https://doi.org/10.1038/s41467-022-30456-8, 2022
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 CH 4 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 CH 4 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 4 burden due to halogens (up to 700 Tg or 8% by 2100) is equivalent to the observed atmospheric CH 4 growth during the last three to four decades. Notably, the halogen-driven enhancement in CH 4 radiative forcing is 0.05 W/m 2 at present and is projected to increase in the future (0.06 W/m 2 by 2100); such enhancement equals ~10% of present-day CH4 radiative forcing and one-third of N2 O 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 4 projections.