Methane (CH4) is a greenhouse gas with the second-largest contribution to global warming after CO2. Unlike the inert gas CO2, CH4 actively participates in atmospheric chemistry. A better understanding of the global CH4 budget (i.e., sources and sinks) is vital to constrain its atmospheric levels and radiative forcing in the 21st century, as well as to guide climate mitigation efforts, e.g., policies to fulfil the Paris agreement. Previous studies focus on CH4 sources with less attention on its losses, which have been reported to be the direct reactions with the OH radical (the dominant sink) and Chlorine atoms (Cl). Current chemistry-climate models tend to underestimate the lifetime of CH4, suggesting uncertainties in its sources and sinks.
Over the past decades, a large and growing body of observational evidence has demonstrated the ubiquitous existence of reactive halogen species (RHS; Cl, Br, and I containing species with a lifetime < 180 days) in the troposphere. Reactive halogens (1) supply Cl atoms, directly increasing CH4 loss; and (2) substantially perturb the budget of tropospheric OH, indirectly affecting CH4 loss. However, the existing climate projections of CH4 only consider the direct effect of halogens (Cl-driven) but neglect the indirect effect (via OH).
This work adopts a state-of-the-art Earth system model (CESM), coupled with the online modelling of RHS sources and chemistry, and demonstrates that reactive halogen chemistry increases the global CH4 lifetime by 6-9% during the 21st century. This effect arises from the significant halogen-mediated decrease 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. 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.
Our results suggest that the RHS effects on CH4 parameters (loss rate, lifetime, burden, and radiative forcing) are not constant but rather variable under the changing climate in the 21st century, which is associated with the projected trend in CH4 emission as well as the online simulated future evolution of halogen burdens. In light of these significant results, we recommend that both direct (Cl-driven) and indirect (via OH) impacts of halogens should be included in future CH4 projections.
Qinyi Li, Rafael P. Fernandez, Ryan Hossaini, Fernando Iglesias-Suarez, Carlos A. Cuevas, Eric C. Apel, Douglas E. Kinnison, Jean-François Lamarque, and Alfonso Saiz-Lopez. Reactive halogens increase the global methane lifetime and radiative forcing in the 21st century. Nature Communications, DOI: 10.1038/s41467-022-30456-8.