• Energy Research

AbstractObservational evidence shows the ubiquitous presence of ocean-emitted short-lived halogens in the global atmosphere1–3. Natural emissions of these chemical compounds have been anthropogenically amplified since pre-industrial times4–6, while, in addition, anthropogenic short-lived halocarbons are currently being emitted to the atmosphere7,8. Despite their widespread distribution in the atmosphere, the combined impact of these species on Earth’s radiative balance remains unknown. Here we show that short-lived halogens exert a substantial indirect cooling effect at present (−0.13 ± 0.03 watts per square metre) that arises from halogen-mediated radiative perturbations of ozone (−0.24 ± 0.02 watts per square metre), compensated by those from methane (+0.09 ± 0.01 watts per square metre), aerosols (+0.03 ± 0.01 watts per square metre) and stratospheric water vapour (+0.011 ± 0.001 watts per square metre). Importantly, this substantial cooling effect has increased since 1750 by −0.05 ± 0.03 watts per square metre (61 per cent), driven by the anthropogenic amplification of natural halogen emissions, and is projected to change further (18–31 per cent by 2100) depending on climate warming projections and socioeconomic development. We conclude that the indirect radiative effect due to short-lived halogens should now be incorporated into climate models to provide a more realistic natural baseline of Earth’s climate system.

<p><span lang="EN-US">Reactive atmospheric halogens destroy tropospheric ozone (O<sub>3</sub>), an air pollutant and greenhouse gas. The primary source of natural halogens is emissions from marine phytoplankton and algae, as well as abiotic sources from ocean and tropospheric chemistry, but how their fluxes will change under climate warming –and the resulting impacts on O<sub>3</sub>– are not well known. Here we use an Earth system model to estimate that natural halogens deplete approximately 13 % of tropospheric O<sub>3</sub> in the present-day climate. Despite increased levels of natural halogens through the twenty-first century, this fraction remains stable due to compensation from hemispheric, regional, and vertical heterogeneity in tropospheric O<sub>3</sub>loss. Notably, this halogen-driven O<sub>3</sub> buffering is projected to be greatest over polluted and populated regions, mainly due to iodine chemistry, with important implications for air quality.</span></p>

AbstractCH4is 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 CH4loss. However, such an effect of atmospheric halogens is not considered in existing climate projections of CH4burden and radiative forcing. Here, we demonstrate that reactive halogen chemistry increases the global CH4lifetime by 6–9% during the 21st century. This effect arises from significant halogen-mediated decrease, mainly by iodine and bromine, in OH-driven CH4loss that surpasses the direct Cl-induced CH4sink. This increase in CH4lifetime helps to reduce the gap between models and observations and results in a greater burden and radiative forcing during this century. The increase in CH4burden due to halogens (up to 700 Tg or 8% by 2100) is equivalent to the observed atmospheric CH4growth during the last three to four decades. Notably, the halogen-driven enhancement in CH4radiative forcing is 0.05 W/m2at present and is projected to increase in the future (0.06 W/m2by 2100); such enhancement equals ~10% of present-day CH4radiative 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 CH4projections.

AbstractIn contrast to the general stratospheric ozone recovery following international agreements, recent observations show an ongoing net ozone depletion in the tropical lower stratosphere (LS). This depletion is thought to be driven by dynamical transport accelerated by global warming, while chemical processes have been considered to be unimportant. Here we use a chemistry–climate model to demonstrate that halogenated ozone-depleting very short-lived substances (VSLS) chemistry may account for around a quarter of the observed tropical LS negative ozone trend in 1998–2018. VSLS sources include both natural and anthropogenic emissions. Future projections show the persistence of the currently unaccounted for contribution of VSLS to ozone loss throughout the twenty-first century in the tropical LS, the only region of the global stratosphere not projecting an ozone recovery by 2100. Our results show the need for mitigation strategies of anthropogenic VSLS emissions to preserve the present and future ozone layer in low latitudes.