Natural short-lived halogens exert an indirect cooling effect on climate

Abstract

Observational 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.

Comparing the Effect of Anthropogenically Amplified Halogen Natural Emissions on Tropospheric Ozone Chemistry Between Pre‐Industrial and Present‐Day

“…These changes fall within the uncertainty range of the anthropogenically amplified halogen natural emissions estimated by Saiz‐Lopez et al. (2023). Note that even though there is an increase in global Br y heterogeneous recycling from PI to PD, the result is a decrease in Br y budget due to enhanced bromine wet‐removal by changes in Br y partitioning.…”

“…These changes in the inorganic halogen budget are directly linked to the PI to PD changes in halogen emission sources: (a) inorganic iodine (I 2 + HOI: 105%), (b) halogen heterogeneous recycling (Br y : 8.3%; Cl y : 16.3%), (c) acid-displacement heterogeneous on SSA (Cl y : 313%), and (d) long-lived halocarbons (see Table 2). These changes fall within the uncertainty range of the anthropogenically amplified halogen natural emissions estimated by Saiz-Lopez et al (2023). Note that even though there is an increase in global Br y heterogeneous recycling from PI to PD, the result is a decrease in Br y budget due to enhanced bromine wet-removal by changes in Br y partitioning.…”

Isotopic Evidence Unveils Fossil Fuels Contribution to Atmospheric Iodine

“…We quantified the contribution of fossil fuel combustion to iodine in the winter atmosphere, aiming to enhance our understanding of the impact of human activities on terrestrial atmospheric iodine concentrations. This understanding is pertinent not only to health issues related to iodine deficiency but also to tropospheric ozone loss, especially during the winter season when ocean emissions of iodine are relatively lower. − …”

Frequency Dependence of the Correlation between Ozone and Temperature Oscillations in the Middle Atmosphere

“…Thus, simultaneous measurements of ozone and temperature are valuable to estimate in how far the ozone increase can be attributed to the decrease in chlorine in the upper stratosphere after the ban of chlorofluorocarbons emissions by the Montreal Protocol in 1987 [18]. Further, an indirect radiative effect (cooling) due to short-lived halogens should now be incorporated into climate models to provide a more realistic natural baseline of Earth's climate system [19].…”