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

The study investigates how natural and anthropogenic short-lived halogen species (chlorine, bromine and iodine compounds with atmospheric lifetimes under six months) influence Earth's radiative balance. Although climate research has long emphasized greenhouse gases and sulfur-derived aerosols, the role of reactive halogens in altering atmospheric oxidation capacity and regulating short-lived climate forcers (SLCF) has been underexplored. Observations over the past two decades demonstrate ubiquitous SLH in the global atmosphere, emitted mainly from the oceans, polar ice and the biosphere, and recently augmented by rising anthropogenic halogen emissions. SLH chemistry affects ozone through catalytic loss cycles, modifies OH and HOx/NOx balances, influences methane lifetime, and alters aerosol formation pathways (including DMS oxidation and iodine-driven new particle formation). Yet CMIP6-class climate models and IPCC assessments have generally omitted explicit SLH emissions and chemistry. The purpose of this work is to quantify the indirect radiative effect of SLH across pre-industrial, present-day and future climates, thereby assessing their net impact on Earth's energy balance and highlighting implications for climate modeling and projections.

Extensive literature documents sources, chemistry and impacts of SLH. Global observations have confirmed widespread halogen radicals and reservoirs in the troposphere and lower stratosphere, including iodine, bromine and chlorine species from marine, cryospheric and biospheric sources. Prior modeling studies suggest halogens can reduce tropospheric ozone by roughly 10–20%, lowering ozone radiative forcing, and influence OH, thereby modulating methane lifetime. Iodine oxides and oxoacids can nucleate particles, while halogen oxidation of DMS can suppress CCN formation in remote marine air. Feedbacks between anthropogenic ozone pollution and oceanic iodine emissions indicate that deposition of O3 onto the ocean enhances iodine release (AANE), documented in ice cores and tree rings. Rapid increases in anthropogenic short-lived chlorocarbons (for example, CH2Cl2, CHCl3, CHBr3) have been observed, posing additional risks to stratospheric ozone. Despite these insights, comprehensive Earth-system-scale quantification of the net radiative effect of combined natural and anthropogenic SLH, separating contributions from gases and aerosols across time periods and scenarios, has been lacking.

The authors used the Community Earth System Model (CESM) with CAM-Chem chemistry and the RRTMG radiation module to compute SLH-mediated radiative effects (RE) at the top of model for all-sky conditions. Time-slice simulations represent pre-industrial (year 1750), present-day (2020), and future (2100) climates under RCP6.0 and RCP8.5. SLH sources were partitioned into: NAT (natural emissions), AANE (anthropogenically amplified natural emissions driven by anthropogenic pollutants altering ocean/ice/aerosol chemistry and partitioning), and ANT (direct anthropogenic SLH emissions). Pre-industrial simulations included NAT only (oceanic, polar and biogenic sources such as HOI, CHBr3, BrCl, Cl2). Present-day and future simulations included ANT (e.g., HCl, CH2Cl2 from industry, combustion, waste burning, etc.) and AANE linked to anthropogenic precursors (NOx, VOCs, O3, HNO3) that increase halogen release from natural reservoirs. The model calculated halogen-induced perturbations in key SLCF: ozone (O3), methane (CH4), aerosols (sulfate, SOA, ammonium nitrate), and stratospheric water vapour (H2Ostrat). Uncertainties were assessed through a suite of sensitivity experiments spanning SLH burdens and processes, with RE uncertainty reported as the half-range across sensitivities for each period. The analysis also quantified the change in RE relative to pre-industrial (ARE) and decomposed contributions of AANE versus ANT. Spatial and latitudinal distributions of RE were diagnosed to reveal geographic patterns and hotspots.

• Present-day net indirect radiative effect of SLH (gases + aerosols) is -0.13 ± 0.03 W m⁻² (cooling).
• Component contributions at present: O3: -0.24 ± 0.02 W m⁻² (cooling); CH4: +0.09 ± 0.01 W m⁻² (warming); aerosols: +0.03 ± 0.01 W m⁻² (warming); stratospheric H2O: +0.011 ± 0.001 W m⁻² (warming).
• Since 1750, the SLH-driven cooling has increased by -0.05 ± 0.03 W m⁻² (61%), largely due to AANE rather than direct ANT emissions, and is projected to change by 18–31% by 2100 depending on scenario.
• Pre-industrial SLH RE: net -0.08 ± 0.02 W m⁻², dominated by gases (-0.11 ± 0.02 W m⁻²) with aerosol warming offset of +0.03 ± 0.01 W m⁻².
• Ozone: With halogens, pre-industrial tropospheric and stratospheric O3 decrease by -3.3 DU and -3.9 DU; RE change -0.16 ± 0.01 W m⁻². Present-day reductions: -4.9 DU (troposphere) and -5.2 DU (stratosphere), RE -0.24 ± 0.02 W m⁻². By 2100: O3 RE -0.19 ± 0.01 W m⁻² (RCP6.0; total O3 loss -8.5 DU) and -0.24 ± 0.02 W m⁻² (RCP8.5; -10.7 DU).
• Methane: SLH increase CH4 burden by +14% (pre-industrial) and +9% (present-day), yielding +0.09 ± 0.01 W m⁻² RE in both. By 2100: +0.10 ± 0.01 W m⁻² (RCP6.0) and +0.11 ± 0.01 W m⁻² (RCP8.5), with burden increases of 464 Tg (11%) and 936 Tg (7%) relative to corresponding no-SLH cases.
• Stratospheric water vapour: Halogen-driven increase via CH4 oxidation gives +0.011 ± 0.001 W m⁻², similar magnitude in future.
• Aerosols: SLH reduce OH and modify oxidant fields, decreasing formation of secondary aerosols (sulfate, SOA, NH4NO3), thereby weakening aerosol cooling and producing +0.03 ± 0.01 W m⁻² warming (pre-industrial and present-day). Regional aerosol RE hotspots occur over Europe, eastern North America and East Asia.
• Spatial/latitudinal patterns: CH4 warming peaks in low latitudes; O3 cooling strongest at high latitudes and over polar regions; aerosol signal peaks over the Southern Ocean. Net SLH cooling is much stronger at high latitudes than in the tropics.
• Source attribution relative to pre-industrial (ARE): Present-day total ARE -0.05 ± 0.03 W m⁻² with ~30% ANT and ~70% AANE. By 2100: ARE -0.01 ± 0.03 W m⁻² (RCP6.0; ~51% ANT, ~49% AANE) and -0.02 ± 0.03 W m⁻² (RCP8.5; ~17% ANT, ~83% AANE). For O3 ARE, ANT contributes ~26% at present, projected to ~17% (RCP6.0) and ~6% (RCP8.5) by 2100; for CH4 ARE, present ~42% ANT/~58% AANE, shifting to AANE-dominant by 2100 (~87% RCP6.0; ~96% RCP8.5).
• Forcing comparison: Industrial-era SLH forcing (-0.05 W m⁻²) is comparable to dust increase (-0.07 W m⁻²) and opposite in sign to combined contrail and contrail-induced cirrus forcing (+0.06 W m⁻²).

The results demonstrate that short-lived halogens impart a persistent, indirect cooling of the climate system primarily by depleting ozone in both the troposphere and lower stratosphere. This cooling is partly offset by halogen-induced increases in methane burden and stratospheric water vapour, and by reduced aerosol formation that weakens aerosol cooling. The interplay among these competing effects yields a net cooling that is strongest at high latitudes, indicating potential implications for meridional heat transport and polar amplification. Crucially, the amplification of natural halogen emissions by anthropogenic pollution (AANE) dominates the change in SLH radiative impacts from pre-industrial to present and into the future. Consequently, models lacking SLH chemistry likely overestimate warming from SLCF since pre-industrial times and mischaracterize regional radiative perturbations. The decomposition between AANE and ANT clarifies that much of the radiative change arises indirectly via anthropogenic modification of natural emissions (e.g., enhanced iodine release due to O3 deposition and acidification effects), rather than from direct anthropogenic SLH emissions alone. This highlights policy-relevant linkages: controls on conventional air pollutants (O3 precursors, acids) can modulate the halogen feedbacks and, in turn, SLCF radiative forcing. The strong latitudinal and regional structure of the SLH effects underscores the need for comprehensive representation in Earth-system models to capture climate feedbacks and radiative baselines accurately across scenarios.

Short-lived halogens exert a net indirect cooling effect on Earth’s climate (-0.13 ± 0.03 W m⁻² at present), predominantly through ozone depletion, partly offset by warming contributions from methane, stratospheric water vapour and reduced aerosol cooling. This cooling has intensified since 1750 (-0.05 ± 0.03 W m⁻²), largely driven by anthropogenic amplification of natural halogen emissions, and is projected to continue evolving (18–31% change by 2100) under different emissions scenarios. The findings indicate that neglecting SLH in climate models biases estimates of SLCF-driven warming and regional radiative impacts. The authors recommend integrating comprehensive natural and anthropogenic SLH sources and chemistry into climate models to reduce uncertainties in historical and future radiative balance. Future work should better constrain SLH emissions (particularly AANE mechanisms), halogen-oxidant interactions affecting OH and CH4, aerosol formation pathways and their radiative effects, and regional processes (especially in polar regions) to refine projections.

Uncertainties remain in global secondary aerosol responses to halogen chemistry, leading to notable uncertainty in the aerosol radiative effect. Projections of future SLH impacts carry additional uncertainty due to scenario-dependent halogen levels and oxidant fields. The attribution between AANE and ANT is based on model sensitivities and depends on assumptions about anthropogenic pollutant emissions, deposition, and multiphase chemistry. Spatial heterogeneity and complex nonlinear chemistry introduce further model-dependent variability, and observational constraints on some halogen processes and burdens are limited in certain regions and in the stratosphere.