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In 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.

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.

In 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.

Modern climate projections lack adequate spatial and temporal resolution due to computational constraints. A consequence is inaccurate and imprecise predictions of critical processes such as storms. Hybrid methods that combine physics with machine learning (ML) have introduced a new generation of higher fidelity climate simulators that can sidestep Moore's Law by outsourcing compute-hungry, short, high-resolution simulations to ML emulators. However, this hybrid ML-physics simulation approach requires domain-specific treatment and has been inaccessible to ML experts because of lack of training data and relevant, easy-to-use workflows. We present ClimSim, the largest-ever dataset designed for hybrid ML-physics research. It comprises multi-scale climate simulations, developed by a consortium of climate scientists and ML researchers. It consists of 5.7 billion pairs of multivariate input and output vectors that isolate the influence of locally-nested, high-resolution, high-fidelity physics on a host climate simulator's macro-scale physical state. The dataset is global in coverage, spans multiple years at high sampling frequency, and is designed such that resulting emulators are compatible with downstream coupling into operational climate simulators. We implement a range of deterministic and stochastic regression baselines to highlight the ML challenges and their scoring. The data (https://huggingface.co/datasets/LEAP/ClimSim_high-res) and code (https://leap-stc.github.io/ClimSim) are released openly to support the development of hybrid ML-physics and high-fidelity climate simulations for the benefit of science and society., Comment: NeurIPS 2023 Outstanding Datasets and Benchmarks Track Paper

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 CH4 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 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. Notably, 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. Both direct (Cl-driven) and indirect (via OH) impacts of halogens should be included in future CH4 projections.

Observational and modeling evidence suggest a recent acceleration of the stratospheric Brewer-Dobson circulation (BDC), driven by climate change and stratospheric ozone depletion. However, slowly varying natural variability can compromise our ability to detect such forced changes over the relatively short observational record. Using observations and chemistry-climate model simulations, we demonstrate a link between multi-decadal variability in the strength of the BDC and the Interdecadal Pacific Oscillation (IPO), with knock-on impacts for composition in the stratosphere. After accounting for the IPO-like variability in the BDC, the modeled trend is approximately 7%–10% dec−1 over 1979–2010. Furthermore, we find that sea surface temperatures explain up to 50% of the simulated decadal variability in tropical mid-stratospheric ozone. Our findings demonstrate strong links between low-frequency variability in the oceans, troposphere and stratosphere, as well as their potential importance in detecting structural changes in the BDC and future ozone recovery.

Tropospheric ozone ((Formula presented.)) is an important greenhouse gas and a surface pollutant. The future evolution of (Formula presented.) abundances and chemical processing are uncertain due to a changing climate, socioeconomic developments, and missing chemistry in global models. Here, we use an Earth System Model with natural halogen chemistry to investigate the changes in the (Formula presented.) budget over the 21st century following Representative Concentration Pathway (RCP)6.0 and RCP8.5 climate scenarios. Our results indicate that the global tropospheric (Formula presented.) net chemical change (NCC, chemical gross production minus destruction) will decrease (Formula presented.), notwithstanding increasing or decreasing trends in ozone production and loss. However, a wide range of surface NCC variations (from −60 (Formula presented.) to 150 (Formula presented.)) are projected over polluted regions with stringent abatements in (Formula presented.) precursor emissions. Water vapor and iodine are found to be key drivers of future tropospheric (Formula presented.) destruction, while the largest changes in (Formula presented.) production are determined by the future evolution of peroxy radicals. We show that natural halogens, currently not considered in climate models, significantly impact on the present-day and future global (Formula presented.) burden reducing (Formula presented.) 30–35 Tg (11–15 (Formula presented.)) of tropospheric ozone throughout the 21st century regardless of the RCP scenario considered. This highlights the importance of including natural halogen chemistry in climate model projections of future tropospheric ozone.

Observational and modeling evidence suggest a recent acceleration of the stratospheric Brewer-Dobson circulation (BDC), driven by climate change and stratospheric ozone depletion. However, slowly varying natural variability can compromise our ability to detect such forced changes over the relatively short observational record. Using observations and chemistry-climate model simulations, we demonstrate a link between multi-decadal variability in the strength of the BDC and the Interdecadal Pacific Oscillation (IPO), with knock-on impacts for composition in the stratosphere. After accounting for the IPO-like variability in the BDC, the modeled trend is approximately 7%–10% dec over 1979–2010. Furthermore, we find that sea surface temperatures explain up to 50% of the simulated decadal variability in tropical mid-stratospheric ozone. Our findings demonstrate strong links between low-frequency variability in the oceans, troposphere and stratosphere, as well as their potential importance in detecting structural changes in the BDC and future ozone recovery.

Biogenic very short-lived bromocarbons (VSLBr) currently represent 25% of the total stratospheric bromine loading. Owing to their much shorter lifetime compared to anthropogenic long-lived bromine (e.g. halons) and chlorine (e.g. chlorofluorocarbons), the impact of VSLBr on ozone peaks in the lowermost stratosphere, which is a key climatic and radiative atmospheric region. Here we present a modelling study of the evolution of stratospheric ozone and its chemical loss within the tropics and at mid-latitudes during the 21st century. Two different experiments are explored: considering and neglecting the additional stratospheric injection of 5 ppt biogenic bromine naturally released from the ocean. Our analysis shows that the inclusion of VSLBr results in a realistic stratospheric bromine loading and improves the agreement between the model and satellite observations of the total ozone column (TOC) for the 1980 2015 period at mid-latitudes. We show that the overall ozone response to VSLBr at mid-latitudes follows the stratospheric evolution of long-lived inorganic chlorine and bromine throughout the 21st century. Additional ozone loss due to VSLBr is maximized during the present-day period (1990 2010), with TOC differences of 8DU (3 %) and 5:5DU (2 %) for the Southern Hemisphere and Northern Hemisphere midlatitudes (SH-MLs and NH-MLs), respectively. Moreover, the projected TOC differences at the end of the 21st century are 50% lower than the values found for the present-day period. We find that seasonal VSLBr impact on lowermost stratospheric ozone at mid-latitude is influenced by the seasonality of the heterogeneous inorganic-chlorine reactivation processes on ice crystals. Indeed, due to the more efficient reactivation of chlorine reservoirs (mainly ClONO2 and HCl) within the colder SH-ML lowermost stratosphere, the seasonal VSLBr impact shows a small but persistent hemispheric asymmetry through the whole modelled period. Our results indicate that, although the overal

Reactive atmospheric halogens destroy tropospheric ozone (O3), 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 O3, are not well known. Here, we use an Earth system model to estimate that natural halogens deplete approximately 13% of tropospheric O3 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 O3 loss. Notably, this halogen-driven O3 buffering is projected to be greatest over polluted and populated regions, due mainly to iodine chemistry, with important implications for air quality.

The contribution of very short-lived bromine (VSLBr) represent 5 ± 2 ppt ( ~ 25%) of total stratospheric bromine (WMO, 2018), which is still nowadays dominated by long-lived bromocarbons that do not impact on tropospheric chemistry. Due to their shorter lifetimes, the overall injection to the stratosphere of VSLBr compounds possesses two distinct pathways: i) Source Gas Injection (SGI), where the brominated species are injected as they were emit- ted at the surface; and ii) Product Gas Injection (PGI), where the photochemical processing of reactive species aris- ing from SG degradation must be considered. Depending on the partitioning and distribution of SGI and PGI, the chemical impact of VSLBr on tropospheric and lowermost stratospheric ozone, HO x and other oxidizing species can be very different. Many Chemistry Climate Models (CCMs) include a simplified treatment of tropospheric VSLBr sources by as- suming a long-lived halocarbon (usually CH3Br) as a Surrogate for VSLBr. Even though these surrogate models possess a consistent evolution of the stratospheric bromine loading, CCMs including an explicit VSLBr represen- tation compare better with organic and inorganic bromine observations in the lowermost stratosphere (Wales et al., 2018). Here we used the halogenated version of the CAM-Chem model (Fernandez et al., 2014) to evaluate the chemical impact of considering an explicit treatment of VSLBr versus considering a simplified tropospheric treatment of long-lived CH3Br as surrogate of VSLBr. The explicit mechanism considers a full gas- and aerosol- phase chem- ical scheme (including sea-salt dehalogenation) as well as time-dependent and geographically-distributed VSLBr emissions inventory (Ordoñez et al., 2012), which replaces the typical lower-boundary surface conditions for long- lived compounds usually considered in CCMs. An additional baseline simulation neglecting the contribution of VSLBr is also considered. First we show the differences in the overall inorg

Tropospheric ozone (O3) is of particular interest for climate and air quality studies. O3 is a radiatively active gas and, in urban areas, O3 is a surface pollutant. Key drivers of future O3 abundances and distributions are thought to be ozone precursors, ozone depleting substances and climate (Banerjee et al., 2016; Iglesias-Suarez et al., 2018). Changes in climate impact O3 and its budget - chemical production and loss, deposition to the surface, and stratosphere-troposphere exchange - through changes in dynamics and chemistry. Hence, its evolution during the next century is of important interest for climate change and air quality. The three-dimensional chemistry-climate model CAM-Chem (Community Atmospheric Model with Chemistry, version 4.0)(Lamarque et al., 2012), included in the CESM framework (Community Earth System Model, version 1.1.1) is used to investigate the impact of climate change on tropospheric odd oxygen (Ox) budget - defined here as the sum of species that rapidly interconvert with O3- from present day to 2100 for both, the Representative Con- centration Pathways (RCP) 6.0 and 8.5 scenarios. Our model set-up includes a full halogen chemistry mechanism (in the troposphere and the stratosphere) along with varying oceanic emissions of Very Short Lived Halocarbons (VSLH) and zonally averaged distributions of long-lived halocarbons (Ordóñez et al., 2012; Fernández et al., 2017; Saiz-López 2014). Here we present a comprehensive analysis of the main chemical channels for each term of the Ox budget (over space, time and altitude) and identify geographical areas where climate change plays a key role on the evolution and partitioning of the Ox family.

Arctic feedbacks accelerate climate change through carbon releases from thawing permafrost and higher solar absorption from reductions in the surface albedo, following loss of sea ice and land snow. Here, we include dynamic emulators of complex physical models in the integrated assessment model PAGE-ICE to explore nonlinear transitions in the Arctic feedbacks and their subsequent impacts on the global climate and economy under the Paris Agreement scenarios. The permafrost feedback is increasingly positive in warmer climates, while the albedo feedback weakens as the ice and snow melt. Combined, these two factors lead to significant increases in the mean discounted economic effect of climate change: +4.0% ($24.8 trillion) under the 1.5 °C scenario, +5.5% ($33.8 trillion) under the 2 °C scenario, and +4.8% ($66.9 trillion) under mitigation levels consistent with the current national pledges. Considering the nonlinear Arctic feedbacks makes the 1.5 °C target marginally more economically attractive than the 2 °C target, although both are statistically equivalent.