These substances have minimal direct climate effects but trigger chemical reactions that can lead to warming.

The Antarctic ozone hole was first reported in 1985, and small ozone losses at the global scale were also observed in the late 1980s. The combination of field and laboratory measurements, together with modeling, quickly established anthropogenic chlorofluorocarbons (CFCs) as the cause of both the Antarctic and global ozone depletion. However, when, where, and why the earliest ozone depletion could have been detected has not been determined. Here, we conduct a thought experiment to investigate when human-induced ozone depletion could have first been detectable, assuming the availability of accurate stratospheric ozone observations from 1950 onward. We find that human-caused ozone depletion was likely identifiable as early as 1957 in the tropical upper stratosphere. This region’s low internal variability enables the earliest detection of the anthropogenic signal, even though tropical ozone losses in the upper stratosphere were smaller than those in higher-latitude regions. Our results highlight the key role of considering both internal variability (“noise”) and the forced response (“signal”) in detection studies. Further, while CFCs are widely recognized as the primary drivers of current ozone depletion, we find that early ozone loss was primarily caused by human-made carbon tetrachloride (CCl 4 ), used mainly as a solvent. These findings suggest that a clear human influence on the stratospheric ozone layer began nearly 70 y ago, even before substantial emissions of CFCs from spray cans or air conditioning.

Abstract Tropospheric ozone influences Earth's radiative energy budget and has increased in recent decades. With initial‐condition ensembles from a single chemistry‐climate model, we show that global surface anthropogenic NOx emissions explain about 90% of the simulated 1995–2014 tropospheric ozone increase (1.8 DU) and that this increase exceeds those arising internally from natural climate variability. South and East Asian NOx emissions account for about 40% of the global NOx‐driven ozone increase but contribute about 80% of the associated net positive stratospheric‐adjusted radiative forcing (SARF; tropospheric ozone plus methane), reflecting a weaker methane response to Asian NOx than to tropical surface NOx emissions. Considering both CO + NMVOC and NOx emissions from Asia produces a larger net SARF than from global emissions (+0.032 vs. +0.023 W m−2). Tropospheric ozone changes over 1995–2014 from global CO + NMVOC emissions or methane concentrations alone are too small to be detected relative to internal climate variability.

Abstract. Achieving net-zero emissions over the coming decades requires unprecedented reductions in anthropogenic emissions of greenhouse gases (GHGs) complemented by a rapid ramp-up in the magnitude of global carbon dioxide removal (CDR). The carbon credit market (CCM) is emerging as a means to finance both emissions reductions and carbon dioxide removal from the atmosphere. To achieve necessary growth on these fronts, the total scope and diversity of projects that are candidates for inclusion in the CCM must expand, necessitating a means of comprehensively assessing the quality of carbon credit projects (CCPs) based on their ability to make quantifiable reductions to GHG concentrations in the atmosphere. Toward a comprehensive quality assessment, we propose a framework to assess and differentiate CCPs based on their estimated impact on atmospheric GHG composition. In parallel, we propose a path towards verification of the aggregated atmospheric impact of CCM actions, since a detectable and attributable signal in atmospheric GHG composition can be viewed as the clearest measure of their climate forcing and, therefore, effectiveness.

Tropospheric ozone remains a critical but uncertain driver of terrestrial productivity loss, and land surface models (LSMs) diverge markedly in how they represent vegetation ozone stress. We conduct a global, mechanistically consistent evaluation of three prominent ozone stress parameterization schemes, Sitch, Lombardozzi, and Li, within the Community Land Model version 5 (CLM5). Using unified meteorological and ozone forcing from CAM-chem and GSWP3.1, we designed five experiments to isolate the roles of ozone flux threshold selection and response function form. The mixed experiments using thresholds and response functions derived from the Sitch and Lombardozzi schemes were implemented without additional recalibration, allowing structural sensitivities to be evaluated consistently within the Li framework. Model output is benchmarked against MODIS and FLUXNET gross primary production (GPP) across spatial gradients, biomes, and among plant functional types (PFTs). All parameterizations capture the ozone-induced reduction in GPP relative to the ozone-free baseline, but their accuracy varies widely. The Li scheme, featuring PFT-specific thresholds and separate nonlinear responses for photosynthesis and stomatal conductance, best agrees with observed GPP patterns across scales. In contrast, the Lombardozzi scheme produces much larger reductions in high-flux regions. Analysis reveals that the structures of ozone response functions and memory-decay mechanisms primarily determine improvements in GPP simulation. Our results support a shift toward ozone parameterizations that couple stomatal flux with canopy phenology, dynamic water constraints, and regionally calibrated thresholds. These findings provide a transferable framework for quantifying ozone-carbon coupling in LSMs and highlight priorities for improving terrestrial biosphere models under atmospheric change.

We present a protocol for scenario simulations of marine cloud brightening (MCB) solar radiation modification (SRM), which we design for inclusion as a bridge simulation in the Geoengineering Model Intercomparison Project (GeoMIP). This protocol, named G6-1.5K-MCB, parallels the existing G6-1.5K-SAI, but it simulates injecting sea salt aerosol (iSSA) into the lower marine boundary layer to create a MCB scenario. Using information taken from recent modeling studies, we propose to apply MCB iSSA emissions in the midlatitudes, which can produce a surface temperature response that more closely resembles the opposite of the greenhouse gas (GHG) warming pattern without invoking the La Ni & ntilde;a response that has been predicted in previous studies. In many ways, this approach is analogous to the choice of emissions at 30 degrees N and 30 degrees S for stratospheric aerosol injection (SAI) in G6-1.5K-SAI. Owing to substantial uncertainty in the aerosol-cloud forcing from MCB, we outline recommended benchmark simulations to facilitate similar simulations of cloud brightening across different models. We present simulations of the G6-1.5K-MCB protocol using three Earth System Models (ESMs). All three ESMs show that for an intermediate baseline GHG emission trajectory, midlatitude MCB can maintain 21st century global mean surface temperature (GMST) at 2020-2039 temperatures. The iSSA emission rates required to maintain this target vary by a factor of 20 across the ESMs due to differences in the size distribution of the emitted iSSA and in the representations of aerosol-cloud interactions, demonstrating the importance of benchmark simulations for both understanding uncertainties and setting up the scenario simulations. Temperature and precipitation anomalies are greatly reduced relative to the GHG warming background, with most regions experiencing no statistically significant changes relative to the reference period. In some regions, there is a notable seasonal cycle in the residual climate change, though the anomalies are still much smaller than the GHG warming impact. On the basis of the promising results from this three-model testbed, we propose that the G6-1.5K-MCB serve as a basis for future model intercomparison protocols. This will enable further estimation of the structural uncertainties of ESMs in the climate response to MCB and provide a valuable dataset for more detailed analysis of the potential impacts of MCB.

Effective adaptation and mitigation strategies for climate change require high-resolution projections to inform strategic decision-making. Conventional global climate models, which typically operate at resolutions of 150 to 200 kilometers, lack the capacity to represent essential regional processes. IPSL-AID is a global to regional downscaling tool based on a denoising diffusion probabilistic model designed to address this limitation. Trained on ERA5 reanalysis data, it generates 0.25 degree resolution fields for temperature, wind, and precipitation using coarse inputs and their spatiotemporal context. It also models probability distributions of fine-scale features to produce plausible scenarios for uncertainty quantification. The model accurately reconstructs statistical distributions, including extreme events, power spectra, and spatial structures. This work highlights the potential of generative diffusion models for efficient climate downscaling with uncertainty

Halogen (chlorine, bromine, and iodine) species actively interact with other atmospheric constituents such as nitrogen oxides, organic gases, and ozone. Previous field observations confirmed the ubiquitous existence of halogens in various environments and modeling simulations quantified halogen impacts on air pollutants at present time. However, the abundance and impacts of continental inorganic halogens (CIH) on global air quality throughout history remain unexplored. Here, we present a global inventory of CIH emissions from anthropogenic and biomass-burning sources from 1970 to 2015, with average fluxes of 4302, 207, and 98 kt/yr for chlorine, bromine, and iodine, respectively, representing ~15%, 5-10%, and ~3% of the corresponding global short-lived chlorine, bromine, and iodine emissions. Incorporating these emissions into a global chemistry-climate model reveals a substantial increase ( > 50%) in inorganic halogen levels over continents, helping to reproduce the observed halogen levels. Our results also suggest substantial effects of CIH on air pollutant concentrations with significant spatio-temporal variations, e.g., the largest perturbations in nitrogen oxides and secondary aerosols migrating from Europe and North America in the 1980s-1990s, to East Asia in the 2000s, then to South Asia in the 2010s, suggesting a movement of CIH-associated air quality impact hotspots from the developed regions to the developing areas. This study highlights the under-appreciated role of halogen chemistry in air quality and its evolution in the recent four decades and calls for attention to the potential CIH impacts in emerging regions where field observations are not yet available.

In a rapidly changing climate, evidence-based decision-making benefits from up-to-date and timely information. We track twelve key sets of indicators of the state of the climate system, closely following Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment report (AR6) methods, to produce our fourth annual publication. One of the indicators, the Earth's energy imbalance (EEI) provides a crucial integrative measure of the overall heating of the planet and the pace of climate change - this has more than doubled since the 1976-1995 period. A newly added indicator of temperature extremes, the number of days experiencing marine heatwaves, has more than tripled between 1991 and 2025.For the 2016-2025 decade average, observed warming relative to 1850-1900 was 1.26 [1.13 to 1.36] degrees C, of which 1.24 [1.0 to 1.5] degrees C was human-induced. Human-induced warming reached 1.37 degrees C relative to 1850-1900 in the year 2025, increasing at a rate of 0.27 [0.2-0.4] degrees C per decade over 2016-2025. This high rate of warming, which matches the all-time high seen last year in the instrumental record, was caused by a combination of greenhouse gas emissions being at an all-time high of 54.6 +/- 5.5 GtCO2e yr-1 over the last decade (2015-2024), as well as reductions in the strength of aerosol cooling. Despite this, there is evidence that CO2 emission growth is slowing. The continuation of these annual updates could track decreases or increases in the rate of human influence and climatic changes presented here, reflecting the outcomes of societal choices during the critical 2020s decade.The data presented herein can provide a useful reference point for the drafting of the IPCC seventh assessment report. In total, we employ analysis from over 40 global datasets (10.5281/zenodo.20499280, Smith et al., 2026a). Future monitoring of these indicators, such as ocean and satellite measurements of the Earth's energy imbalance, are threatened by geopolitical and public funding decisions. Our ability to consistently track many of the indicators requires the continuity of observation programs and coordination mechanisms, including the Global Climate Observing System (GCOS) program, that enable their effective integration and use.

Atmospheric methane is a potent greenhouse gas that is photochemically active. The addition of chlorine to the atmosphere has been proposed to mitigate global warming through methane reduction by increasing its chemical loss. However, the potential environmental impacts of such climate mitigation remain unexplored. We explore the possible effects of increasing reactive chlorine emissions on the methane budget, atmospheric composition and radiative forcing. Due to non-linear chemistry we found that achieving effective methane reduction require a minimum 3-fold increase in chlorine atoms compared to present-day levels. Our highest scenario, 50-fold present-day chlorine levels, led to a reduction of the surface temperature by 0.6°C in the year 2050. Beyond the direct effects on methane and temperature, our results show significant alterations in other climate forcers, particularly a large decrease in tropospheric ozone. This translates into a reduction in radiative forcing of a similar magnitude as of the methane removed. Additionally, the Antarctic stratosphere ozone burden during September and October was reduced by up to 40% with the highest chlorine addition. Consequently, the implementation of such strategies requires careful consideration of various factors, including the quantity and method of chlorine addition, as well as potential environmental impacts on air quality and ocean acidity.

Since the 1970s, air pollutant emissions controls in the United States (US) have lowered concentrations of ozone (O3) and aerosols, which have opposing radiative effects on surface temperature. Using a pair of initial-condition ensembles generated by a fully-coupled chemistry-climate model, we simulate the “world avoided” by US air pollution controls. In this counterfactual world, we find tropospheric column O3 increases, robust to natural internal variability, extending across the Northern Hemisphere. Robust aerosol increases, dominated by sulfate, remain localized near the US. Wintertime Northwest Atlantic cloud droplet number concentration is particularly sensitive to US aerosol. While an ensemble mean US surface cooling signal (−0.4 °C) implies that aerosol-driven cooling prevails over any O3-induced warming, we find that large regional internal variability will confound its detection in any single transient realization. Larger signal-to-noise ratios for composition versus climate variables underscore the greater detectability of emissions-driven changes in tropospheric composition compared to their associated climate impacts.

Processes occurring over the oceans, including greenhouse gas (GHG) fluxes and cloud-aerosol interactions, are a major source of uncertainty in the present climate state and hence in estimating near-term warming. To reduce this uncertainty, more comprehensive and specific observations over the oceans are needed. Due to the limited supply of dedicated research vessels, platforms of opportunity are essential to fill these observational gaps. We discuss here the Ships of Opportunity for Atmospheric Research (SOAR) program, a science infrastructure program built in collaboration with OceansX. SOAR’s purpose is to expand atmospheric observations in under-observed oceanic regions, providing new measurements to existing climate observation programs that provide open data to the global community. We present current pilot projects under SOAR. A GHG flask sampler from NOAA Global Monitoring Lab is deployed on the Maersk Kentucky, which is taking samples as the ship transits the tropical Pacific. Sensors from NASA’s Maritime Aerosol Network (AERONET MAN), are also deployed on two other Maersk vessels and a Smyril Line vessel, where volunteer sailors are collecting aerosol optical depth measurements that are now accessible on the AERONET/MAN webpage. Finally, in collaboration with NOAA Global Monitoring Laboratory, SilverLining is developing a version of the NOAA Federated Aerosol Network (NFAN) package for deployment on ships of opportunity. This effort includes integrating instruments into a system adapted to marine environments and validating it against the NFAN technical standards. This project serves as a proof of concept for including additional higher-complexity atmospheric instrumentation packages in ships of opportunity programs

Abstract. The CMIP6 project was the most expansive and ambitious Model Intercomparison Project (MIP), the latest in a long history, extending back four decades. CMIP has captivated and engaged a broad, growing community focused on improving our climate understanding. It has anchored our ability to quantify and attribute the drivers and responses of the observed climate changes we are experiencing today. The project's profound impact has been achieved by combining the latest climate science and technology. This has enabled the production of latest-generation climate simulations and the dissemination of their output, which has seen increased community attention in every successive phase. The review emphasizes the pragmatics of progressively scaling up efforts, the evolution of how the MIPs were implemented, and the coordinated efforts to establish a minimal infrastructure to make that possible, most recently delivering CMIP6.

The overestimation of surface ozone concentration in low‐resolution global atmospheric chemistry and climate models has been a long‐standing issue. We first update the ozone dry deposition scheme in both high‐ (0.25°) and low‐resolution (1°) Community Earth System Model (CESM) version 1.3 runs, by adding the effects of leaf area index and correcting the sunlit and shaded fractions of stomatal resistances. With this update, 5‐year‐long summer simulations (2015–2019) using the low‐resolution CESM still exhibit substantial ozone overestimation (by 6.0–16.2 ppbv) over the U.S., Europe, eastern China, and ozone pollution hotspots. The ozone dry deposition scheme is further improved by adjusting the leaf cuticle conductance, reducing the mean ozone bias by 19%, and increasing the model resolution further reduces the ozone overestimation by 43%. We elucidate the mechanism by which model grid spacing influences simulated ozone, revealing distinctive pathways in urban versus rural areas. In rural areas, grid spacing mainly affects daytime ozone levels, where additional NO x emissions from nearby urban areas result in an ozone boost and overestimation in low‐resolution simulations. In contrast, over urban areas, daytime ozone overestimation follows a similar mechanism due to the influence of volatile organic compounds from surrounding rural areas. However, nighttime ozone overestimation is closely linked to weakened NO titration owing to the redistribution of urban NO x to rural areas. Additionally, stratosphere‐troposphere exchange may also contribute to reducing ozone bias in high‐resolution simulations, warranting further investigation. This optimized high‐resolution CESM may enhance understanding of ozone formation mechanisms, sources, and changes in a warming climate.

Observations have demonstrated the ubiquity of short-lived halogens (SLHs)-defined as organic and inorganic chlorine, bromine and iodine compounds with an overall atmospheric lifetime of less than 6 months-in the global atmosphere. They are primarily emitted naturally from the ocean, cryosphere, volcanoes, salt lakes and the biosphere. However, unregulated anthropogenic sources are increasingly contributing to their atmospheric loading. Some of their natural emissions have increased over time due to anthropogenic pollution, for example, the increased oceanic emissions of iodine compounds due to the deposition of ozone on the sea surface. SLHs affect chemical processes, such as ozone and methane chemistry, and therefore influence air quality and climate. Nevertheless, some of their sources and chemistry are not included in air-quality and climate models used in international assessment reports. Here we describe in detail the various impacts of SLHs on air quality and climate, and make a case for the inclusion of more comprehensive SLH chemistry in future atmospheric, air-quality and climate assessments. In doing so, we also identify gaps in our knowledge of SLH emissions, chemistry, and environmental and climate impacts.

The hydroxyl radical (OH) lies at the nexus of climate and air quality as the primary oxidant for both reactive greenhouse gases and many hazardous air pollutants. To better understand the role of climate variability on spatiotemporal patterns of OH, we utilize a 13-member ensemble of the Community Earth System Model version 2-Whole Atmosphere Community Climate Model version 6 (CESM2-WACCM6), a fully coupled chemistry-climate model, spanning the years 1950-2014. Ensemble members vary only in their initial conditions of the climate state in 1950. We focus on the final decade of the simulation, 2005-2014, when prior studies disagree on the signs of the global OH trends. The ensemble mean global airmass-weighted mean tropospheric column OH ( Omega TOH ), which is an estimate of the forced signal, increases by 0.06%/year between 2005 and 2014 while regional Omega TOH trends range from -0.56%/year over Southern Europe to +0.64%/year over South America. We show that ten-year Omega TOH trends are strongly affected by internal climate variability, as the spread of Omega TOH trends across the ensemble varies between 0.23%/year in Asia and 1.53%/year in South America. We train a fully connected neural network to emulate the Omega TOH simulated by the CESM2-WACCM6 model and combine it with satellite observations to interpret the role of OH chemical proxies. While the OH chemical proxies are subject to internal variability, the impact of internal variability on Omega TOH trends is primarily due to the meteorological parameters except for South America. Forced trends in global mean Omega TOH do not unambiguously emerge from trends driven by internal variability over the 2005-2014 period. The observation-constrained Omega TOH presents opposite trends due to climate variability, resulting in varying conclusions on the attribution of OH to CH4 trends.

Tropospheric ozone (O3) is a strong greenhouse gas, particularly in the upper troposphere (UT). Limited observations point to a continuous increase in UT O3 in recent decades, but the attribution of UT O3 changes is complicated by large internal climate variability. We show that the anthropogenic signal ("fingerprint") in the patterns of UT O3 increases is distinguishable from the background noise of internal variability. The time-invariant fingerprint of human-caused UT O3 changes is derived from a 16-member initial-condition ensemble performed with a chemistry-climate model (CESM2-WACCM6). The fingerprint is largest between 30°S and 40°N, especially near 30°N. In contrast, the noise pattern in UT O3 is mainly associated with the El Niño-Southern Oscillation (ENSO). The UT O3 fingerprint pattern can be discerned with high confidence within only 13 years of the 2005 start of the OMI/MLS satellite record. Unlike the UT O3 fingerprint, the lower tropospheric (LT) O3 fingerprint varies significantly over time and space in response to large-scale changes in anthropogenic precursor emissions, with the highest signal-to-noise ratios near 40°N in Asia and Europe. Our analysis reveals a significant human effect on Earth's atmospheric chemistry in the UT and indicates promise for identifying fingerprints of specific sources of ozone precursors.

While the dominant role of halogens in Arctic ozone loss during spring has been widely studied in the last decades, the impact of sea-ice halogens on surface ozone abundance over the northern hemisphere (NH) mid-latitudes remains unquantified. Here, we use a state-of-the-art global chemistry-climate model including polar halogens (Cl, Br, and I), which reproduces Arctic ozone seasonality, to show that Arctic sea-ice halogens reduce surface ozone in the NH mid-latitudes (47°N to 60°N) by ~11% during spring. This background ozone reduction follows the southward export of ozone-poor and halogen-rich air masses from the Arctic through polar front intrusions toward lower latitudes, reducing the springtime tropospheric ozone column within the NH mid-latitudes by ~4%. Our results also show that the present-day influence of Arctic halogens on surface ozone destruction is comparatively smaller than in preindustrial times driven by changes in the chemical interplay between anthropogenic pollution and natural halogens. We conclude that the impact of Arctic sea-ice halogens on NH mid-latitude ozone abundance should be incorporated into global models to improve the representation of ozone seasonality.

Short-term exposure to ground-level ozone in cities is associated with increased mortality and is expected to worsen with climate and emission changes. However, no study has yet comprehensively assessed future ozone-related acute mortality across diverse geographic areas, various climate scenarios, and using CMIP6 multi-model ensembles, limiting our knowledge on future changes in global ozone-related acute mortality and our ability to design targeted health policies. Here, we combine CMIP6 simulations and epidemiological data from 406 cities in 20 countries or regions. We find that ozone-related deaths in 406 cities will increase by 45 to 6,200 deaths/year between 2010 and 2014 and between 2050 and 2054, with attributable fractions increasing in all climate scenarios (from 0.17% to 0.22% total deaths), except the single scenario consistent with the Paris Climate Agreement (declines from 0.17% to 0.15% total deaths). These findings stress the need for more stringent air quality regulations, as current standards in many countries are inadequate.