Ambitious climate and air quality mitigation cuts anthropogenic aerosol emissions, lowering fine particulate matter (PM 2.5 ) and mortality, but the magnitude, regional distribution, and uncertainty of these reductions remain insufficiently quantified. Using nine Earth System Models from the Regional Aerosol Model Intercomparison Project (RAMIP), we quantify the PM 2.5 mortality benefits of switching anthropogenic aerosols alone from a low mitigation (SSP3-7.0) to a high mitigation (SSP1-2.6) scenario. The switch averts 2.36 million (95% CI: 1.48–2.57 million) deaths/year globally, 38% of SSP3-7.0 PM 2.5 -attributable mortality. Benefits are largest in East Asia, South Asia, and Africa and smaller in North America and Europe. Global land annual mean PM 2.5 falls by 2%–17% across the RAMIP experiments, reaching 33–38% near Asian sources. Cuts in East Asia alone avert 48% of source region and 22% of global PM 2.5 mortality. Uncertainty is dominated by inter-model variability, followed by health response functions and natural climate variability. Overall, sustainability-pathway aerosol reductions can avert nearly 40% of projected PM2.5 deaths by 2050, with health benefits that are large but highly unevenly spatially distributed.

In response to continued greenhouse gas (GHG) increases, the Atlantic Meridional Overturning Circulation (AMOC) is expected to weaken through the 21st century. However, AMOC impacts associated with efforts to improve air quality are less well understood. Here, eight models from the Regional Aerosol Model Intercomparison Project are examined to quantify mid-21st century AMOC changes resulting from global and regional anthropogenic aerosol and precursor gas (AA) emissions reductions (industrial and biomass burning), by comparing strong air pollution control shared socioeconomic pathway (SSP1-2.6) to a baseline with weak air pollution control (SSP3-7.0). Global AA reductions and subsequent warming yield multi-model mean AMOC weakening of 6% ( -0.98 +/- 0.40 Sv; 1 Sv = 106 m3 s-1) by the last 12 years of the simulation (2039-2050). This is 1/3 of the magnitude of the corresponding weakening associated with the high GHG emissions scenario SSP3-7.0. Of the regional perturbations, combined North American and European AA reductions drive the largest AMOC weakening, followed by combined African and Middle Eastern reductions and then East Asian reductions, with South Asian reductions yielding non-significant weakening. Across these experiments, AMOC weakening is significantly correlated with the North Atlantic Ocean aerosol effective radiative forcing ( r=-0.95) and aerosol optical depth response ( r=1.0). AMOC weakening under AA reductions is associated with a thermally driven reduction in buoyancy in the subpolar North Atlantic, which is largely driven by surface shortwave radiation increases, consistent with the forcing from AA reductions. Africa + Middle East AA reductions also involve excitation of a negative North Atlantic Oscillation pattern, which contributes to AMOC weakening. Our results show that efforts to improve air quality, particularly around the Atlantic basin but also far away in East Asia, will contribute to future AMOC weakening.

Anthropogenic aerosol and associated precursor gas emissions are already declining in many regions and are likely to decline significantly by 2050, with major implications for regional climate. Unlike greenhouse gases, aerosol impacts are spatially heterogeneous and can influence climate both near emission sources and through remote teleconnections. This is particularly important for the South Asian monsoon system, where both local and remote aerosol changes can significantly affect precipitation patterns. Using simulations from the Regional Aerosol Model Intercomparison Project, we examine how local and remote aerosol emission reductions influence South Asian climate across both pre-monsoon and monsoon seasons, including weakening of the elevated heat pump (EHP) mechanism during pre-monsoon months. Our analysis employs 10-member ensembles from 10 CMIP6-era models to compare three experiments with global, South Asian, and East Asian aerosol reductions relative to a high-emission baseline (SSP3-7.0). This experimental design allows us to isolate and quantify the distinct impacts of regional emission changes. Results reveal that global aerosol reductions produce a larger all-India precipitation increase ( +0.28 mm day -1) than South Asian reductions alone ( +0.19 mm day -1), with the strongest regional responses over the northern Bay of Bengal, the Western Ghats and Indo-Gangetic Plains. East Asian reductions show uncertain but modest precipitation reductions of 0.2-0.6 mmday-1 over parts of west-central and eastern Indian regions. Pre-monsoon carbonaceous aerosol reductions systematically weaken the EHP, cooling the mid-troposphere over the Himalayan foothills by up to 0.4 K and producing localised anomalous descent that opposes the climatological pre-monsoon ascent. Substantial inter-model diversity exists in the spatial patterns and magnitudes of these responses, highlighting key uncertainties in aerosol-monsoon interactions. The South Asian precipitation response is driven by enhanced land-sea thermal contrast, while the pre-monsoon EHP weakening is attributable exclusively to local South Asian carbonaceous aerosol forcing rather than remote teleconnections. These findings have direct implications for air quality and climate adaptation planning across South Asia as regional aerosol emissions diverge under different development pathways.

The Regional Aerosol Model Intercomparison Project (RAMIP) is designed to quantify the forcing and climate impacts of mid-21st century anthropogenic aerosol and precursor gas (AA) emissions reductions (both industrial and biomass burning), by comparing a weak (SSP3-7.0) versus strong (SSP1-2.6) level of air quality control aerosol emissions pathway. AA emissions reductions experiments include global (GLO), East Asia (EAS), South Asia, Africa and the Middle East (AFR), and North America and Europe (NAE). Here, we use RAMIP time-slice simulations with fixed sea surface temperatures and sea-ice distributions from nine models to quantify the aerosol effective radiative forcing (ERF), including aerosol radiation (ERFari) and aerosol cloud interactions (ERFaci). The multi-model global mean net ERFari+aci is 0.77 +/- 0.25 W m-2 for GLO, and three of the four regional perturbations yield a significant positive net ERFari+aci (up to 0.15 +/- 0.07 W m-2 for EAS). In all cases, net ERFari+aci is dominated by aerosol-cloud interactions, which are largely due to reduced cloud scattering. Of the four regions, NAE yields the largest forcing efficiency whereas AFR yields the weakest. Although the areas outside our four target regions contribute 25% to the GLO aerosol optical depth reduction, they disproportionately contribute 44% to the GLO net ERFari+aci. The multimodel regional mean net ERFari+aci for three regional perturbations is much larger (up to 1.64 +/- 1.36 W m-2 for EAS) than the corresponding global mean value. However, these regional values are even larger (up to 2.69 +/- 1.72 W m-2 for EAS) under global aerosol reductions, implying remote emission reductions represent a sizable contribution (up to 1.05 +/- 0.56 W m-2 for EAS). These large regional ERFs will in turn drive relatively large regional climate impacts, which continue to be underappreciated in most policy discussions.

Decreases in anthropogenic aerosols will reduce fine particulate matter (PM2.5); however, meteorological feedbacks alter dust emissions, modifying air quality gains. We use eight Earth System Models from the Regional Aerosol Model Intercomparison Project (RAMIP) simulations to assess African climate and air quality responses to anthropogenic aerosol emission perturbations, including meteorological feedbacks on dust emissions. By 2050, African and global emissions reductions drive the largest continent-average PM2.5 decrease (0.92 +/- 0.17 mu g m-3; 5% and 1.35 +/- 0.50 mu g m-3; 7%, respectively) relative to SSP3-7.0, though regional dust increases partially offset these reductions. Anthropogenic emissions reductions in the U.S. and Europe also lower African PM2.5 by 0.29 +/- 0.32 mu g m-3 (2%) due to teleconnections of Northern Hemisphere warming influencing the Intertropical Convergence Zone. Inter-model variability in dust and total PM2.5 reflects differences in meteorological responses and dust emission parameterizations. Meteorological responses explain 90% of dust emissions variability across regions. Aerosol-driven climate feedbacks on dust account for up to 70% of total PM2.5 changes in the Sahara and Namib, offsetting up to 20% of anthropogenic PM2.5 reductions across Africa. Under 2050 global and Africa-wide anthropogenic aerosol reductions, 96,000 (95% CI: 54,000-137,000) and 84,000 (95% CI: 43,000-125,000) PM2.5-related deaths are avoided in Africa, respectively. Dust PM2.5 contributes an uncertain 3.4% of the avoided deaths under global reductions and has no net effect under Africa-wide reductions. Aerosol-driven climate feedbacks may partially offset direct air quality gains, though their continental-scale contribution remains small and uncertain.

Anthropogenic aerosol emissions are expected to change rapidly over the coming decades, with complex geographical and seasonal patterns. This is expected to drive strong, spatially varying trends in temperature, hydroclimate, and extreme events, both near and far from emission sources. These changes are poorly constrained in current models, and very sparsely represented in climate risk assessments, partly because of a lack of dedicated emission pathways and multi-model investigations. The Regional Aerosol Model Intercomparison Project (RAMIP) is designed to quantify and bound the role of regional aerosol emissions changes in near-term climate projections. RAMIP experiments are based on the SSPs commonly used in CMIP6 Endorsed MIPs, but are designed to explore sensitivities to aerosol type and location, and provide improved constraints on uncertainties driven by aerosol radiative forcing and the dynamical response to aerosol changes. The core experiments assess the effects of different aerosol emission pathways in East Asia, South Asia, Africa and the Middle East, and North America and Europe through 2051, using a multi-ensemble-member approach in a set of 10 Earth System Models. Based on early output from a subset of participating RAMIP models, we highlight regions where current and future aerosol reductions may lead to changes in seasonal mean climate and the frequency and severity of extreme events. We will also show examples of how the near-future evolution of temperature and precipitation extremes in Europe and Asia may be influenced by local air quality policies, and those further afield.

Global surface warming has accelerated since around 2010, relative to the preceding half century1, 2-3. This has coincided with East Asian efforts to reduce air pollution through restricted atmospheric aerosol and precursor emissions4,5. A direct link between the two has, however, not yet been established. Here we show, using a large set of simulations from eight Earth System Models, how a time-evolving 75% reduction in East Asian sulfate emissions partially unmasks greenhouse gas-driven warming and influences the spatial pattern of surface temperature change. We find a rapidly evolving global, annual mean warming of 0.07 +/- 0.05 degrees C, sufficient to be a main driver of the uptick in global warming rate since 2010. We also find North-Pacific warming and a top-of-atmosphere radiative imbalance that are qualitatively consistent with recent observations. East Asian aerosol cleanup is thus likely a key contributor to recent global warming acceleration and to Pacific warming trends.

Global surface warming has accelerated since around 2010, relative to the preceding half century1-3. This has coincided with China’s efforts to reduce air pollution through restricted atmospheric aerosol and precursor emissions4,5. A direct link between the two has, however, not yet been established. Here we show, using a large set of simulations from eight Earth System Models, how a time evolving 75% reduction in Chinese sulfate emissions partially unmasks greenhouse driven warming and influences the pattern of surface temperature change. We find a rapidly evolving global, annual-mean warming of 0.07 ± 0.05 ºC, sufficient to explain a majority of the uptick in global warming rate since 2010. We also find North-Pacific warming and a top-of-atmosphere radiative imbalance that are consistent with recent observations. China’s aerosol cleanup is thus likely a key contributor to recent global warming acceleration, and to Pacific warming trends.

Changes in anthropogenic aerosol emissions have strongly contributed to global and regional trends in temperature, precipitation, and other climate characteristics and have been one of the dominant drivers of decadal trends in Asian and African precipitation. These and other influences on regional climate from changes in aerosol emissions are expected to continue and potentially strengthen in the coming decades. However, a combination of large uncertainties in emission pathways, radiative forcing, and the dynamical response to forcing makes anthropogenic aerosol a key factor in the spread of near-term climate projections, particularly on regional scales, and therefore an important one to constrain. For example, in terms of future emission pathways, the uncertainty in future global aerosol and precursor gas emissions by 2050 is as large as the total increase in emissions since 1850. In terms of aerosol effective radiative forcing, which remains the largest source of uncertainty in future climate change projections, CMIP6 models span a factor of 5, from 0.3 to 1.5Wm 2. Both of these sources of uncertainty are exacerbated on regional scales. The Regional Aerosol Model Intercomparison Project (RAMIP) will deliver experiments designed to quantify the role of regional aerosol emissions changes in near-term projections. This is unlike any prior MIP, where the focus has been on changes in global emissions and/or very idealised aerosol experiments. Perturbing regional emissions makes RAMIP novel from a scientific standpoint and links the intended analyses more directly to mitigation and adaptation policy issues. From a science perspective, there is limited information on how realistic regional aerosol emissions impact local as well as remote climate conditions. Here, RAMIP will enable an evaluation of the full range of potential influences of realistic and regionally varied aerosol emission changes on near-future climate. From the policy perspective, RAMIP addresses the burning question of how local and remote decisions affecting emissions of aerosols influence climate change in any given region. Here, RAMIP will provide the information needed to make direct links between regional climate policies and regional climate change. RAMIP experiments are designed to explore sensitivities to aerosol type and location and provide improved constraints on uncertainties driven by aerosol radiative forcing and the dynamical response to aerosol changes. The core experiments will assess the effects of differences in future global and regional (Africa and the Middle East, East Asia, North America and Europe, and South Asia) aerosol emission trajectories through 2051, while optional experiments will test the nonlinear effects of varying emission locations and aerosol types along this future trajectory. All experiments are based on the shared socioeconomic pathways and are intended to be performed with 6th Climate Model Intercomparison Project (CMIP6) generation models, initialised from the CMIP6 historical experiments, to facilitate comparisons with existing projections. Requested outputs will enable the analysis of the role of aerosol in near-future changes in, for example, temperature and precipitation means and extremes, storms, and air quality.

This supplement contains an example of using the CDO functions, mergegrid and sellonlatbox, to modify SSP emission files for the RAMIP simulations.In the example below, mergegrid is used to insert SSP1-2.6 data from a specified geographical region into an array containing SSP3-7.0 data everywhere else.

The Earth system model EC-Earth3 for contributions to CMIP6 is documented here, with its flexible coupling framework, major model configurations, a methodology for ensuring the simulations are comparable across different high-performance computing (HPC) systems, and with the physical performance of base configurations over the historical period. The variety of possible configurations and sub-models reflects the broad interests in the EC-Earth community. EC-Earth3 key performance metrics demonstrate physical behavior and biases well within the frame known from recent CMIP models. With improved physical and dynamic features, new Earth system model (ESM) components, community tools, and largely improved physical performance compared to the CMIP5 version, EC-Earth3 represents a clear step forward for the only European community ESM. We demonstrate here that EC-Earth3 is suited for a range of tasks in CMIP6 and beyond.

The uncertainty in aerosol radiative forcing is currently the largest source of uncertainty in estimates of the magnitude of the total anthropogenic forcing on climate, and changes in aerosol emissions are likely important for regional climate over the next few decades. This is especially the case for Africa and Asia where large aerosol emission changes are anticipated, and where aerosol has played an important role in historical changes. Uncertainty in near-term projections due to the substantial spread in aerosol (or their precursor) emissions pathways is compounded by uncertainty in the simulated response to these emissions, so a multi-model framework is needed to identify robust changes. Several earlier studies have explored the climate response to regional aerosol perturbations, with interesting, but not always consistent, results. Using these studies to inform our understanding of the potential role of aerosol in near-future changes is not straightforward. Many are based around equilibrium experiments that are challenging to use to interpret transient simulations, and the effects of different experimental designs are difficult to separate from the effects of structural differences between the models. In Regional Aerosol MIP, we will perform a set of transient experiments based on emissions from the Shared Socioeconomic Pathways. Regional Aerosol MIP will better enable us to assess the potential contribution of aerosol to near-future climate change, to describe the robust features of the response to regional aerosol changes, and to identify where the key uncertainties lie. In this presentation we will introduce the experiment design, alongside some early analysis.

We investigate how a regionally confined radiative forcing of South and East Asian aerosols translate into local and remote surface temperature responses across the globe. To do so, we carry out equilibrium climate simulations with and without modern day South and East Asian anthropogenic aerosols in two climate models with independent development histories (ECHAM6.1 and NorESM1). We run the models with the same anthropogenic aerosol representations via MACv2-SP (a simple plume implementation of the 2nd version of the Max Planck Institute Aerosol Climatology). This leads to a near identical change in instantaneous direct and indirect aerosol forcing due to removal of Asian aerosols in the two models. We then robustly decompose and compare the energetic pathways that give rise to the global and regional surface temperature effects in the models by a novel temperature response decomposition method, which translated the changes in atmospheric and surface energy fluxes into surface temperature responses by using a concept of planetary emissivity.We find that the removal of South and East Asian anthropogenic aerosols leads to strong local warming response from increased clear-sky shortwave radiation over the region, combined with opposing warming and cooling responses due to changes in cloud longwave and shortwave radiation. However, the local warming response is strongly modulated by the changes in horizontal atmospheric energy transport. Atmospheric energy transport and changes in clear-sky longwave radiation redistribute the surface temperature responses efficiently across the Northern hemisphere, and to a lesser extent also over the Southern hemisphere. The model-mean global surface temperature response to Asian anthropogenic aerosol removal is 0.26±0.04 °C (0.22±0.03 for ECHAM6.1 and 0.30±0.03 °C for NorESM1) of warming. Model-to-model differences in global surface temperature response mainly arise from differences in longwave cloud (0.01±0.01 for ECHAM6.1 and 0.05±0.01 °C for NorESM1) and shortwave cloud (0.03±0.03 for ECHAM6.1 and 0.07±0.02 °C for NorESM1) responses. The differences in cloud responses between the models also dominate the differences in regional temperature responses. In both models, the Northern hemispheric surface warming amplifies towards the Arctic, where the total temperature response is highly seasonal and modulated by seasonal changes in oceanic heat exchange and clear-sky longwave radiation.We estimate that under a strong Asian aerosol mitigation policy tied with strong greenhouse gas mitigation (Shared Socioeconomic Pathway 1-1.9) the Asian aerosol reductions can add around 8 years’ worth of current day global warming during the next few decades.

This paper documents the global climate model EC-Earth3-AerChem, one of the members of the EC-Earth3 family of models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). EC-Earth3-AerChem has interactive aerosols and atmospheric chemistry and contributes to the Aerosols and Chemistry Model Intercomparison Project (AerChemMIP). In this paper, we give an overview of the model, describe in detail how it differs from the other EC-Earth3 configurations, and outline the new features compared with the previously documented version of the model (EC-Earth 2.4). We explain how the model was tuned and spun up under preindustrial conditions and characterize the model's general performance on the basis of a selection of coupled simulations conducted for CMIP6. The net energy imbalance at the top of the atmosphere in the preindustrial control simulation is on average −0.09 W m−2 with a standard deviation due to interannual variability of 0.25 W m−2, showing no significant drift. The global surface air temperature in the simulation is on average 14.08 ∘C with an interannual standard deviation of 0.17 ∘C, exhibiting a small drift of 0.015 ± 0.005 ∘C per century. The model's effective equilibrium climate sensitivity is estimated at 3.9 ∘C, and its transient climate response is estimated at 2.1 ∘C. The CMIP6 historical simulation displays spurious interdecadal variability in Northern Hemisphere temperatures, resulting in a large spread across ensemble members and a tendency to underestimate observed annual surface temperature anomalies from the early 20th century onwards. The observed warming of the Southern Hemisphere is well reproduced by the model. Compared with the ECMWF (European Centre for Medium-Range Weather Forecasts) Reanalysis version 5 (ERA5), the surface air temperature climatology for 1995–2014 has an average bias of −0.86 ± 0.05 ∘C with a standard deviation across ensemble members of 0.35 ∘C in the Northern Hemisphere and 1.29 ± 0.02 ∘C with a corresponding standard deviation of 0.05 ∘C in the Southern Hemisphere. The Southern Hemisphere warm bias is largely caused by errors in shortwave cloud radiative effects over the Southern Ocean, a deficiency of many climate models. Changes in the emissions of near-term climate forcers (NTCFs) have significant effects on the global climate from the second half of the 20th century onwards. For the SSP3-7.0 Shared Socioeconomic Pathway, the model gives a global warming at the end of the 21st century (2091–2100) of 4.9 ∘C above the preindustrial mean. A 0.5 ∘C stronger warming is obtained for the AerChemMIP scenario with reduced emissions of NTCFs. With concurrent reductions of future methane concentrations, the warming is projected to be reduced by 0.5 ∘C.

South and East Asian anthropogenic aerosols mostly reside in an air mass extending from the Indian Ocean to the North Pacific. Yet the surface temperature effects of Asian aerosols spread across the whole globe. Here, we remove Asian anthropogenic aerosols from two independent climate models (ECHAM6.1 and NorESM1) using the same representation of aerosols via MACv2-SP (a simple plume implementation of the second version of the Max Planck Institute Aerosol Climatology). We then robustly decompose the global distribution of surface temperature responses into contributions from atmospheric energy flux changes. We find that the horizontal atmospheric energy transport strongly moderates the surface temperature response over the regions where Asian aerosols reside. Atmospheric energy transport and changes in clear-sky longwave radiation redistribute the temperature effects efficiently across the Northern Hemisphere and to a lesser extent also over the Southern Hemisphere. The model-mean global surface temperature response to Asian anthropogenic aerosol removal is 0.26±0.04 ∘C (0.22±0.03 for ECHAM6.1 and 0.30±0.03 ∘C for NorESM1) of warming. Model-to-model differences in global surface temperature response mainly arise from differences in longwave cloud (0.01±0.01 for ECHAM6.1 and 0.05±0.01 ∘C for NorESM1) and shortwave cloud (0.03±0.03 for ECHAM6.1 and 0.07±0.02 ∘C for NorESM1) responses. The differences in cloud responses between the models also dominate the differences in regional temperature responses. In both models, the northern-hemispheric surface warming amplifies towards the Arctic, where the total temperature response is highly seasonal and weakest during the Arctic summer. We estimate that under a strong Asian aerosol mitigation policy tied with strong climate mitigation (Shared Socioeconomic Pathway 1-1.9) the Asian aerosol reductions can add around 8 years' worth of current-day global warming during the next few decades.

Biogenic secondary organic aerosol (SOA) is formed as a result of the atmospheric oxidation of gas-phase biogenic volatile organic compounds (BVOCs). Here, we evaluate the ability of five European Earth System Models (CNRM-ESM2-1, EC-Earth3, IPSL-CM6, NorESM1.2, UKESM1) to capture the amount, and behaviour, of biogenic SOA in the atmosphere. The ESMs cover a range of complexity in terms of their representation of the sources and processing of biogenic SOA (i.e., from a fixed climatology of SOA amount to an interactive BVOC emission scheme followed by atmospheric processing). We combine station measurements of BVOC emission and atmospheric BVOC concentrations with remotely sensed isoprene emission estimates to evaluate the models’ representation of the sources of biogenic SOA. We use organic aerosol mass and particle number concentration measurements from a number of forested sites to evaluate the ability of the models to capture the seasonal cycle in the amount of biogenic SOA present, as well as its impact on the aerosol size distribution. Whilst the models appear to capture the seasonal cycle in organic aerosol well for a boreal forest site, the ESMs consistently over-predict the amount of organic aerosol present at a tropical forest location. Finally, we explore the ability of these models to capture the observed relationships between organic aerosol mass, or particle number, and temperature. We find that the ESMs equipped with vegetation models that generate BVOC emissions interactively are able to capture well the strength of the observed relationship between temperature and organic aerosol mass. This lends confidence to the ability of these ESMs to accurately represent changes in atmospheric composition driven by climate.

Mineral dust aerosols participate in the climate system and biogeochemistry processes due to its interactions with key components of Earth Systems: radiation, clouds, soil and chemical components. A central element to improve our understanding of mineral dust is through its modeling with Earth Systems Models where all these interactions are included. However, current simulations of dust variability exhibit important uncertainties and biases, which are model-dependent, whose cause is our imperfect knowledge about how to best represent the dust life cycle. For these reasons a continuous evaluation of the performance and properties of the different models compared against measurements is a crucial step to improve our knowledge of the dust cycle and its role in the climate system and biogeochemical cycles. Here we present an exhaustive evaluation of mineral dust aerosols in CRESCEND-ESMs over global, regional and local scales. We compare models against three networks of instruments for total dust deposition flux, yearly surface concentrations, and optical depths. Global and regional dust optical depths are compared with MODIS and MISR derived products. Specific analyses are done over the Sahel region where improved and compressive dust observational datasets are available. The results indicate that all the models capture the general properties of the global dust cycle, although the role of larger particles remains challenging. Differences are partially due to surface winds as nudged simulations improve the inter-model comparison and the performance in optical depth compared to MODIS. At the regional scale, there is an optical depth reasonable agreement over main source areas, but a joint inter-comparison including fluxes and concentration indicates larger differences. At the local scale, the uncertainties increase and current models are not able to reproduce together several observables at the same time.

Abstract. Many modelling groups that contribute to CMIP6 (Coupled Model Intercomparison Project Phase 6) have found a larger equilibrium climate sensitivity (ECS) with their latest model versions compared with the values obtained with the earlier versions used in CMIP5. This is also the case for the EC-Earth model. Therefore, in this study, we investigate what developments since the CMIP5 era could have caused the increase in the ECS in this model. Apart from increases in the horizontal and vertical resolution, the EC-Earth model has also substantially changed the representation of aerosols; in particular, it has introduced a more sophisticated description of aerosol indirect effects. After testing the model with some of the recent updates switched off, we find that the ECS increase can be attributed to the more advanced treatment of aerosols, with the largest contribution coming from the effect of aerosols on cloud microphysics (cloud lifetime or second indirect effect). The increase in climate sensitivity is unrelated to model tuning, as all experiments were performed with the same tuning parameters and only the representation of the aerosol effects was changed. These results cannot be generalised to other models, as their CMIP5 and CMIP6 versions may differ with respect to aspects other than the aerosol–cloud interaction, but the results highlight the strong sensitivity of ECS to the details of the aerosol forcing.