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Abstract Atmospheric rivers (ARs), intrusions of warm and moist air, can effectively drive weather extremes over the Arctic and trigger subsequent impact on sea ice and climate. What controls the observed multi-decadal Arctic AR trends remains unclear. Here, using multiple sources of observations and model experiments, we find that, contrary to the uniform positive trend in climate simulations, the observed Arctic AR frequency increases by twice as much over the Atlantic sector compared to the Pacific sector in 1981-2021. This discrepancy can be reconciled by the observed positive-to-negative phase shift of Interdecadal Pacific Oscillation (IPO) and the negative-to-positive phase shift of Atlantic Multidecadal Oscillation (AMO), which increase and reduce Arctic ARs over the Atlantic and Pacific sectors, respectively. Removing the influence of the IPO and AMO can reduce the projection uncertainties in near-future Arctic AR trends by about 24%, which is important for constraining projection of Arctic warming and the timing of an ice-free Arctic.

Abstract Stratospheric aerosol injection (SAI) has been proposed as a potential supplement to mitigate some climate impacts of anthropogenic warming. Using Community Earth System Model ensemble simulation results, we analyze the response of temperature and precipitation extremes to two different SAI strategies: one injects SO2 at the equator to stabilize global mean temperature and the other injects SO2 at multiple locations to stabilize global mean temperature as well as the interhemispheric and equator‐to‐pole temperature gradients. Our analysis shows that in the late 21st century, compared with the present‐day climate, both equatorial and multi‐location injection lead to reduced hot extremes in the tropics, corresponding to overcooling of the mean climate state. In mid‐to‐high latitude regions, in comparison to the present‐day climate, substantial decreases in cold extremes are observed under both equatorial and multi‐location injection, corresponding to residual winter warming of the mean climate state. Both equatorial and multi‐location injection reduce precipitation extremes in the tropics below the present‐day level, associated with the decrease in mean precipitation. Overall, for most regions, temperature and precipitation extremes show reduced change in response to multi‐location injection than to equatorial injection, corresponding to reduced mean climate change for multi‐location injection. In comparison with equatorial injection, in response to multi‐location injection, most land regions experience fewer years with significant change in cold extremes from the present‐day level, and most tropical regions experience fewer years with significant change in hot extremes. The design of SAI strategies to mitigate anthropogenic climate extremes merits further study.

Abstract Previous modeling studies indicate that even though marine cloud brightening under a susceptibility‐based strategy is effective in reducing the global average surface temperature, it triggers a La Niña‐like sea‐surface temperature response with cooling mostly confined within lower latitudes. Here we explore a different cloud seeding strategy involving seeding of regions with low susceptibility, primarily over the ocean in midlatitudes during the winter. Simulations with the Community Earth System Model, version 2 reveal that because the regional forcing is weaker and more widespread, cooling is more evenly distributed over the globe. This new strategy also does not result in the La Niña‐like state seen in the other strategies.

Abstract Climate change is expected to have far‐reaching effects at both the global and regional scale, but local effects are difficult to determine from coarse‐resolution climate studies. Dynamical downscaling can provide insight into future climate projections on local scales. Here, we present a new dynamically downscaled dataset for Indiana and the surrounding regions. Output from the Community Earth System Model (CESM) version 1 is downscaled using the Weather Research and Forecasting model (WRF). Simulations are run with a 24‐hr reinitialization strategy and a 12‐hr spin‐up window. WRF output is bias corrected to the National Centers for Environmental Protection/National Center for Atmospheric Research 40‐year Reanalysis project (NCEP) using a modified quantile mapping method. Bias‐corrected 2‐m air temperature and accumulated precipitation are the initial focus, with additional variables planned for future releases. Regional climate change signals agree well with larger global studies, and local fine‐scaled features are visible in the resulting dataset, such as urban heat islands, frontal passages, and orographic temperature gradients. This high‐resolution climate dataset could be used for down‐stream applications focused on impacts across the domain, such as urban planning, energy usage, water resources, agriculture and public health.

Abstract The impacts of Stratospheric Aerosol Injection (SAI) on the atmosphere and surface climate depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. We use a set of CESM2(WACCM6) SAI simulations achieving three different levels of global mean surface cooling and demonstrate that unlike some direct surface climate impacts driven by the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in the high latitude circulation and ozone are more complex and could be non‐linear. This manifests in our simulations by disproportionally larger Antarctic springtime ozone loss, significantly larger intra‐ensemble spread of the Arctic stratospheric jet and ozone responses, and non‐linear impacts on the extratropical modes of surface climate variability under the strongest‐cooling SAI scenario compared to the weakest one. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI.

Purpose – Three Coupled Model Intercomparison Project Phase 5 models involved in the G4 experiment of the Geoengineering Model Inter-comparison Project (GeoMIP) project were used to investigate the impact of stratospheric aerosol injection (SAI) on the mean surface air temperature and precipitation extremes in Africa. Design/methodology/approach – This impact was examined under G4 and Representative Concentration Pathway (RCP) 4.5 scenarios on the total precipitation, the number of rainy days (RR1) and of days with heavy rainfall (R20 mm), the rainfall intensity (SDII), the maximum length of consecutive wet (CWD) and dry (CDD) days and on the maximum rainfall in five consecutive days (Rx5day) across four regions: Western Africa (WAF), Eastern Africa (EAF), Northern Africa and Southern Africa (SAF). Findings – During the 50 years (2020–2069) of SAI, mean continental warming is −0.40°C lower in G4 than under RCP4.5. During the post-injection period (2070–2090), the temperature continues to increase, but at a lower rate (−0.19°C) than in RCP4.5. During SAI, annual rainfall in G4 is significantly greater than in RCP4.5 over the high latitudes (especially over SAF) and lower over the tropics. The termination of SAI leads to a significant increase of rainfall over Sahel and EAF and a decrease over SAF and Guinea Coast (WAF). Practical implications – Compared to RCP4.5, SAI will contribute to reducing significantly regional warming but with a significant decrease of rainfall in the tropics where rainfed agriculture account for a large part of the economies. After the SAI period, the risk of drought over the extratropical regions (especially in SAF) will be mitigated, while the risk of floods will be exacerbated in the Central Sahel. Originality/value – To meet the Paris Agreement, African countries will implement mitigation measures to contribute to keep the surface air temperature below 2°C. Geoengineering with SAI is suggested as an option to meet this challenge, but its implication on the African climate system needs a deep investigation in the aim to understand the impacts on temperature and precipitation extremes. To the best of the authors’ knowledge, this study is the first to investigate the potential impact of SAI using the G4 experiment of GeoMIP on temperature and precipitation extremes of the African continent.

Abstract Stratospheric aerosol injection (SAI) of reflective sulfate aerosols has been proposed to temporarily reduce the impacts of global warming. In this study, we compare two SAI simulations which inject at different altitudes to provide the same amount of cooling, finding that lower‐altitude SAI requires 64% more injection. SAI at higher altitudes cools the surface more efficiently per unit injection than lower‐altitude SAI through two primary mechanisms: the longer lifetimes of SO2 and SO4 at higher altitudes, and the water vapor feedback, in which lower‐altitude SAI causes more heating in the tropical cold point tropopause region, thereby increasing water vapor transport into the stratosphere and trapping more terrestrial infrared radiation that offsets some of the direct aerosol‐induced cooling. We isolate these individual mechanisms and find that the contribution of lifetime effects to differences in cooling efficiency is approximately five to six times larger than the contribution of the water vapor feedback.

The economy of Central African countries strongly depends on rain-fed agriculture and hydropower generation. However, most countries in this subregion do not yet have the irrigation technologies that are already applied in many more advanced nations, which further exposes them to the serious risk of severe drought caused by global warming. This study investigates the potential impact of solar radiation modification (SRM) geoengineering on the water availability over the four major river basins that cross most of Central African countries (i.e. Niger Basin, Lake Chad Basin, Cameroon Atlantic Basin (CAB) and Congo Basin). For this purpose a potential water availability index was computed based on an ensemble-mean simulations carried out in the framework of Phase 6 of the Geoengineering Model Intercomparison Project considering two SRM simulation experiments: the stratospheric sulphate aerosol injection (G6sulfur) and the global solar dimming (G6solar). The climate change simulation results in a robust decreases by up to 60% in water availability, most pronounced over the CAB under the high radiative forcing scenario. Therefore, in a business-as-usual world, the reduction in water availability combined with the rapid population growth expected by 2050 in the studied subregion, could result in a significant water deficit over Central African countries towards the end of the 21st century. This water deficit could affect all activities that depend on water resources, such as water supply, agriculture and hydropower generation. Furthermore, the results also show that SRM methods have the potential to significantly reduce this deficit by increasing water availability (as compared to climate change) by up to 50% over the affected river basins, with a more accentuated increase found in the CAB when the global solar dimming is applied. These results suggest good possibilities of adaptation for populations living in the geographical areas of these river basins.

Stratospheric aerosol injection (SAI) would involve the addition of sulfate aerosols in the stratosphere to reflect part of the incoming solar radiation, thereby cooling the climate. Studies trying to explore the impacts of SAI have often focused on idealized scenarios without explicitly introducing what we call ‘inconsistencies’ in a deployment. A concern often discussed is what would happen to the climate system after an abrupt termination of its deployment, whether inadvertent or deliberate. However, there is a much wider range of plausible inconsistencies in deployment than termination that should be evaluated to better understand associated risks. In this work, we simulate a few representative inconsistencies in a pre-existing SAI scenario: an abrupt termination, a decade-long gradual phase-out, and 1 year and 2 year temporary interruptions of deployment. After examining their climate impacts, we use these simulations to train an emulator, and use this to project global mean temperature response for a broader set of inconsistencies in deployment. Our work highlights the capacity of a finite set of explicitly simulated scenarios that include inconsistencies to inform an emulator that is capable of expanding the space of scenarios that one might want to explore far more quickly and efficiently.

Climate change is a prevalent threat, and it is unlikely that current mitigation efforts will be enough to avoid unwanted impacts. One potential option to reduce climate change impacts is the use of stratospheric aerosol injection (SAI). Even if SAI is ultimately deployed, it might be initiated only after some temperature target is exceeded. The consequences of such a delay are assessed herein. This study compares two cases, with the same target global mean temperature of ∼1.5° C above preindustrial, but start dates of 2035 or a ‘delayed’ start in 2045. We make use of simulations in the Community Earth System Model version 2 with the Whole Atmosphere Coupled Chemistry Model version 6 (CESM2-WACCM6), using SAI under the SSP2-4.5 emissions pathway. We find that delaying the start of deployment (relative to the target temperature) necessitates lower net radiative forcing (−30%) and thus larger sulfur dioxide injection rates (+20%), even after surface temperatures converge, to compensate for the extra energy absorbed by the Earth system. Southern hemisphere ozone is higher from 2035 to 2050 in the delayed start scenario, but converges to the same value later in the century. However, many of the surface climate differences between the 2035 and 2045 start simulations appear to be small during the 10–25 years following the delayed SAI start, although longer simulations would be needed to assess any longer-term impacts in this model. In addition, irreversibilities and tipping points that might be triggered during the period of increased warming may not be adequately represented in the model but could change this conclusion in the real world.

Abstract Stratospheric aerosol geoengineering has been proposed as a potential solution to reduce climate change and its impacts. Here, we explore the responses of the Hadley circulation (HC) intensity and the intertropical convergence zone (ITCZ) using the strategic stratospheric aerosol geoengineering, in which sulfur dioxide was injected into the stratosphere at four different locations to maintain the global-mean surface temperature and the interhemispheric and equator-to-pole temperature gradients at present-day values (baseline). Simulations show that, relative to the baseline, strategic stratospheric aerosol geoengineering generally maintains northern winter December–January–February (DJF) HC intensity under RCP8.5, while it overcompensates for the greenhouse gas (GHG)-forced southern winter June–July–August (JJA) HC intensity increase, producing a 3.5 ± 0.4% weakening. The residual change of southern HC intensity in JJA is mainly associated with stratospheric heating and tropospheric temperature response due to enhanced stratospheric aerosol concentrations. Geoengineering overcompensates for the GHG-driven northward ITCZ shifts, producing 0.7° ± 0.1° and 0.2° ± 0.1° latitude southward migrations in JJA and DJF, respectively relative to the baseline. These migrations are affected by tropical interhemispheric temperature differences both at the surface and in the free troposphere. Further strategies for reducing the residual change of HC intensity and ITCZ shifts under stratospheric aerosol geoengineering could involve minimizing stratospheric heating and restoring and preserving the present-day tropical tropospheric interhemispheric temperature differences.

Abstract Stratospheric aerosol injection (SAI) has been shown in climate models to reduce some impacts of global warming in the Arctic, including the loss of sea ice, permafrost thaw, and reduction of Greenland Ice Sheet (GrIS) mass; SAI at high latitudes could preferentially target these impacts. In this study, we use the Community Earth System Model to simulate two Arctic‐focused SAI strategies, which inject at 60°N latitude each spring with injection rates adjusted to either maintain September Arctic sea ice at 2030 levels (“Arctic Low”) or restore it to 2010 levels (“Arctic High”). Both simulations maintain or restore September sea ice to within 10% of their respective targets, reduce permafrost thaw, and increase GrIS surface mass balance by reducing runoff. Arctic High reduces these impacts more effectively than a globally focused SAI strategy that injects similar quantities of SO2 at lower latitudes. However, Arctic‐focused SAI is not merely a “reset button” for the Arctic climate, but brings about a novel climate state, including changes to the seasonal cycles of Northern Hemisphere temperature and sea ice and less high‐latitude carbon uptake relative to SSP2‐4.5. Additionally, while Arctic‐focused SAI produces the most cooling near the pole, its effects are not confined to the Arctic, including detectable cooling throughout most of the northern hemisphere for both simulations, increased mid‐latitude sulfur deposition, and a southward shift of the location of the Intertropical Convergence Zone. For these reasons, it would be incorrect to consider Arctic‐focused SAI as “local” geoengineering, even when compared to a globally focused strategy.

Abstract The response of the Indian Summer Monsoon (ISM) to global warming, solar geoengineering and its termination is examined using the multi-model mean of seven global climate model simulations from G2 experiment of the Geoengineering Model Intercomparison Project. Under the global warming scenario, land–ocean temperature contrasts and low-level monsoon circulation progressively strengthen accompanied by enhanced precipitation over the Indian subcontinent. Notably, in the solar geoengineered scenario, marginal surface cooling is projected over the majority of the ISM region, and there is strengthening of both upper and lower level circulation. However, preferential precipitation near Western Ghats leads to dry bias over majority of Indian land. Upon the termination of the geoengineering, the climatic conditions—temperature, precipitation, winds and moisture would abruptly change to what it would have been under the global warming scenario. Thus, this may be important to note that such changes may need attention for the future mitigation and adaptation purposes if solar geoengineering is required to implement in future.

Regional geoengineering, by reflecting sunlight over a very limited spatial domain, might be considered as a means to target specific regional impacts of climate change. One of the obvious concerns raised by such approaches is the extent to which the resulting effects would be detectable well beyond the targeted region (e.g. in neighbouring countries). A few studies have explored this question for targeted regions that are still comparatively large. We consider idealized simulations with increased ocean albedo over relatively small domains; the Gulf of Mexico (0.23% of Earth's surface) and over the Australian Great Barrier Reef (0.07%), both with negligible global radiative forcing. Applied over these very small domains, the only statistically significant non-local changes we find are some limited reduction on summer precipitation in Florida in the Gulf of Mexico case (adjacent to the targeted region). The lack of transboundary effects suggests that governance needs for such targeted interventions are quite distinct from those for more global sunlight reflection.

Abstract We document the configuration, tuning, and evaluation of a modified version of the Community Earth System Model version 1 (Hurrell et al., 2013, https://doi.org/10.1175/BAMS-D-12), introduced here as E3SMv0‐HiLAT, intended for study of high‐latitude processes. E3SMv0‐HiLAT incorporates changes to the atmospheric model affecting aerosol transport to high northern latitudes and to reduce shortwave cloud bias over the Southern Ocean. An updated sea ice model includes biogeochemistry that is coupled to an extended version of the ocean model's biogechemistry. This enables cloud nucleation to depend on the changing marine emissions of aerosol precursors, which may be expected in scenarios with strongly changing sea ice extent, oceanic stratification and associated nutrient availability, and atmospheric state. An evaluation of the basic preindustrial state of E3SMv0‐HiLAT is presented in order to ensure that its climate is adequate to support future experimentation. Additional capability is not achieved without some cost, relative to the extraordinarily well‐tuned model from which it was derived. In particular, a reduction of bias in cloud forcing achieved over the Southern Hemisphere also allows for greater Southern Ocean sea ice extent, a tendency that has been partially but not fully alleviated through experimentation and tuning. The most interesting change in the behavior of the model may be its response to greenhouse gas forcing: While the climate sensitivity is found to be essentially unchanged from that of Community Earth System Model version 1, the adjusted radiative forcing has increased from within one standard deviation above that of Coupled Model Intercomparison Project Phase 5 models to nearly two standard deviations.

Abstract Simple climate models (SCMs) are computationally efficient and capable of emulating global mean output of more complex Earth system models (ESMs). In doing so, SCMs can play a critical role in climate research as stand‐ins for the computationally more expensive models, especially in studies involving low, spatial, and/or temporal resolution, providing more computationally efficient sources of climate data. Here we use Hector v2.5.0 to emulate the multiforcing historical and RCP scenario output for 31 concentration and seven emission‐driven ESMs. When calibrating Hector, sufficient calibration data must be used to constrain the model; otherwise, climate and/or carbon parameters affecting physical processes may be able to trade off with one another, allowing for solutions to use physically unreasonable fitted parameter values as well as limiting the application of the SCM as an emulator. We also present a novel methodology that uses the ESM range as a calibration data, which can be adopted when faced with missing variable output from a specific model.

Earth System Models (ESMs) are excellent tools for quantifying many aspects of future climate dynamics but are too computationally expensive to produce large collections of scenarios for downstream users of ESM data. In particular, many researchers focused on the impacts of climate change require large collections of ESM runs to rigorously study the impacts to both human and natural systems of low-frequency high-importance events, such as multi-year droughts. Climate model emulators provide an effective mechanism for filling this gap, reproducing many aspects of ESMs rapidly but with lower precision. The fldgen v1.0 R package quickly generates thousands of realizations of gridded temperature fields by randomizing the residuals of pattern scaling temperature output from any single ESM, retaining the spatial and temporal variance and covariance structures of the input data at a low computational cost. The fldgen v2.0 R package described here extends this capability to produce joint realizations of multiple variables, with a focus on temperature and precipitation in an open source software package available for community use (https://github.com/jgcri/fldgen). This substantially improves the fldgen package by removing the requirement that the ESM variables be normally distributed, and will enable researchers to quickly generate covarying temperature and precipitation data that are synthetic but faithful to the characteristics of the original ESM.

Geoengineering by injecting sulfur dioxide (SO _2 ) into the lower stratosphere has been suggested to reduce anthropogenically induced warming. While impacts of such geoengineering on climate have been investigated in recent decades, few modeling studies have considered biogeochemical feedbacks resulting from such intervention. This study comprehensively characterizes responses and feedbacks of terrestrial ecosystems, from an ensemble of coupled high-resolution Earth system model climate change simulations, under the highest standard greenhouse gas scenario with an extreme geoengineering mitigation strategy. Under this strategy, temperature increases beyond 2020 levels due to elevated anthropogenic carbon dioxide (CO _2 ) were completely offset by the SO _2 injection. Carbon cycle feedbacks can alter the trajectory of atmospheric CO _2 levels by storing or releasing additional carbon on land and in the ocean, thus moderating or amplifying climate change. We assess terrestrial biogeochemical feedbacks to climate in response to geoengineering, using model output from the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS) project. Results indicate terrestrial ecosystems become a stronger carbon sink globally because of lower ecosystem respiration and diminished disturbance effects under geoengineering. An additional 79 Pg C would be stored on land by the end of the twenty-first century, yielding as much as a 4% reduction in atmospheric CO _2 mole fraction without marine biogeochemical feedbacks, compared to the high greenhouse gas scenario without geoengineering.

Abstract Stress‐induced emissions of biogenic volatile organic compounds (VOCs) from terrestrial ecosystems may be one of the dominant sources of VOC emissions worldwide. Understanding the ecosystem stress response could reveal how ecosystems will respond and adapt to climate change and, in turn, quantify changes in the atmospheric burden of VOC oxidants and secondary organic aerosols. Here, we argue, based on preliminary evidence from several opportunistic measurement sources, that chemical signatures of stress can be identified and quantified at the ecosystem scale. We also outline future endeavors that we see as next steps toward uncovering quantitative signatures of stress, including new advances in both VOC data collection and analysis of “big data.”

Climate modification by stratospheric SO2 injections, to form sulfate aerosols, may alter the spectral and angular distributions of the solar ultraviolet and visible radiation that reach the Earth’s surface, with potential consequences to environmental photobiology and photochemistry. We used modeling results from the CESM1(WACCM) stratospheric aerosol geoengineering large ensemble (GLENS) project, following the RCP8.5 emission scenario, and one geoengineering experiment with SO2 injections in the stratosphere, designed to keep surface temperatures at 2020 levels. Zonally and monthly averaged vertical profiles of O3, SO2, and sulfate aerosols, at 30 N and 70 N, served as input into a radiative transfer model, to compute biologically active irradiances for DNA damage (iDNA), UV index (UVI), photosynthetically active radiation (PAR), and two key tropospheric photodissociation coefficients (jO1D for O3 + hν (λ < 330 nm) → O(1D) + O2; and jNO2 for NO2 + hν (λ < 420 nm) → O(3P) + NO). We show that the geoengineering scenario is accompanied by substantial reductions in UV radiation. For example, comparing March 2080 to March 2020, iDNA decreased by 25% to 29% in the subtropics (30 N) and by 26% to 33% in the polar regions (70 N); UVI decreased by 19% to 20% at 30 N and 23% to 26% at 70 N; and jO1D decreased by 22% to 24% at 30 N and 35% to 40% at 70 N, with comparable contributions from sulfate scattering and stratospheric O3 recovery. Different responses were found for processes that depend on longer UV and visible wavelengths, as these are minimally affected by ozone; PAR and jNO2 were only slightly lower (9⁻12%) at 30 N, but much lower at 70 N (35⁻40%). Similar reductions were estimated for other months (June, September, and December). Large increases in the PAR diffuse-direct ratio occurred in agreement with previous studies. Absorption by SO2 gas had a small (~1%) effect on jO1D, iDNA, and UVI, and no effect on jNO2 and PAR.

Global-scale solar geoengineering is the deliberate modification of the climate system to offset some amount of anthropogenic climate change by reducing the amount of incident solar radiation at the surface. These changes to the planetary energy budget result in differential regional climate effects. For the first time, we quantitatively evaluate the potential for regional disparities in a multi-model context using results from a model experiment that offsets the forcing from a quadrupling of CO _2 via reduction in solar irradiance. We evaluate temperature and precipitation changes in 22 geographic regions spanning most of Earthʼs continental area. Moderate amounts of solar reduction (up to 85% of the amount that returns global mean temperatures to preindustrial levels) result in regional temperature values that are closer to preindustrial levels than an un-geoengineered, high CO _2 world for all regions and all models. However, in all but one model, there is at least one region for which no amount of solar reduction can restore precipitation toward its preindustrial value. For most metrics considering simultaneous changes in both variables, temperature and precipitation values in all regions are closer to the preindustrial climate for a moderate amount of solar reduction than for no solar reduction.

Solar geoengineering has been proposed as a method of meeting climate objectives, such as reduced globally averaged surface temperatures. However, because of incomplete understanding of the effects of geoengineering on the climate system, its implementation would be in the presence of substantial uncertainties. In our study, we use two fully coupled atmosphere–ocean general circulation models: one in which the geoengineering strategy is designed, and one in which geoengineering is implemented (a real-world proxy). We show that regularly adjusting the amount of solar geoengineering in response to departures of the observed global mean climate state from the predetermined objective (sequential decision making; an explicit feedback approach) can manage uncertainties and result in achievement of the climate objective in both the design model and the real-world proxy. This approach results in substantially less error in meeting global climate objectives than using a predetermined time series of how much geoengineering to use, especially if the estimated sensitivity to geoengineering is inaccurate.