Aerosol Geoengineering Research Articles

Sensitivity of the global hydrological cycle to the altitude of stratospheric sulphate aerosol layer

Abstract Stratospheric aerosol geoengineering (SAG) has been proposed as one of the potential options to offset the impacts of anthropogenically induced climate change. Previous modelling studies have shown that the efficacy of the cooling via SAG increases with altitude of the aerosol layer. It has been also shown that the stratospheric heating associated with SAG could stabilize the tropical atmosphere and weaken the tropical hydrological cycle. Using a global climate model, we perform a systematic study by prescribing volcanic sulphate aerosols at three different altitudes (22 km, 18 km and 16 km) and assess the sensitivity of the global and tropical mean precipitation to the altitude. We find that even though the efficacy of cooling increases with altitude of the aerosol layer, the global and tropical mean precipitation changes are less sensitive to the height of the aerosol layer. This is because the magnitude of both the global and tropical mean precipitation reduction increases with aerosol altitude in response to increasing efficacy of aerosols, but this sensitivity related to the slow response is nearly offset by the sensitivity of fast precipitation adjustments to aerosol altitude. A perspective and analysis based on atmospheric energy budget is presented to explain the lack of sensitivity of the hydrological cycle to the altitude of the stratospheric sulphate aerosol layer.

Projected Thermally Driven Elderly Mortality for Beijing Under Greenhouse Gas and Stratospheric Aerosol Geoengineering Scenarios

AbstractBeijing is undergoing multiple challenges including urbanization, warming and aging. The Beijing megalopolis of 20 million people now suffers more cold‐related than heat‐related deaths. Stratospheric aerosol injection (SAI) geoengineering is designed to lower surface temperatures, so if SAI were ever done, it may reduce future heat‐related mortality, while also increasing cold‐related mortality. Here we use four Earth System Models (ESM) downscaled to 10 km resolution with the Weather Research and Forecasting (WRF) system to capture urban temperature, humidity and wind speeds. Temperature‐related mortality risk were calculated using a distributed lag nonlinear model (DLNM) of the elderly (over 65s) under the dynamically downscaled moderate (RCP4.5) and extreme (RCP8.5) greenhouse gas, and the G4 SAI scenarios. We used population demographics for all five shared socioeconomic pathways (SSP) and various adaptation measures. Heat‐related excess deaths under G4 are 630∼3,160 per year fewer than RCP4.5, while cold‐related deaths are 370∼1,990 more than RCP4.5 during 2060–2069, with a marginally significant net reduction. G4 significantly reduces the excess deaths relative to RCP8.5. Both heat‐related and cold‐related mortality will increase by 240∼490% when the aging population is accounted for, and decrease by 11%, 23% and 44% under low, medium and high adaptation relative to a no adaptation scenario. Dynamical downscaling produces better quality climate simulations than commonly used statistical approaches, and in the case of Beijing, significantly fewer heat‐related deaths. The marginal health benefits of modest future SAI in Beijing may be representative of the population impacts in the extra‐tropics where deaths due to cold are more than those caused by heat.

Response of the Southern Hemisphere extratropical cyclone climatology to climate intervention with stratospheric aerosol injection

Little is known about how climate intervention through stratospheric aerosol injection (SAI) may affect the climatology of the Southern Hemisphere extratropical cyclones under warming scenarios. To address this knowledge gap, we tracked extratropical cyclones from 2015 to 2099 in a set of projections of three international projects: the Assessing Responses and Impacts of Solar Climate Intervention on the Earth System with Stratospheric Aerosol Injection (ARISE), the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS), and the Geoengineering Model Intercomparison Project (GeoMIP/G6sulfur). Comparisons were performed between no-SAI and SAI scenarios as well as between different timeslices and their reference period (2015–2024). Among the findings, both no-SAI and SAI project a decrease in cyclone frequency towards the end of the century although weaker under SAI scenarios. On the other hand, cyclones tend to be stronger under no-SAI scenarios while keeping their intensity more similar to the reference period under SAI scenarios. This means that under SAI scenarios the climatology of cyclones is less affected by global warming than under no-SAI. Other features of these systems, such as travelling distance, lifetime, and mean velocity show small differences between no-SAI and SAI scenarios and between reference and future periods.

An investigation of the relationship between tropical monsoon precipitation changes and stratospheric sulfate aerosol optical depth

Abstract Stratospheric aerosol geoengineering (SAG) is one of the several solar geoengineering options that have been proposed to counteract climate change. In the case of SAG, reflective aerosols injected into the stratosphere would reflect more sunlight and cool the planet. When assessing the potential efficacy and risks of SAG, the sensitivity of tropical monsoon precipitation changes should be also considered. Using a climate model, we perform several stylized simulations with different meridional distributions and amounts of volcanic sulfate aerosols in the stratosphere. Because tropical monsoon precipitation responds to global mean and interhemispheric difference in radiative forcing or temperature, we quantify the sensitivity of tropical monsoon precipitation to SAG in terms of two parameters: global mean aerosol optical depth (GMAOD) and interhemispheric AOD difference (IHAODD). For instance, we find that the simulated northern hemisphere monsoon precipitation has a sensitivity of −1.33 ± 0.95% per 0.1 increase in GMAOD and −7.62 ± 0.27% per 0.1 increase in IHAODD. Our estimated precipitation changes in terms of the two sensitivity parameters for the global mean precipitation and for the indices of tropical, northern hemisphere, southern hemisphere and Indian summer monsoon precipitation are in good agreement with the model simulated precipitation changes. Similar sensitivity estimates are also made for unit changes in global mean and interhemispheric differences in effective radiative forcing and surface temperature. Our study based on planetary energetics provides a simpler framework for understanding the tropical monsoon precipitation response to external forcing agents.

Injection strategy – a driver of atmospheric circulation and ozone response to stratospheric aerosol geoengineering

Abstract. Despite offsetting global mean surface temperature, various studies demonstrated that stratospheric aerosol injection (SAI) could influence the recovery of stratospheric ozone and have important impacts on stratospheric and tropospheric circulation, thereby potentially playing an important role in modulating regional and seasonal climate variability. However, so far, most of the assessments of such an approach have come from climate model simulations in which SO2 is injected only in a single location or a set of locations. Here we use CESM2-WACCM6 SAI simulations under a comprehensive set of SAI strategies achieving the same global mean surface temperature with different locations and/or timing of injections, namely an equatorial injection, an annual injection of equal amounts of SO2 at 15∘ N and 15∘ S, an annual injection of equal amounts of SO2 at 30∘ N and 30∘ S, and a polar strategy injecting SO2 at 60∘ N and 60∘ S only in spring in each hemisphere. We demonstrate that despite achieving the same global mean surface temperature, the different strategies result in contrastingly different magnitudes of the aerosol-induced lower stratospheric warming, stratospheric moistening, strengthening of stratospheric polar jets in both hemispheres, and changes in the speed of the residual circulation. These impacts tend to maximise under the equatorial injection strategy and become smaller as the aerosols are injected away from the Equator into the subtropics and higher latitudes. In conjunction with the differences in direct radiative impacts at the surface, these different stratospheric changes drive different impacts on the extratropical modes of variability (Northern and Southern Annular modes), including important consequences on the northern winter surface climate, and on the intensity of tropical tropospheric Walker and Hadley circulations, which drive tropical precipitation patterns. Finally, we demonstrate that the choice of injection strategy also plays a first-order role in the future evolution of stratospheric ozone under SAI throughout the globe. Overall, our results contribute to an increased understanding of the fine interplay of various radiative, dynamical, and chemical processes driving the atmospheric circulation and ozone response to SAI and lay the foundation for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons.

Stratospheric Sulfate Aerosols Impacts on West African Monsoon Precipitation Using GeoMIP Models

AbstractStratospheric Aerosol Geoengineering (SAG) is proposed to offset global warming; however, the use of this approach can an impact on the hydrological cycle. We used simulations from Coupled Model Intercomparison Project (CMIP5) and Geoengineering Model Intercomparison Project (G3 simulation) to analyze the impacts of SAG on precipitation (P) and to determine its responsible causes in West Africa and Sahel region. CMIP5 historical data were first validated, the results obtained are consistent with observational data. Under the Representative Concentration Pathway scenario RCP4.5, a slight increase is found in the West Africa Region relative to present‐day climate. The dynamic processes especially, the monsoon shifts are responsible for this precipitation change. Under RCP4.5, during the monsoon period, reductions in P are 0.86%, 0.80% relative to the present‐day climate in the Northern and Southern Sahel, respectively, while precipitation is increased by 1.04% in the West African Region. Under SAG, we find a 3.71% decrease of precipitation in the West African Region while the precipitation decrease is 17.4% and 8.47% respectively in the North Sahel and South Sahel. This decrease in monsoon precipitation is mainly explained by changes in dynamics, which lead to weakened monsoon circulation and a shift in the distribution of monsoon precipitation. This result suggests that SAG deployment to balance all warming can be harmful to rainfall in WAR if the amount of SO2 to be injected into this tropical area is not taken into consideration.

Quantifying the Efficiency of Stratospheric Aerosol Geoengineering at Different Altitudes

AbstractStratospheric 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.

Potential impact of stratospheric aerosol geoengineering on projected temperature and precipitation extremes in South Africa

Stratospheric aerosol injection (SAI) is the theoretical deployment of sulphate particles into the stratosphere to reflect incoming solar radiation and trigger a cooling impact at the Earth’s surface. This study assessed the potential impact of SAI geoengineering on temperature and precipitation extremes over South Africa (SAF) and its climatic zones in the future (2075–2095) using simulations from the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS) project. We analyse three different experiments from the GLENS project, each of which simulate stratospheric SO2 injection under the representative concentration pathway 8.5 (RCP8.5) emissions scenario: (i) tropical injection around 22.8–25 km altitude (GLENS), (ii) tropical injection around 1 km above the tropopause (GLENS_low), and (iii) injection near the equator around 20–25 km (GLENS_eq). The study used a set of the Expert Team on Climate Change Detection and Indices describing temperature and rainfall extremes to assess the impact of the three SAI experiments on extreme weather in the future over SAF. The results of this study indicate that, relative to the baseline period (2010–2030), all three SAI experiments are mostly over-effective in offsetting the projected RCP8.5 increase in the frequency of hot (up to −60%) and decrease (up to +10%) in cold temperature extremes over SAF and its climatic zones. These findings suggest that SAI could cause over-cooling in SAF. However, SAI impact on precipitation extremes is less linear and varies across the country’s climatic zones. For example, SAI could reinforce the projected decrease in precipitation extremes across most of SAF, although it could exacerbate heavy precipitation over the KwaZulu-Natal Coast. These findings are consistent across SAI experiments except in magnitude, as GLENS_eq and GLENS_low could cause larger decreases in precipitation extremes than GLENS. These findings imply that SAI could alleviate heat stress on human health, agriculture, and vulnerable communities while simultaneously decreasing infrastructure and crops’ vulnerability to flooding. It is, however, essential to interpret these findings cautiously as they are specific to the SAI experiments and modelling settings considered in the GLENS project.

The Sensitivity of Moisture Flux Partitioning in the Cold‐Point Tropopause to External Forcing

AbstractThe dryness of the stratosphere is the result of air entering through the cold tropical tropopause layer (TTL). However, our understanding of the moisture flux partitioning into water vapor and frozen hydrometeors is incomplete. This raises concerns regarding the ability of General Circulation Models to accurately predict changes in stratospheric water vapor following perturbations in the radiative budget due to volcanic aerosol or stratospheric geoengineering. We present the first results using a global storm‐resolving model investigating the sensitivity of moisture fluxes within the TTL to an additional heating source. We address the question how the partitioning of moisture fluxes into water vapor and frozen hydrometeors changes under perturbations. The analysis reveals the resilience of the TTL, keeping the flux partitioning constant even at an average cold‐point warming exceeding 8 K. In the control and perturbed simulations, water vapor contributes around 80% of the moisture entering the stratosphere.

Quantification of tropical monsoon precipitation changes in terms of interhemispheric differences in stratospheric sulfate aerosol optical depth

Stratospheric Aerosol Geoengineering (SAG) is one of the solar geoengineering approaches that have been proposed to offset some of the impacts of anthropogenic climate change. Past studies have shown that SAG may have adverse impacts on the global hydrological cycle. Using a climate model, we quantify the sensitivity of the tropical monsoon precipitation to the meridional distribution of volcanic sulfate aerosols prescribed in the stratosphere in terms of the changes in aerosol optical depth (AOD). In our experiments, large changes in summer monsoon precipitation in the tropical monsoon regions are simulated, especially over the Indian region, in association with meridional shifts in the location of the intertropical convergence zone (ITCZ) caused by changes in interhemispheric AOD differences. Based on our simulations, we estimate a sensitivity of − 1.8° ± 0.0° meridional shift in global mean ITCZ and a 6.9 ± 0.4% reduction in northern hemisphere (NH) monsoon index (NHMI; summer monsoon precipitation over NH monsoon regions) per 0.1 interhemispheric AOD difference (NH minus southern hemisphere). We also quantify this sensitivity in terms of interhemispheric differences in effective radiative forcing and interhemispheric temperature differences: 3.5 ± 0.3% change in NHMI per unit (Wm−2) interhemispheric radiative forcing difference and 5.9 ± 0.4% change per unit (°C) interhemispheric temperature difference. Similar sensitivity estimates are also made for the Indian monsoon precipitation. The establishment of the relationship between interhemispheric AOD (or radiative forcing) differences and ITCZ shift as discussed in this paper will further facilitate and simplify our understanding of the effects of SAG on tropical monsoon rainfall.

Impact of Stratospheric Geoengineering on Sea Surface Temperature in the Northern Gulf of Guinea

Among techniques proposed to limit global warming, there is Stratospheric Aerosol Geoengineering (SAG) which is aiming to increase Earth-atmosphere albedo by injecting sulfur dioxide into the stratosphere in order to reduce the solar radiation that reaches the earth. This study aims to assess the potential impact of SAG on Sea Surface Temperature (SST) in the Northern Gulf of Guinea and its causes using GLENS (Geoengineering Large Ensemble) simulations performed under a high anthropogenic emission scenario (RCP8.5). Here, we focus on two dynamically different regions: Sassandra Upwelling in Côte d’Ivoire (SUC, located east of Cape Palmas) and Takoradi Upwelling in Ghana (TUG, located east of Cape Three Points). Results show that in the SUC region, under climate change, there is an increase in SST (referred to as the current climate) all year long (by 1.52 °C on average) mainly due to an increase in net heat flux (lead by the decrease in longwave radiation) and also in weak vertical mixing (caused by strong stratification which dominates the vertical shear). Under SAG, SST decreases all the seasonal cycle with its maximum in December (−0.4 °C) due to a reduction in the net heat flux (caused by a diminution of solar radiation) and an increase in vertical advection (due to an increase in vertical temperature gradient and vertical velocity). In the TUG region, under climate change, SST warming is a little more intense than in the SUC region and SST changes are driven by an increase in the net heat flux and strong stratification. The cooling of the SST in TUG is similar to the SUC region, but contrary to this region, the cooling under SAG is not only explained by a decrease in the net heat flux but also by the remote forcing of wind changes at the western equatorial Atlantic.

Impacts of Solar Radiation Management on Hydro-Climatic Extremes in Southeast Asia

Solar radiation management (SRM), or solar geoengineering, reduces the earth’s temperature by reflecting more sunlight back to space. However, the impacts of SRM remain unclear, making it difficult to project the benefits as well as consequences should this approach be adopted to combat climate change. To provide novel insight into the SRM impact on hydro-climatic extremes in Southeast Asia, this study conducts a simulation experiment for the Kelantan River Basin (KRB) in Malaysia by incorporating three bias-corrected Stratospheric Aerosol Geoengineering Large Ensemble (GLENS) members into the Soil and Water Assessment Tool Plus (SWAT+) model. The study found that SRM practices could generate substantial cooling effects on regional temperatures, leading to a reduction in projected annual precipitation and monthly precipitation during the flooding season (from November to mid-January) under SRM relative to the Representative Concentration Pathway 8.5 (RCP8.5) scenario. In addition, SRM could reduce the number of days with heavy precipitation as well as the intensity of maximum daily precipitation as compared to RCP8.5, during the 2045–2064 and 2065–2084 periods, leading to a reduction in high flows. Nevertheless, under SRM impacts, the driest months from February to May would experience comparable decreases in monthly precipitation and streamflow.

Implication of stratospheric aerosol geoengineering on compound precipitation and temperature extremes in Africa

Three Coupled Model Intercomparison Project 5 (CMIP5) models that simulated the G4 experiment of the Geoengineering Model Intercomparison Project (GeoMIP) were used to investigate the impact of stratospheric aerosol injection (SAI) on combined temperature and precipitation extremes in Africa that can have greater negative impacts on human and the environment than individual rainfall or temperature extremes. The examined compound extremes included the dry (Rwarm׀dry and Rcold׀dry) and wet (Rwarm׀wet and Rcold׀wet) modes assessed during the injection (SAI, 2050–2069) and post-injection (postSAI, 2070–2089) periods compared with the historical period (1986–2005). We found a significant projected change in the occurrence of both wet and dry modes during SAI and postSAI related to the historical period. The magnitude and sign of this change depend on the season and the geographical location. During the SAI and postSAI, the wet (Rwarm׀wet and Rcold׀wet) modes are projected to be significantly lower while the dry modes are noted to increase in a large part of African continent depending on the season and the geographical location and may consequently leads to an increase of the droughts prone areas. The termination effect is noted to reduce the occurrence of dry modes, which may reduce the potential negative effects of the injection after halting. As the effect may vary from one region to another and according to the season, it suggested assessing the key sector impacts of SAI. Thus, this change in dry modes due to SAI could affect all activities which depend on water resources such as water supply, agriculture and food production, energy demand, and production with adverse effects on health, security, and sustainable development, but this needs to be assessed and quantified at regional scales.

Detecting Changes in Global Extremes Under the GLENS‐SAI Climate Intervention Strategy

AbstractAs anthropogenic activities continue to drive increases in extreme events, the fundamental solution of reducing greenhouse gas emissions remains elusive. Thus, there is growing interest in stratospheric aerosol injection (SAI) to offset some of the most dangerous consequences of climate change. Although global SAI deployment would likely be easy to detect by some metrics, the detectability of SAI on extreme events might be more difficult. We examine this question in climate model simulations of SAI; specifically, the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS‐SAI) scenario. We train a logistic regression model to predict whether a map of global extremes came from climate simulations with or without SAI. The timing of accurate predictions is a quantification of the time required to detect SAI impacts. We find that regional changes in extreme temperature and precipitation under GLENS are robustly detected within 1 and 15 years of initial SAI injection, respectively.

Collapse of Earth’s Biosphere: A Case of Planetary Treason

Earth’s life support systems are breaking down, including the stratospheric ozone layer, which protects all higher life on the planet from deadly ultraviolet radiation. This breakdown is a direct result of human activities including the large-scale manipulation of processes that affect Earth’s climate, otherwise known as geoengineering. We present further evidence that coal fly ash, utilized in tropospheric aerosol geoengineering, is the primary cause of stratospheric ozone depletion, not chlorofluorocarbons, as “decreed” by the Montreal Protocol. The misdiagnosis was a potentially fatal mistake by mankind. Coal fly ash particles, uplifted to the stratosphere, are collected and trapped by polar stratospheric clouds. In springtime, as these clouds begin to melt/evaporate, multiple coal fly ash compounds and elements are released to react with and consume stratospheric ozone. Contrary to the prevailing narrative, the stratospheric ozone layer has already been badly damaged and now increasingly deadly ultraviolet radiation, UV-B and UV-C, penetrates to Earth’s surface. Our time is short to permanently end all geoengineering activities, and to reduce and/or eliminate all sources of aerosolized coal fly ash, including first and foremost the jet-sprayed emplacements into the troposphere that are systematically breaking down Earth’s support systems and poisoning life on this planet.

Influence of stratospheric aerosol geoengineering on temperature mean and precipitation extremes indices in Africa

PurposeThree 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/approachThis 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).FindingsDuring 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 implicationsCompared 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/valueTo 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.

Impact of Stratospheric Aerosol Geoengineering on Extreme Precipitation and Temperature Indices in West Africa Using GLENS Simulations

AbstractThis study assesses changes in extremes precipitation and temperature in West Africa under a high greenhouse gas scenario, that is, a representative concentration pathway 8.5, and under a scenario of stratospheric aerosol geoengineering (SAG) deployment using the NCAR Community Earth System Model version 1. We use results from the Geoengineering Large Ensemble simulations (GLENS), where SAG is deployed to keep global surface temperatures at present day values. This impact study evaluates changes in some of the extreme climate indices recommended by the Expert Team Monitoring on Climate Change Detection and Indices. The results indicate that SAG would effectively keep surface temperatures at present day‐conditions across a range of indices compared to the control (CRTL) period, including Cold days, Cold nights and Cold Spell Duration Indicator which show no significant increase compared to the CRTL period. Regarding the extremes precipitation, GLENS shows mostly a statistically significant increase in annual precipitation and statistically significant decrease in the number of heavy and very heavy precipitation events relative to the CRTL period in some regions of Gulf of Guinea. In the Sahel, we notice a mix of statistically significant increase and decrease in Max 1‐day and Max 5‐days precipitation amount relative to the CRTL period at the end of the 21st century when large amounts of SAG has been applied. The changes in extreme precipitation indices are linked to changes in Atlantic Multidecadal Oscillation, NINO3.4 and Indian Ocean Dipole and these changes in extreme precipitation are driven by change in near surface specific humidty and atmospheric circulation.

Changes in Hadley circulation and intertropical convergence zone under strategic stratospheric aerosol geoengineering

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.

An interactive stratospheric aerosol model intercomparison of solar geoengineering by stratospheric injection of SO<sub>2</sub> or accumulation-mode sulfuric acid aerosols

Abstract. Studies of stratospheric solar geoengineering have tended to focus on modification of the sulfuric acid aerosol layer, and almost all climate model experiments that mechanistically increase the sulfuric acid aerosol burden assume injection of SO2. A key finding from these model studies is that the radiative forcing would increase sublinearly with increasing SO2 injection because most of the added sulfur increases the mass of existing particles, resulting in shorter aerosol residence times and aerosols that are above the optimal size for scattering. Injection of SO3 or H2SO4 from an aircraft in stratospheric flight is expected to produce particles predominantly in the accumulation-mode size range following microphysical processing within an expanding plume, and such injection may result in a smaller average stratospheric particle size, allowing a given injection of sulfur to produce more radiative forcing. We report the first multi-model intercomparison to evaluate this approach, which we label AM-H2SO4 injection. A coordinated multi-model experiment designed to represent this SO3- or H2SO4-driven geoengineering scenario was carried out with three interactive stratospheric aerosol microphysics models: the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM2) with the Whole Atmosphere Community Climate Model (WACCM) atmospheric configuration, the Max-Planck Institute's middle atmosphere version of ECHAM5 with the HAM microphysical module (MAECHAM5-HAM) and ETH's SOlar Climate Ozone Links with AER microphysics (SOCOL-AER) coordinated as a test-bed experiment within the Geoengineering Model Intercomparison Project (GeoMIP). The intercomparison explores how the injection of new accumulation-mode particles changes the large-scale particle size distribution and thus the overall radiative and dynamical response to stratospheric sulfur injection. Each model used the same injection scenarios testing AM-H2SO4 and SO2 injections at 5 and 25 Tg(S) yr−1 to test linearity and climate response sensitivity. All three models find that AM-H2SO4 injection increases the radiative efficacy, defined as the radiative forcing per unit of sulfur injected, relative to SO2 injection. Increased radiative efficacy means that when compared to the use of SO2 to produce the same radiative forcing, AM-H2SO4 emissions would reduce side effects of sulfuric acid aerosol geoengineering that are proportional to mass burden. The model studies were carried out with two different idealized geographical distributions of injection mass representing deployment scenarios with different objectives, one designed to force mainly the midlatitudes by injecting into two grid points at 30∘ N and 30∘ S, and the other designed to maximize aerosol residence time by injecting uniformly in the region between 30∘ S and 30∘ N. Analysis of aerosol size distributions in the perturbed stratosphere of the models shows that particle sizes evolve differently in response to concentrated versus dispersed injections depending on the form of the injected sulfur (SO2 gas or AM-H2SO4 particulate) and suggests that prior model results for concentrated injection of SO2 may be strongly dependent on model resolution. Differences among models arise from differences in aerosol formulation and differences in model dynamics, factors whose interplay cannot be easily untangled by this intercomparison.

Impact of Stratospheric Aerosol Geoengineering on Meteorological Droughts in West Africa

This study assesses changes in meteorological droughts in West Africa under a high greenhouse gas scenario, i.e., a representative concentration pathway 8.5 (RCP8.5), and under a scenario of stratospheric aerosol geoengineering (SAG) deployment. Using simulations from the Geoengineering Large Ensemble (GLENS) project that employed stratospheric sulfate aerosols injection to keep global mean surface temperature, as well as the interhemispheric and equator-to-pole temperature gradients at the 2020 level (present-day climate), we investigated the impact of SAG on meteorological droughts in West Africa. Analysis of the meteorological drought characteristics (number of drought events, drought duration, maximum length of drought events, severity of the greatest drought events and intensity of the greatest drought event) revealed that over the period from 2030–2049 and under GLENS simulations, these drought characteristics decrease in most regions in comparison to the RCP8.5 scenarios. On the contrary, over the period from 2070–2089 and under GLENS simulations, these drought characteristics increase in most regions compared to the results from the RCP8.5 scenarios. Under GLENS, the increase in drought characteristics is due to a decrease in precipitation. The decrease in precipitation is largely driven by weakened monsoon circulation due to the reduce of land–sea thermal contrast in the lower troposphere.