Stratospheric Aerosol Injection Research Articles

Thermosteric and dynamic sea level under solar geoengineering

The IPCC sixth assessment report forecasts sea level rise (SLR) of up to 2 m along coasts by 2100 relative to 1995–2014 following business as usual (SSP585) scenarios. Geoengineering may reduce this threat. We use five Earth System Models simulations of two different solar geoengineering methods (solar dimming and stratospheric sulfate aerosol injection), that offset radiative forcing differences between SSP585 “no-mitigation” and the modest mitigation SSP245 greenhouse gas scenarios, to analyze the impact on global mean thermosteric and dynamic regional sea levels. By 2080–2099, both forms of geoengineering reduce global mean thermosteric sea level by 36–41% (11.2–12.6 cm) relative to SSP585, bringing the global mean SLR under SSP585 in line with that under SSP245, but do not perfectly restore regional SLR patterns. Some of the largest reductions (∼18 cm) are on densely populated coasts of eastern Northern America and Japan and along vulnerable Arctic coastal permafrost.

Optimizing Injection Locations Relaxes Altitude‐Lifetime Trade‐Off for Stratospheric Aerosol Injection

AbstractStratospheric Aerosol Injection (SAI) aims to offset some climate hazards by injecting aerosols into the stratosphere to reflect solar radiation. The lifetime of injected particles influences SAI's radiative efficacy—the ratio of radiative forcing to particle mass flux. We employ a Lagrangian trajectory model with particle sedimentation to simulate how background circulations influence the transport of passive particles (without microphysical growth) in the stratosphere and quantify sensitivities of particle lifetime to injection locations. At 20 km, optimizing injection locations can increase particle lifetime by >40%. Injection strategies can be constrained to maintain an interhemispheric balance of particle lifetime without significantly decreasing total lifetime. Generally, increasing injection altitude increases particle lifetime while also increasing costs and environmental impacts of deployment aircraft. Optimizing injection latitude and longitude can relax this altitude‐lifetime trade‐off by increasing lifetime without needing to increase altitude, which warrants further testing in global climate models with aerosol microphysics.

Climate intervention on a high-emissions pathway could delay but not prevent West Antarctic Ice Sheet demise

Solar radiation modification (SRM) is increasingly discussed as a tool to reduce or avert global warming and concomitantly the risk of ice-sheet collapse, as is considered possible for the West Antarctic Ice Sheet (WAIS). Here we analyse the impact of stratospheric aerosol injections on the centennial-to-millennial Antarctic sea-level contribution using an ice-sheet model. We find that mid-twenty-first-century large-scale SRM could delay but ultimately not prevent WAIS collapse in a high-emissions scenario. On intermediate-emissions pathways, SRM could be an effective tool to delay or even prevent an instability of WAIS if deployed by mid-century. However, SRM interventions may be associated with substantial risks, commitments and unintended side effects; therefore, emissions reductions to prevent WAIS collapse seem to be the more practical and sensible approach at the current stage.

The Choice of Baseline Period Influences the Assessments of the Outcomes of Stratospheric Aerosol Injection

AbstractThe specifics of the simulated injection choices in the case of stratospheric aerosol injections (SAI) are part of the crucial context necessary for meaningfully discussing the impacts that a deployment of SAI would have on the planet. One of the main choices is the desired amount of cooling that the injections are aiming to achieve. Previous SAI simulations have usually either simulated a fixed amount of injection, resulting in a fixed amount of warming being offset, or have specified one target temperature, so that the amount of cooling is only dependent on the underlying trajectory of greenhouse gases. Here, we use three sets of SAI simulations achieving different amounts of global mean surface cooling while following a middle‐of‐the‐road greenhouse gas emission trajectory: one SAI scenario maintains temperatures at 1.5°C above preindustrial levels (PI), and two other scenarios which achieve additional cooling to 1.0°C and 0.5°C above PI. We demonstrate that various surface impacts scale proportionally with respect to the amount of cooling, such as global mean precipitation changes, changes to the Atlantic Meridional Overturning Circulation and to the Walker Cell. We also highlight the importance of the choice of the baseline period when comparing the SAI responses to one another and to the greenhouse gas emission pathway. This analysis leads to policy‐relevant discussions around the concept of a reference period altogether, and to what constitutes a relevant, or significant, change produced by SAI.

Public response to solar geoengineering: how media frames about stratospheric aerosol injection affect opinions

Public response to solar geoengineering: how media frames about stratospheric aerosol injection affect opinions

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.

Identifying key-psychological factors influencing the acceptance of yet emerging technologies–A multi-method-approach to inform climate policy

The best combination of possible climate policy options (mitigation, adaptation and different climate engineering technologies) to tackle climate change is unknown. Climate policy is facing a hard decision in answering the question whether climate engineering technologies should be researched, limitedly deployed or even deployed at global scale. Such technologies bear large epistemic and ethical uncertainties and their use as well as non-use might have severe consequences. To deal with such uncertainties, the (ethical) assessment of climate engineering technologies should include the perspectives of various stakeholders including laypersons to inform climate policy. To facilitate (ethical) technology assessment, we propose a novel 2-step methodology to collect and analyze data on ethical concerns and the acceptability of climate engineering technologies. Thereby we focus on Stratospheric Aerosol Injection (SAI) as an use case. We propose an innovative combination of newly developed methods consisting of two data collection tools (Cognitive-Affective Mapping and large-scale survey) and two types of data analyses (using graph theory and factor analysis). Applying this multi-method approach we were able to identify (1) central ethical and governance related concerns regarding SAI (by Cognitive-Affective Maps) and (2) to estimate the relative importance of core constructs (positive and negative affect, risk and benefit perception, trust) on the acceptability of SAI (by large-scale survey).

AI climate tipping‐point discovery (ACTD)

AI climate tipping‐point discovery (ACTD)

Solar Geoengineering in the Polar Regions: A Review

AbstractSolar geoengineering refers to proposals, including stratospheric aerosol injection (SAI), to slow or reverse climate change by reflecting away incoming sunlight. The rapid changes ongoing in the Arctic and Antarctic, and the risk of exceeding tipping points in the cryosphere within decades, make limiting such changes a plausible objective of solar geoengineering. Here, we review the impacts of SAI on polar climate and cryosphere, including the dependence of these impacts on the latitude(s) of injection, and make recommendations for future research directions. SAI would cool the polar regions and reduce many changes in polar climate under future warming scenarios. Some under‐cooling of the polar regions relative to the global mean is expected under SAI without high latitude injection, due to latitudinal variation in insolation and CO2 forcing, the forcing dependence of the polar lapse rate feedback, and altered atmospheric dynamics. There are also potential limitations in the effectiveness of SAI to arrest changes in winter‐time polar climate and to prevent sea‐level rise from the Antarctic ice sheet. Finally, we also review the prospects for three other solar geoengineering proposals targeting the poles: marine cloud brightening, cirrus cloud thinning, and sea‐ice albedo modification. Sea‐ice albedo modification appears unlikely to be viable on pan‐Arctic or Antarctic scales. Whether marine cloud brightening or cirrus cloud thinning would be effective in the polar regions remains uncertain. Solar geoengineering is an increasingly prominent proposal and a robust understanding of its consequences in the polar regions is needed to inform climate policy in the coming decades.

Normative Uncertainty in Solar Climate Engineering Research Governance

ABSTRACT This paper explores what kind of uncertainty a research program governing solar climate engineering through Stratospheric Aerosol Injection (SAI) needs to account for. Specifically, it tries to answer two central issues with regards to SAI research and it’s ethical evaluation: One, what irreducible uncertainties remain throughout the decision-process, and, two, how do these remaining uncertainties affect the ethical evaluation of SAI research. The main assumption is that decisions on SAI research governance will be made under normative uncertainty, i.e. situations under irreducible knowledge-constraints that arise in concrete, practical decision-contexts. These decision-contexts are multi-lateral and empirically ambiguous, and the decision-makers need to reconcile a plurality of values. While normative uncertainty complicates the ethical evaluation of policy decisions, I argue that moral considerations can be accommodated through the inclusion of recognitional, participatory justice approaches, as well as adaptive and anticipatory governance methods.

Opinion: The scientific and community-building roles of the Geoengineering Model Intercomparison Project (GeoMIP) – past, present, and future

Abstract. The Geoengineering Model Intercomparison Project (GeoMIP) is a coordinating framework, started in 2010, that includes a series of standardized climate model experiments aimed at understanding the physical processes and projected impacts of solar geoengineering. Numerous experiments have been conducted, and numerous more have been proposed as “test-bed” experiments, spanning a variety of geoengineering techniques aimed at modifying the planetary radiation budget: stratospheric aerosol injection, marine cloud brightening, surface albedo modification, cirrus cloud thinning, and sunshade mirrors. To date, more than 100 studies have been published that used results from GeoMIP simulations. Here we provide a critical assessment of GeoMIP and its experiments. We discuss its successes and missed opportunities, for instance in terms of which experiments elicited more interest from the scientific community and which did not, and the potential reasons why that happened. We also discuss the knowledge that GeoMIP has contributed to the field of geoengineering research and climate science as a whole: what have we learned in terms of intermodel differences, robustness of the projected outcomes for specific geoengineering methods, and future areas of model development that would be necessary in the future? We also offer multiple examples of cases where GeoMIP experiments were fundamental for international assessments of climate change. Finally, we provide a series of recommendations, regarding both future experiments and more general activities, with the goal of continuously deepening our understanding of the effects of potential geoengineering approaches and reducing uncertainties in climate outcomes, important for assessing wider impacts on societies and ecosystems. In doing so, we refine the purpose of GeoMIP and outline a series of criteria whereby GeoMIP can best serve its participants, stakeholders, and the broader science community.

Assessing Outcomes in Stratospheric Aerosol Injection Scenarios Shortly After Deployment

AbstractStratospheric aerosol injection (SAI) is a proposed form of climate intervention that would release reflective particles into the stratosphere, thereby reducing solar insolation and cooling the planet. The climate response to SAI is not well understood, particularly on short‐term time horizons frequently used by decision‐makers and planning practitioners to assess climate information. We demonstrate two framings to explore the climate response in the decade after SAI deployment in modeling experiments with parallel SAI and no‐SAI simulations. The first framing, which we call a snapshot around deployment, displays change over time within the SAI scenarios and applies to the question “What happens before and after SAI is deployed in the model?” The second framing, the intervention impact, displays the difference between the SAI and no‐SAI simulations, corresponding to the question “What is the impact of a given intervention relative to climate change with no intervention?” We apply these framings to annual mean 2 m temperature, precipitation, and a precipitation extreme during the 10 yr after deployment in two large ensembles of Earth system model simulations that comprehensively represent both the SAI injection process and climate response, and connect these results to implications for other climate variables. We show that SAI deployment robustly reduces changes in many high‐impact climate variables even on these short timescales where the forced response is relatively small, but that details of the climate response depend on the model version, greenhouse gas emissions scenario, and other aspects of the experimental design.

Stratospheric ozone, UV radiation, and climate interactions

This assessment provides a comprehensive update of the effects of changes in stratospheric ozone and other factors (aerosols, surface reflectivity, solar activity, and climate) on the intensity of ultraviolet (UV) radiation at the Earth’s surface. The assessment is performed in the context of the Montreal Protocol on Substances that Deplete the Ozone Layer and its Amendments and Adjustments. Changes in UV radiation at low- and mid-latitudes (0–60°) during the last 25 years have generally been small (e.g., typically less than 4% per decade, increasing at some sites and decreasing at others) and were mostly driven by changes in cloud cover and atmospheric aerosol content, caused partly by climate change and partly by measures to control tropospheric pollution. Without the Montreal Protocol, erythemal (sunburning) UV irradiance at northern and southern latitudes of less than 50° would have increased by 10–20% between 1996 and 2020. For southern latitudes exceeding 50°, the UV Index (UVI) would have surged by between 25% (year-round at the southern tip of South America) and more than 100% (South Pole in spring). Variability of erythemal irradiance in Antarctica was very large during the last four years. In spring 2019, erythemal UV radiation was at the minimum of the historical (1991–2018) range at the South Pole, while near record-high values were observed in spring 2020, which were up to 80% above the historical mean. In the Arctic, some of the highest erythemal irradiances on record were measured in March and April 2020. For example in March 2020, the monthly average UVI over a site in the Canadian Arctic was up to 70% higher than the historical (2005–2019) average, often exceeding this mean by three standard deviations. Under the presumption that all countries will adhere to the Montreal Protocol in the future and that atmospheric aerosol concentrations remain constant, erythemal irradiance at mid-latitudes (30–60°) is projected to decrease between 2015 and 2090 by 2–5% in the north and by 4–6% in the south due to recovering ozone. Changes projected for the tropics are ≤ 3%. However, in industrial regions that are currently affected by air pollution, UV radiation will increase as measures to reduce air pollutants will gradually restore UV radiation intensities to those of a cleaner atmosphere. Since most substances controlled by the Montreal Protocol are also greenhouse gases, the phase-out of these substances may have avoided warming by 0.5–1.0 °C over mid-latitude regions of the continents, and by more than 1.0 °C in the Arctic; however, the uncertainty of these calculations is large. We also assess the effects of changes in stratospheric ozone on climate, focusing on the poleward shift of climate zones, and discuss the role of the small Antarctic ozone hole in 2019 on the devastating “Black Summer” fires in Australia. Additional topics include the assessment of advances in measuring and modeling of UV radiation; methods for determining personal UV exposure; the effect of solar radiation management (stratospheric aerosol injections) on UV radiation relevant for plants; and possible revisions to the vitamin D action spectrum, which describes the wavelength dependence of the synthesis of previtamin D3 in human skin upon exposure to UV radiation.Graphical abstract

The Slippery Slopes of Climate Engineering Research

Climate engineering research attracts Slippery Slope concerns – the idea that initial research will inevitably lead to inappropriate deployment. Some have dismissed it as an unrealistic, unproductive critique. However, extant climate engineering discussions of the Slippery Slope discuss an unorganised set of different causal mechanisms with little detail. These range from technological cost reduction, to the creation of special interest lobby groups, to normalisation across society and policymakers. Dismissing the Slippery Slope may be premature if its causal nature is unclear, especially given the potentially high impacts and controversy of global climate engineering deployment. Disaggregating and clarifying the Slippery Slope can reduce unnecessary ambiguity, promote productive debate, and highlight risks that require further attention. Drawing on previous Slippery Slope literature and mechanisms of change from range of disciplines, this paper creates a typology of Slippery Slopes for application to stratospheric aerosol injection and other emerging technologies. Initial research can lead to deployment by 1) sparking price-performance improvements and sunk cost biases, 2) contributing to normalisation and legitimisation, 3) altering power structures, 4) sparking hype, and 5) incrementally progressing development. These feedback loops may currently seem unlikely, but unforeseen dynamics could still trigger rapid development and implementation of stratospheric aerosol injection. Conversely, there is no guarantee one of these Slippery Slopes will occur. The point is that they could – the future is too uncertain to fully dismiss non-linear change, particularly for high impact and accessible technologies like stratospheric aerosol injection. This can provide direction and clarity for effective technology governance and Slippery Slope discussion. Furthermore, this typology differentiates the Slippery Slope from lock-in and highlights their interaction points. Slippery Slope dynamics are processes that can (but are not guaranteed to) lead to different types of lock-in. Lock-in is when a technology is entrenched in existing sociotechnical systems. Given the risks of unchecked undesired lock-in, lock-in is a state to be encouraged instead of avoided.

Identifying the regional emergence of climate patterns in the ARISE-SAI-1.5 simulations

Stratospheric aerosol injection is a proposed form of solar climate invention (SCI) that could potentially reduce the amount of future warming from externally-forced climate change. However, more research is needed, as there are significant uncertainties surrounding the possible impacts of SCI, including unforeseen effects on regional climate patterns. In this study, we consider a climate model simulation of the deployment of stratospheric aerosols to maintain the global mean surface temperature at 1.5 ∘C above pre-industrial levels (ARISE-SAI-1.5). Leveraging two different machine learning methods, we evaluate when the effects of SCI would be detectable at regional scales. Specifically, we train a logistic regression model to classify whether an annual mean map of near-surface temperature or total precipitation is from future climate change under the influence of SCI or not. We then design an artificial neural network to predict how many years it has been since the deployment of SCI by inputting the regional maps from the climate intervention scenario. In both detection methods, we use feature attribution methods to spatially understand the forced climate patterns that are important for the machine learning model predictions. The differences in regional temperature signals are detectable in under a decade for most regions in the SCI scenario compared to greenhouse gas warming. However, the influence of SCI on regional precipitation patterns is more difficult to distinguish due to the presence of internal climate variability.

Considering stratospheric aerosol injections beyond an environmental frame: The intelligible ‘emergency’ techno-fix and preemptive security

AbstractStratospheric Aerosol Injection (SAI), is often referred to as a ‘Plan B’ if mitigation strategies to reduce emissions fail and the need to rapidly reduce global temperatures becomes urgent. In theory, SAI would help buy more time to bring carbon and other emissions down while also cooling or keeping the planet below the threshold for dangerous warming, though it is not a solution to the problem of climate change in itself. What little attention it has received in International Relations (IR) is usually focused on the need for governance of the technology and assumes that development and use of the technology will be driven primarily by vulnerability to climate impacts. Through an analysis of common security assumptions and preemptive security framings the article shows that while current assessments of SAI focus on the technology’s environmental impact, broader political and security dynamics, particularly the desire to render climate change more intelligible as a security problem with a solution may have substantial influence on how the technology is used and by whom.

Impact of Stratospheric Aerosol Injection on the East Asian Winter Monsoon

AbstractStratospheric aerosol injection (SAI) has been proposed as a potential measure to alleviate global warming; however, uncertainties exist in its regional climate impacts. This study investigates the influence of the SAI on the East Asian winter monsoon (EAWM) by analyzing the G6sulfur simulation from the Geoengineering Model Intercomparison Project. Our results show that the SAI intensifies the Siberian high and Aleutian low, enhances the low‐level northerly winds along the coast of East Asia and deepens the East Asian trough during 2080–2099, reversing the weakening of EAWM in high‐emission Shared Socioeconomic Pathway scenario (SSP5‐8.5). Comparison between the G6sulfur and SSP5‐8.5 simulations indicates that, the SAI‐induced strong cooling in the central equatorial Pacific excites a poleward‐propagating Rossby wave train and strengthen the EAWM. Meanwhile, the negative vorticity advection in the middle troposphere and the radiative cooling over the Siberian high region caused by SAI also intensify the EAWM.

Particle number concentrations and size distributions in the stratosphere: implications of nucleation mechanisms and particle microphysics

Abstract. While formation and growth of particles in the troposphere have been extensively studied in the past two decades, very limited efforts have been devoted to understanding these in the stratosphere. Here we use both Cosmics Leaving OUtdoor Droplets (CLOUD) laboratory measurements taken under very low temperatures (205–223 K) and Atmospheric Tomography Mission (ATom) in situ observations of particle number size distributions (PNSDs) down to 3 nm to constrain nucleation mechanisms and to evaluate model-simulated particle size distributions in the lowermost stratosphere (LMS). We show that the binary homogenous nucleation (BHN) scheme used in most of the existing stratospheric aerosol injection (a proposed method of solar radiation modification) modeling studies overpredicts the nucleation rates by 3–4 orders of magnitude (when compared to CLOUD data) and particle number concentrations in the background LMS by a factor ∼ 2–4 (when compared to ATom data). Based on a recently developed kinetic nucleation model, which gives rates of both ion-mediated nucleation (IMN) and BHN at low temperatures in good agreement with CLOUD measurements, both BHN and IMN occur in the stratosphere. However, IMN rates are generally more than 1 order of magnitude higher than BHN rates and thus dominate nucleation in the background stratosphere. In the Southern Hemisphere (SH) LMS with minimum influence of anthropogenic emissions, our analysis shows that ATom-measured PNSDs generally have four apparent modes. The model captures reasonably well the two modes (Aitken mode and the first accumulation mode) with the highest number concentrations and size-dependent standard deviations. However, the model misses an apparent second accumulation mode peaking around 300–400 nm, which is in the size range important for aerosol direct radiative forcing. The bimodal structure of accumulation mode particles has also been observed in the stratosphere well above tropopause and in the volcano-perturbed stratosphere. We suggest that this bimodal structure may be caused by the effect of charges on coagulation and growth, which is not yet considered in any existing models and may be important in the stratosphere due to high ionization rates and the long lifetime of aerosols. Considering the importance of accurate PNSDs for projecting a realistic radiation forcing response to stratospheric aerosol injection (SAI), it is essential to understand and incorporate such potentially important processes in SAI model simulations and to carry out further research to find out what other processes the present models might have missed.

Northern-high-latitude permafrost and terrestrial carbon response to two solar geoengineering scenarios

Abstract. The northern-high-latitude permafrost contains almost twice the carbon content of the atmosphere, and it is widely considered to be a non-linear and tipping element in the earth's climate system under global warming. Solar geoengineering is a means of mitigating temperature rise and reduces some of the associated climate impacts by increasing the planetary albedo; the permafrost thaw is expected to be moderated under slower temperature rise. We analyze the permafrost response as simulated by five fully coupled earth system models (ESMs) and one offline land surface model under four future scenarios; two solar geoengineering scenarios (G6solar and G6sulfur) based on the high-emission scenario (ssp585) restore the global temperature from the ssp585 levels to the moderate-mitigation scenario (ssp245) levels via solar dimming and stratospheric aerosol injection. G6solar and G6sulfur can slow the northern-high-latitude permafrost degradation but cannot restore the permafrost states from ssp585 to those under ssp245. G6solar and G6sulfur tend to produce a deeper active layer than ssp245 and expose more thawed soil organic carbon (SOC) due to robust residual high-latitude warming, especially over northern Eurasia. G6solar and G6sulfur preserve more SOC of 4.6 ± 4.6 and 3.4 ± 4.8 Pg C (coupled ESM simulations) or 16.4 ± 4.7 and 12.3 ± 7.9 Pg C (offline land surface model simulations), respectively, than ssp585 in the northern near-surface permafrost region. The turnover times of SOC decline slower under G6solar and G6sulfur than ssp585 but faster than ssp245. The permafrost carbon–climate feedback is expected to be weaker under solar geoengineering.

Climate response to off-equatorial stratospheric sulfur injections in three Earth system models – Part 2: Stratospheric and free-tropospheric response

Abstract. The paper constitutes Part 2 of a study performing a first systematic inter-model comparison of the atmospheric responses to stratospheric aerosol injection (SAI) at various single latitudes in the tropics, as simulated by three state-of-the-art Earth system models – CESM2-WACCM6, UKESM1.0, and GISS-E2.1-G. Building on Part 1 (Visioni et al., 2023) we demonstrate the role of biases in the climatological circulation and specific aspects of the model microphysics in driving the inter-model differences in the simulated sulfate distributions. We then characterize the simulated changes in stratospheric and free-tropospheric temperatures, ozone, water vapor, and large-scale circulation, elucidating the role of the above aspects in the surface SAI responses discussed in Part 1. We show that the differences in the aerosol spatial distribution can be explained by the significantly faster shallow branches of the Brewer–Dobson circulation in CESM2, a relatively isolated tropical pipe and older tropical age of air in UKESM, and smaller aerosol sizes and relatively stronger horizontal mixing (thus very young stratospheric age of air) in the two GISS versions used. We also find a large spread in the magnitudes of the tropical lower-stratospheric warming amongst the models, driven by microphysical, chemical, and dynamical differences. These lead to large differences in stratospheric water vapor responses, with significant increases in stratospheric water vapor under SAI in CESM2 and GISS that were largely not reproduced in UKESM. For ozone, good agreement was found in the tropical stratosphere amongst the models with more complex microphysics, with lower stratospheric ozone changes consistent with the SAI-induced modulation of the large-scale circulation and the resulting changes in transport. In contrast, we find a large inter-model spread in the Antarctic ozone responses that can largely be explained by the differences in the simulated latitudinal distributions of aerosols as well as the degree of implementation of heterogeneous halogen chemistry on sulfate in the models. The use of GISS runs with bulk microphysics demonstrates the importance of more detailed treatment of aerosol processes, with contrastingly different stratospheric SAI responses to the models using the two-moment aerosol treatment; however, some problems in halogen chemistry in GISS are also identified that require further attention. Overall, our results contribute to an increased understanding of the underlying physical mechanisms as well as identifying and narrowing the uncertainty in model projections of climate impacts from SAI.