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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. Plain Language Summary: The injection of reflective aerosols, or their precursors, into the lower stratosphere (Stratospheric Aerosol Injection, SAI) has been proposed as a temporary measure to offset some of the adverse impacts of climate change whilst atmospheric concentrations of greenhouses are being stabilized and, ultimately, reduced. The impacts of SAI on the atmosphere and surface climate would depend on when and where the sulfate aerosol precursors are injected, as well as on how much surface cooling is to be achieved. Here we analyze SAI impacts on stratospheric climate and ozone in a set of Earth system model simulations under varying magnitudes of the SAI‐induced global mean cooling. We demonstrate that unlike some of the direct surface climate impacts from the reflection of solar radiation by sulfate aerosols, the SAI‐induced changes in stratospheric circulation, chemistry and climate are more complex, with the model simulations pointing toward more non‐linear behavior of the high latitude circulation and ozone under higher SAI scenarios. These potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI. Key Points: Impacts of Stratospheric Aerosol Injection (SAI) depend on how much surface cooling is to be achievedHigh latitude circulation, ozone and modes of extratropical variability can vary non‐linearly with the SAI‐induced global surface coolingThese potential non‐linearities may add to uncertainties in projections of regional surface impacts under SAI [ABSTRACT FROM AUTHOR]

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 SO 2 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 SO 2 at 15 ∘ N and 15 ∘ S, an annual injection of equal amounts of SO 2 at 30 ∘ N and 30 ∘ S, and a polar strategy injecting SO 2 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. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Stratospheric aerosol injection (SAI)—the artificial introduction of reflective droplets, called aerosols, into the middle atmosphere–could reflect a small portion of sunlight and cool the planet in order to temporarily reduce the impacts from global warming. Injecting the aerosols at higher altitudes would be more expensive, but it would also be more efficient because the aerosols would last longer before falling out of the atmosphere. Additionally, injecting at a lower altitude would cause more water vapor to enter the middle atmosphere; since water vapor is a greenhouse gas, this would increase the greenhouse effect, meaning more aerosols would be needed to cool the surface to a desired temperature. In this study, we directly compare high‐altitude SAI to low‐altitude SAI to determine how much more efficient it is to inject at a higher altitude, and we break down the different factors that effect efficiency to see which has the biggest effect. Key Points: We compare two stratospheric aerosol injection strategies which inject SO2 at different altitudes to meet the same temperature targetThe low altitude strategy requires two thirds more injection to provide the same amount of coolingWe isolate and quantify the different factors which cause the high altitude injection strategy to cool the surface more efficiently [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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: 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. In conjunction with the differences in direct radiative impacts at the surface, these drive different impacts on the extratropical modes of variability (Northern and Southern Annular Mode), including important consequences on the northern winter surface climate, as well as 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 response to SAI, as well as lay the ground for designing an optimal SAI strategy that could form a basis of future multi-model intercomparisons. [ABSTRACT FROM AUTHOR]

Stratospheric aerosol injection (SAI) comes with a wide range of possible design choices, such as the location and timing of the injection. Different injection strategies can yield different climate responses; therefore, making informed future decisions on SAI requires an understanding of the range of possible climate outcomes. Yet to date, there has been no systematic exploration of a comprehensive set of SAI strategies. This limits the ability to determine which effects are robust across different strategies and which depend on specific injection choices, or to determine if there are underlying trade-offs between different climate goals. This study systematically explores how the choice of SAI strategy affects climate responses. Here, we introduce four hemispherically-symmetric injection strategies, all of which are designed to maintain the same global mean surface temperature: an annual injection at the equator (EQ), an annual injection of equal amounts of SO2 at 15° N and 15° S (15N+15S), an annual injection of equal amounts of SO2 at 30° N and 30° S (30N+30S), and a polar injection strategy that injects equal amounts of SO2 at 60° N and 60° S only during spring in each hemisphere (60N+60S). We compare these four hemispherically-symmetric SAI strategies with a more complex injection strategy that injects different quantities of SO2 at 30° N, 15° N, 15° S, and 30° S in order to maintain not only the global mean surface temperature but also its large scale horizontal gradients. We find that the choice of SAI strategy notably affects the spatial distribution of aerosol optical depths, injection efficiency, and various surface climate responses. Among other findings, we show that injecting in subtropics produces more global cooling per unit injection, with the EQ and the 60N+60S cases requiring, respectively, 59 % and 50 % more injection than the 30N+30S case to meet the same global mean temperature target. Injecting at higher latitudes results in larger equator-to-pole temperature gradients. While all five strategies restore September Arctic sea ice, the high-latitude injection one is more effective due to the SAI-induced cooling occurring preferentially at higher latitudes. [ABSTRACT FROM AUTHOR]

Stratospheric aerosol injection (SAI) comes with a wide range of possible design choices, such as the location and timing of the injection. Different injection strategies can yield different climate responses; therefore, making informed future decisions on SAI requires an understanding of the range of possible climate outcomes. Yet to date, there has been no systematic exploration of a comprehensive set of SAI strategies. This limits the ability to determine which effects are robust across different strategies and which depend on specific injection choices, or to determine if there are underlying trade-offs between different climate goals. This study systematically explores how the choice of SAI strategy affects climate responses. Here, we introduce four hemispherically-symmetric injection strategies, all of which are designed to maintain the same global mean surface temperature: an annual injection at the equator (EQ), an annual injection of equal amounts of SO2 at 15°N and 15° S (15N+15S), an annual injection of equal amounts of SO2 at 30°N and 30° S (30N+30S), and a polar injection strategy that injects equal amounts of SO2 at 60°N and 60° S only during spring in each hemisphere (60N+60S). We compare these four hemisphericallysymmetric SAI strategies with a more complex injection strategy that injects different quantities of SO2 at 30° N, 15° N, 15° S, and 30° S in order to maintain not only the global mean surface temperature but also its large scale horizontal gradients. We find that the choice of SAI strategy notably affects the spatial distribution of aerosol optical depths, injection efficiency, and various surface climate responses. Among other findings, we show that injecting in subtropics produces more global cooling per unit injection, with the EQ and the 60N+60S cases requiring, respectively, 59 % and 50 % more injection than the 30N+30S case to meet the same global mean temperature target. Injecting at higher latitudes results in larger equator-to-pole temperature gradients. While all five strategies restore September Arctic sea ice, the high-latitude injection one is more effective due to the SAI-induced cooling occurring preferentially at higher latitudes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Modeling experiments reducing surface temperatures via an idealized reduction of the solar constant have often been used as analogs for Stratospheric Aerosol Injection (SAI), thereby implicitly assuming that solar dimming captures the essential physical mechanism through which SAI influences surface climate. While the omission of some important processes that otherwise operate under SAI was identified before, here we demonstrate that the imposed reduction in the incoming solar radiation also induces a different stratospheric dynamical response, manifested through a weakening of the polar vortex, that propagates from the upper stratosphere down to the troposphere. The coupled stratospheric‐tropospheric response exerts a previously overlooked first‐order influence on southern hemispheric surface climate in the solar dimming experiments, including on the position of the tropospheric jet and Hadley Circulation and thus, ultimately, precipitation patterns. This perturbation, opposite to that expected under SAI, highlights the need for caution when attributing responses in idealized experiments. Plain Language Summary: Adverse impacts of climate change have pushed research on temperature reduction strategies like geoengineering into the spotlight. Possible impacts of geoengineering have long been assessed using "solar dimming" climate model experiments, which reduce surface temperatures via an idealized reduction of the incoming solar radiation as an analogue for injecting sulfate aerosols into the lower stratosphere (Stratospheric Aerosol Injection, SAI). We demonstrate that solar dimming induces previously overlooked circulation changes in the stratosphere that propagate down to the troposphere and exert a first‐order influence on the Southern Hemisphere surface climate. This allows for an erroneous attribution of the simulated surface responses to the impacts of SAI geoengineering. The results also emphasize the importance of the stratosphere as an active contributor to the Southern Hemisphere near‐surface climate change, and have implications for paleoclimate studies. Key Points: Solar dimming induces previously overlooked stratospheric dynamical responseThe coupled stratospheric‐tropospheric response exerts a first‐order influence on the Southern Hemisphere (SH) surface climate in the solar dimming experimentsThe results emphasize the importance of the stratosphere, as well as the troposphere, as an active contributor to the SH climate change [ABSTRACT FROM AUTHOR]

Sulfate geoengineering (SG) methods based on lower stratospheric tropical injection of sulfur dioxide (SO2) have been widely discussed in recent years, focusing on the direct and indirect effects they would have on the climate system. Here a potential alternative method is discussed, where sulfur emissions are located at the surface or in the troposphere in the form of carbonyl sulfide (COS) gas. There are two time-dependent chemistry–climate model experiments designed from the years 2021 to 2055, assuming a 40 Tg-Syr-1 artificial global flux of COS, which is geographically distributed following the present-day anthropogenic COS surface emissions (SG-COS-SRF) or a 6 Tg-Syr-1 injection of COS in the tropical upper troposphere (SG-COS-TTL). The budget of COS and sulfur species is discussed, as are the effects of both SG-COS strategies on the stratospheric sulfate aerosol optical depth (∼Δτ=0.080 in the years 2046–2055), aerosol effective radius (0.46 µm), surface SOx deposition (+ 8.9 % for SG-COS-SRF; + 3.3 % for SG-COS-TTL), and tropopause radiative forcing (RF; ∼-1.5 Wm-2 in all-sky conditions in both SG-COS experiments). Indirect effects on ozone, methane and stratospheric water vapour are also considered, along with the COS direct contribution. According to our model results, the resulting net RF is - 1.3 Wm-2 , for SG-COS-SRF, and - 1.5 Wm-2 , for SG-COS-TTL, and it is comparable to the corresponding RF of - 1.7 Wm-2 obtained with a sustained injection of 4 Tg-Syr-1 in the tropical lower stratosphere in the form of SO2 (SG-SO2, which is able to produce a comparable increase of the sulfate aerosol optical depth). Significant changes in the stratospheric ozone response are found in both SG-COS experiments with respect to SG-SO2 (∼5 DU versus + 1.4 DU globally). According to the model results, the resulting ultraviolet B (UVB) perturbation at the surface accounts for - 4.3 % as a global and annual average (versus - 2.4 % in the SG-SO2 case), with a springtime Antarctic decrease of - 2.7 % (versus a + 5.8 % increase in the SG-SO2 experiment). Overall, we find that an increase in COS emissions may be feasible and produce a more latitudinally uniform forcing without the need for the deployment of stratospheric aircraft. However, our assumption that the rate of COS uptake by soils and plants does not vary with increasing COS concentrations will need to be investigated in future work, and more studies are needed on the prolonged exposure effects to higher COS values in humans and ecosystems. [ABSTRACT FROM AUTHOR]

As part of the Geoengineering Model Intercomparison Project a numerical experiment known as G6sulfur has been designed in which temperatures under a high-forcing future scenario (SSP5-8.5) are reduced to those under a medium-forcing scenario (SSP2-4.5) using the proposed geoengineering technique of stratospheric aerosol intervention (SAI). G6sulfur involves introducing sulfuric acid aerosol into the tropical stratosphere where it reflects incoming sunlight back to space, thus cooling the planet. Here, we compare the results from six Earth-system models that have performed the G6sulfur experiment and examine how SAI affects two important modes of natural variability, the northern wintertime North Atlantic Oscillation (NAO) and the Quasi-Biennial Oscillation (QBO). Although all models show that SAI is successful in reducing global mean temperature as designed, they are also consistent in showing that it forces an increasingly positive phase of the NAO as the injection rate increases over the course of the 21st century, exacerbating precipitation reductions over parts of southern Europe compared with SSP5-8.5. In contrast to the robust result for the NAO, there is less consistency for the impact on the QBO, but the results nevertheless indicate a risk that equatorial SAI could cause the QBO to stall and become locked in a phase with permanent westerly winds in the lower stratosphere. [ABSTRACT FROM AUTHOR]

Stratospheric aerosol geoengineering focused on the Arctic could substantially reduce local and worldwide impacts of anthropogenic global warming. Because the Arctic receives little sunlight during the winter, stratospheric aerosols present in the winter at high latitudes have little impact on the climate, whereas stratospheric aerosols present during the summer achieve larger changes in radiative forcing. Injecting SO2 in the spring leads to peak aerosol optical depth (AOD) in the summer. We demonstrate that spring injection produces approximately twice as much summer AOD as year‐round injection and restores approximately twice as much September sea ice, resulting in less increase in stratospheric sulfur burden, stratospheric heating, and stratospheric ozone depletion per unit of sea ice restored. We also find that differences in AOD between different seasonal injection strategies are small compared to the difference between annual and spring injection. Plain Language Summary: Scattering small particles called aerosols into the sky, "geoengineering", could reflect a small amount of sunlight in order to combat global warming. Doing this near the Arctic could help stop sea ice from melting, which would help preserve the Arctic climate. Our study shows that, for Arctic geoengineering, scattering particles in the spring is most efficient because the particles will be present throughout the summer, and the Arctic receives the most sunlight in the summer. Therefore, spring Arctic geoengineering could accomplish the same goals as year‐round Arctic geoengineering, but with fewer particles and thus fewer negative side‐effects. Key Points: Arctic focused stratospheric aerosol injection (SAI) concentrated in spring produces peak aerosol optical depth (AOD) in summer and therefore reflects sunlight more efficiently than year round SAISpring injection at 60°N restores twice as much September sea ice per unit of injection as year round injection at the same latitudeLimiting Arctic injection to spring also results in less ozone loss and stratospheric heating per unit of September sea ice recovered [ABSTRACT FROM AUTHOR]

A new set of stratospheric aerosol geoengineering (SAG) model experiments has been performed with Community Earth System Model version 2 (CESM2) with the Whole Atmosphere Community Climate Model (WACCM6) that are based on the Coupled Model Intercomparison Project phase 6 (CMIP6) overshoot scenario (SSP5-34-OS) as a baseline scenario to limit global warming to 1.5 or 2.0 ∘ C above 1850–1900 conditions. The overshoot scenario allows us to applying a peak-shaving scenario that reduces the needed duration and amount of SAG application compared to a high forcing scenario. In addition, a feedback algorithm identifies the needed amount of sulfur dioxide injections in the stratosphere at four pre-defined latitudes, 30 ∘ N, 15 ∘ N, 15 ∘ S, and 30 ∘ S, to reach three surface temperature targets: global mean temperature, and interhemispheric and pole-to-Equator temperature gradients. These targets further help to reduce side effects, including overcooling in the tropics, warming of high latitudes, and large shifts in precipitation patterns. These experiments are therefore relevant for investigating the impacts on society and ecosystems. Comparisons to SAG simulations based on a high emission pathway baseline scenario (SSP5-85) are also performed to investigate the dependency of impacts using different injection amounts to offset surface warming by SAG. We find that changes from present-day conditions around 2020 in some variables depend strongly on the defined temperature target (1.5 ∘ C vs. 2.0 ∘ C). These include surface air temperature and related impacts, the Atlantic Meridional Overturning Circulation, which impacts ocean net primary productivity, and changes in ice sheet surface mass balance, which impacts sea level rise. Others, including global precipitation changes and the recovery of the Antarctic ozone hole, depend strongly on the amount of SAG application. Furthermore, land net primary productivity as well as ocean acidification depend mostly on the global atmospheric CO2 concentration and therefore the baseline scenario. Multi-model comparisons of experiments that include strong mitigation and carbon dioxide removal with some SAG application are proposed to assess the robustness of impacts on societies and ecosystems. [ABSTRACT FROM AUTHOR]

We propose new testbed model experiments for the Geoengineering Model Intercomparison Project (GeoMIP) that are designed to limit global warming to 1.5 °C or 2.0 °C above 1850-1900 conditions using stratospheric aerosol geoengineering (SAG). The new modeling experiments use the overshoot scenario defined in CMIP6 (SSP5-34-OS) as a baseline scenario and are designed to reduce side effects of SAG in reaching three temperature targets: global mean surface temperature, and inter-hemispheric and pole-to-equator surface temperature gradients. We further compare results to another SAG simulation using a high emission scenario (SSP5-85) as a baseline scenario in order to investigate the dependency of impacts using different injection amounts to offset different amounts of warming by SAG. The new testbed simulations are performed with the CESM2(WACCM6). We use a feedback algorithm that identifies the needed amount of sulfur dioxide injections in the stratosphere at four predefined latitudes, 30° N, 15° N, 15° S, and 30° S, to reach the three temperature targets. Here we analyze climate variables and quantities that matter for societal and ecosystem impacts. We find that changes from present day conditions (2015-2025) in some variables depend strongly on the defined temperature target (1.5 °C vs 2.0 °C). These include surface air temperature and related impacts, the Atlantic Meridional Overturning Circulation (AMOC), which impacts ocean net primary productivity, and changes in ice sheet surface mass balance, which impacts sea-level rise. Others, including global precipitation changes and the recovery of the Antarctic ozone hole, depend strongly on the amount of SAG application. Furthermore, land net primary productivity as well as ocean acidification depend mostly on the global atmospheric CO2 concentration and therefore the baseline scenario. Multi-model comparisons of the experiments proposed here would help identify consequences of scenarios that include strong mitigation, carbon dioxide removal with some SAG application, on societal impacts and ecosystems. [ABSTRACT FROM AUTHOR]

Geoengineering Model Intercomparison Project (GeoMIP) Tier 1 solar geoengineering experiments for CMIP6 only include pathways that are based on high greenhouse gas forcing pathways (SSP5-85 or 4xCO2). However, a scenario with a less extreme deployment of solar geoengineering would be in combination with ambitious mitigation efforts and large carbon dioxide removal (CDR) efforts in order to not exceed temperature levels proposed by the COP21 Paris agreement. A Tier 2 GeoMIP experiment has been proposed, which is based on the SSP5-34 overshoot scenario. This scenario describes an emission pathway following SSP5-85 until 2040 and then includes assumption of large negative CO2 emissions in addition to reductions in greenhouse gas emissions. In this scenario, surface temperatures reach values above 1.5 or 2°C for some time in the future before slowly coming back down. To cut off the peak warming and stay below specific temperature targets, solar geoengineering is applied for a limited time period. It is expected that applying geoengineering to an overshoot scenario is different than using a high greenhouse gas forcing scenario, including impacts on climate, precipitation, but also on chemistry, and stratospheric ozone, due to changes in land surface assumptions and the evolution greenhouse gas and other emissions. Here, we use the NCAR Whole Atmosphere Community Climate Model Version 6 (WACCM6) to simulate the overshoot scenario as the baseline. In a second simulation, we apply solar geoengineering using the same pathway and, in addition, increase the burden of stratospheric sulfate aerosols in a way that temperatures will not exceed 1.5 or 2°C warming above the per-industrial average. We present first results and discuss input datasets that would allow other modeling groups to perform this type of simulation. We further point to the need for designing more scenarios that will achieve critical temperature levels, while combining different mitigation options, including CDR and solar geoengineering. [ABSTRACT FROM AUTHOR]