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Abstract. The Geoengineering Model Intercomparison Project (GeoMIP) has proposed multiple model experiments during phases 5 and 6 of the Climate Model Intercomparison Project (CMIP), with the latest set of model experiments proposed in 2015. With phase 7 of CMIP in preparation and with multiple efforts ongoing to better explore the potential space of outcomes for different solar radiation modifications (SRMs) both in terms of deployment strategies and scenarios and in terms of potential impacts, the GeoMIP community has identified the need to propose and conduct a new experiment that could serve as a bridge between past iterations and future CMIP7 experiments. Here we report the details of such a proposed experiment, named G6-1.5K-SAI, to be conducted with the current generation of scenarios and models from CMIP6 and clarify the reasoning behind many of the new choices introduced. Namely, compared to the CMIP6 GeoMIP scenario G6sulfur, we decided on (1) an intermediate emission scenario as a baseline (the Shared Socioeconomic Pathway 2-4.5), (2) a start date set in the future that includes both considerations for the likelihood of exceeding 1.5 °C above preindustrial levels and some considerations for a likely start date for an SRM implementation, and (3) a deployment strategy for stratospheric aerosol injection that does not inject in the tropical pipe in order to obtain a more latitudinally uniform aerosol distribution. We also offer more details regarding the preferred experiment length and number of ensemble members and include potential options for second-tier experiments that some modeling groups might want to run. The specifics of the proposed experiment will further allow for a more direct comparison between results obtained from CMIP6 models and those obtained from future scenarios for CMIP7. 

Abstract. Solar climate intervention using stratospheric aerosol injection (SAI) has been proposed as a method which could offset some of the adverse effects of global warming. The Assessing Responses and Impacts of Solar climate intervention on the Earth system with Stratospheric Aerosol Injection (ARISE-SAI) set of simulations is based on a moderate-greenhouse-gas-emission scenario and employs injection of sulfur dioxide at four off-equatorial locations using a control algorithm which maintains the global-mean surface temperature at 1.5 K above pre-industrial conditions (ARISE-SAI-1.5), as well as the latitudinal gradient and inter-hemispheric difference in surface temperature. This is the first comparison between two models (CESM2 and UKESM1) applying the same multi-target SAI strategy. CESM2 is successful in reaching its temperature targets, but UKESM1 has considerable residual Arctic warming. This occurs because the pattern of temperature change in a climate with SAI is determined by both the structure of the climate forcing (mainly greenhouse gases and stratospheric aerosols) and the climate models' feedbacks, the latter of which favour a strong Arctic amplification of warming in UKESM1. Therefore, research constraining the level of future Arctic warming would also inform any hypothetical SAI deployment strategy which aims to maintain the inter-hemispheric and Equator-to-pole near-surface temperature differences. Furthermore, despite broad agreement in the precipitation response in the extratropics, precipitation changes over tropical land show important inter-model differences, even under greenhouse gas forcing only. In general, this ensemble comparison is the first step in comparing policy-relevant scenarios of SAI and will help in the design of an experimental protocol which both reduces some known negative side effects of SAI and is simple enough to encourage more climate models to participate. 

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. 

Abstract. As part of the Geoengineering Model IntercomparisonProject a numerical experiment known as G6sulfur has been designed in whichtemperatures under a high-forcing future scenario (SSP5-8.5) are reduced tothose under a medium-forcing scenario (SSP2-4.5) using the proposedgeoengineering technique of stratospheric aerosol intervention (SAI).G6sulfur involves introducing sulfuric acid aerosol into the tropicalstratosphere where it reflects incoming sunlight back to space, thus coolingthe planet. Here, we compare the results from six Earth-system models thathave performed the G6sulfur experiment and examine how SAI affects twoimportant modes of natural variability, the northern wintertime NorthAtlantic Oscillation (NAO) and the Quasi-Biennial Oscillation (QBO).Although all models show that SAI is successful in reducing global meantemperature as designed, they are also consistent in showing that it forcesan increasingly positive phase of the NAO as the injection rate increasesover the course of the 21st century, exacerbating precipitationreductions 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. The realization of the difficulty of limiting global-meantemperatures to within 1.5 or 2.0 ∘C abovepre-industrial levels stipulated by the 21st Conference of Parties inParis has led to increased interest in solar radiation management (SRM)techniques. Proposed SRM schemes aim to increase planetary albedo to reflectmore sunlight back to space and induce a cooling that acts to partiallyoffset global warming. Under the auspices of the Geoengineering ModelIntercomparison Project, we have performed model experiments whereby globaltemperature under the high-forcing SSP5-8.5 scenario is reduced to followthat of the medium-forcing SSP2-4.5 scenario. Two different mechanisms toachieve this are employed: the first via a reduction in the solar constant(experiment G6solar) and the second via modelling injections of sulfurdioxide (experiment G6sulfur) which forms sulfate aerosol in thestratosphere. Results from two state-of-the-art coupled Earth system models(UKESM1 and CESM2-WACCM6) both show an impact on the North AtlanticOscillation (NAO) in G6sulfur but not in G6solar. Both models show apersistent positive anomaly in the NAO during the Northern Hemisphere winterseason in G6sulfur, suggesting an increase in zonal flow and an increase inNorth Atlantic storm track activity impacting the Eurasian continent and leadingto high-latitude warming over Europe and Asia. These results are broadlyconsistent with previous findings which show similar impacts fromstratospheric volcanic aerosol on the NAO and emphasize that detailedmodelling of geoengineering processes is required if accurate impacts of SRMeffects are to be simulated. Differences remain between the two models inpredicting regional changes over the continental USA and Africa, suggestingthat more models need to perform such simulations before attempting to drawany conclusions regarding potential continental-scale climate change underSRM. 

Abstract. Solar geoengineering has been receiving increased attention in recent years as a potential temporary solution to offset global warming. One method of approximating global-scale solar geoengineering in climate models is via solar reduction experiments. Two generations of models in the Geoengineering Model Intercomparison Project (GeoMIP) have now simulated offsetting a quadrupling of the CO2 concentration with solar reduction. This simulation is idealized and designed to elicit large responses in the models. Here, we show that energetics, temperature, and hydrological cycle changes in this experiment are statistically indistinguishable between the two ensembles. Of the variables analyzed here, the only major differences involve highly parameterized and uncertain processes, such as cloud forcing or terrestrial net primary productivity. We conclude that despite numerous structural differences and uncertainties in models over the past two generations of models, including an increase in climate sensitivity in the latest generation of models, the models are consistent in their aggregate climate response to global solar dimming.