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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. We present here results from the Geoengineering Model Intercomparison Project (GeoMIP) simulations for the experiments G6sulfur and G6solar for six Earth system models participating in the Climate Model Intercomparison Project (CMIP) Phase 6. The aim of the experiments is to reduce the warming that results from a high-tier emission scenario (Shared Socioeconomic Pathways SSP5-8.5) to that resulting from a medium-tier emission scenario (SSP2-4.5). These simulations aim to analyze the response of climate models to a reduction in incoming surface radiation as a means to reduce global surface temperatures, and they do so either by simulating a stratospheric sulfate aerosol layer or, in a more idealized way, through a uniform reduction in the solar constant in the model. We find that over the final two decades of this century there are considerable inter-model spreads in the needed injection amounts of sulfate (29 ± 9 Tg-SO2/yr between 2081 and 2100), in the latitudinal distribution of the aerosol cloud and in the stratospheric temperature changes resulting from the added aerosol layer. Even in the simpler G6solar experiment, there is a spread in the needed solar dimming to achieve the same global temperature target (1.91 ± 0.44 %). The analyzed models already show significant differences in the response to the increasing CO2 concentrations for global mean temperatures and global mean precipitation (2.05 K ± 0.42 K and 2.28 ± 0.80 %, respectively, for SSP5-8.5 minus SSP2-4.5 averaged over 2081–2100). With aerosol injection, the differences in how the aerosols spread further change some of the underlying uncertainties, such as the global mean precipitation response (−3.79 ± 0.76 % for G6sulfur compared to −2.07 ± 0.40 % for G6solar against SSP2-4.5 between 2081 and 2100). These differences in the behavior of the aerosols also result in a larger uncertainty in the regional surface temperature response among models in the case of the G6sulfur simulations, suggesting the need to devise various, more specific experiments to single out and resolve particular sources of uncertainty. The spread in the modeled response suggests that a degree of caution is necessary when using these results for assessing specific impacts of geoengineering in various aspects of the Earth system. However, all models agree that compared to a scenario with unmitigated warming, stratospheric aerosol geoengineering has the potential to both globally and locally reduce the increase in surface temperatures. 

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. 

Abstract. Mineral dust is the most abundant aerosol species by massin the atmosphere, and it impacts global climate, biogeochemistry, and humanhealth. Understanding these varied impacts on the Earth system requiresaccurate knowledge of dust abundance, size, and optical properties, and howthey vary in space and time. However, current global models show substantialbiases against measurements of these dust properties. For instance, recentstudies suggest that atmospheric dust is substantially coarser and moreaspherical than accounted for in models, leading to persistent biases inmodelled impacts of dust on the Earth system. Here, we facilitate moreaccurate constraints on dust impacts by developing a new dataset: DustConstraints from joint Observational-Modelling-experiMental analysis(DustCOMM). This dataset combines an ensemble of global model simulationswith observational and experimental constraints on dust size distributionand shape to obtain more accurate constraints on three-dimensional (3-D)atmospheric dust properties than is possible from global model simulationsalone. Specifically, we present annual and seasonal climatologies of the 3-Ddust size distribution, 3-D dust mass extinction efficiency at 550 nm, andtwo-dimensional (2-D) atmospheric dust loading. Comparisons with independentmeasurements taken over several locations, heights, and seasons show thatDustCOMM estimates consistently outperform conventional global modelsimulations. In particular, DustCOMM achieves a substantial reduction in thebias relative to measured dust size distributions in the 0.5–20 µmdiameter range. Furthermore, DustCOMM reproduces measurements of dust massextinction efficiency to almost within the experimental uncertainties,whereas global models generally overestimate the mass extinction efficiency.DustCOMM thus provides more accurate constraints on 3-D dust properties, andas such can be used to improve global models or serve as an alternative toglobal model simulations in constraining dust impacts on the Earth system.