Identifying the sources of uncertainty in climate model simulations of solar radiation modification with the G6sulfur and G6solar Geoengineering Model Intercomparison Project (GeoMIP) simulations

Abstract

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.

The impact of stratospheric aerosol intervention on the North Atlantic and Quasi-Biennial Oscillations in the Geoengineering Model Intercomparison Project (GeoMIP) G6sulfur experiment

“…In GeoMIP experiment G6sulfur, simulated global-mean temperature in a high-forcing scenario is reduced to the level of a medium-forcing scenario by the deployment of SAI geoengineering. The impacts on geographic temperature and precipitation distributions in these simulations have been shown to differ significantly from simulations which achieve the same global-mean temperature goal simply by reducing the solar constant (Jones et al, 2021;Visioni et al, 2021).…”

“…We examine the impact of SAI in the six models which have performed the G6sulfur simulations to date (Table 1); more information can be found in Visioni et al (2021) and references therein. Kravitz et al (2015) were not prescriptive about how SAI should be implemented in the models as the details depend on each model's capabilities, resulting in different approaches that can be grouped into two basic categories.…”

Interactive Stratospheric Aerosol models response to different amount and altitude of SO2 injections during the 1991 Pinatubo eruption

“…The amount of SO 2 required to achieve the proposed cooling varies by a factor of 2 between models and results in a different temporal and latitudinal distribution of aerosols that affects surface temperature and local precipitation differently (Visioni et al, 2021).…”

PInc-PanTher estimates of Arctic permafrost soil carbon under the GeoMIP G6solar and G6sulfur experiments

“…scenarios such as G4, since the amount of SG required to meet the goal varies over time and is dependent on ESM climate and sulfate aerosol sensitivity. The implementation of the G6sulfur experiment varies with the sophistication of the ESM atmospheric physical-chemistry component, ranging from calculations dependent on SO2 emissions to prescribed aerosol optical depth distributions (Visioni et al, 2021). We use the outputs of the six CMIP6 ESMs (Table 1) to drive a dataconstrained, process-based permafrost carbon model and compare the results it gives to the soil carbon changes calculated directly within each of the ESMs.…”

“…The attributes of ESMs listed here are relevant to the soil temperatures (TSL), net primary productivity (NPP) and gross primary productivity (GPP) outputs, which are used to drive a mechanistic soil carbon model for simulating permafrost carbon dynamics. G6sulfur aerosols indicates use of prescribed aerosol optical depth (AOD) or internally calculated from injected SO2 (Visioni et al, 2021). (Burke et al, 2020;Mudryk et al, 2020;Fox-Kemper et al, 2021).…”