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.
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North Atlantic Oscillation response in GeoMIP experiments G6solar and G6sulfur: why detailed modelling is needed for understanding regional implications of solar radiation management
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.
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- NSF-PAR ID:
- 10222320
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 21
- Issue:
- 2
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 1287 to 1304
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-21-1287-2021
