the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Nov 2025
G6-1.5K-SAI and G6sulfur: changes in impacts and uncertainty depending on stratospheric aerosol injection strategy in the Geoengineering Model Intercomparison Project
Abstract. We report initial results for G6-1.5K-SAI, a climate model experiment proposed by the Geoengineering Model Intercomparison Project (GeoMIP). G6-1.5K-SAI, which simulates a stratospheric aerosol injection (SAI) to limit global warming to ∼ 1.5 °C above preindustrial in each model, features several design updates relative to previous GeoMIP experiment G6sulfur, such as hemispherically symmetric subtropical injection (30° N and 30° S) instead of equatorial injection. Due to differences in climate sensitivity, models disagree on the amount of warming to be offset, and therefore on the total injection required. While they agree strongly on the rate of cooling per unit rate of injection (∼ 0.1 °C per Tg SO2 yr−1, a similar value to G6sulfur models with interactive SO2), similarities in aerosol representation and disagreements in aerosol optical depth (AOD) per rate of unit injection and in rate of cooling per unit AOD mean this agreement may not imply accuracy. In all participating models, SAI cools the land surface more than the ocean and offsets mid- and high-latitude precipitation increases under global warming, but models disagree on the magnitude of residual Arctic amplification and changes to tropical precipitation. Relative to G6sulfur, G6-1.5K-SAI cools the Arctic more strongly, and also decreases precipitation less, especially in the tropics and over land. All in all, while the new G6-1.5K-SAI experiment constitutes an update over the older G6sulfur, due to the differences in scenario across these two experiments, any differences in SAI impacts must be evaluated carefully.
Competing interests: EMB is a member of the editorial board of ACP.
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Status: open (until 19 Feb 2026)
- RC1: 'Comment on egusphere-2025-5742', Anonymous Referee #1, 16 Jan 2026 reply
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Data for "G6-1.5K-SAI and G6sulfur: changes in impacts and uncertainty depending on stratospheric aerosol injection strategy in the Geoengineering Model Intercomparison Project" Walker Raymond Lee et al. https://doi.org/10.5281/zenodo.17613419
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- 1
This paper presents initial results from four different Earth system models, comparing the updated G6-1.5K-SAI strategy against the previous GeoMIP experiment, G6sulfur. The findings indicate that while all models successfully meet the target global mean temperature (2020-2039 SSP2-4.5 average), they show significant variations in injection efficiency, aerosol distribution, and regional impacts on precipitation and Arctic temperatures.
Overall this paper is very well written, thorough and explains each step very clearly. In particular I would like to commend the authors for their detailed explanations of previous works and the presentation and explanation of figures 5 onwards. The description of the three time periods and how they relate within each figure, especially compared to the G6sulfur results, is a great way for readers to understand the differences in results.
I only have a few very minor comments, otherwise I recommend this study for publication.
Section 2.1 - I think it would be helpful to have the 2020-2039 average temperature relative to pre-industrial for each model and/or the difference in warming by 2065-2084. This could also be well placed in section 3.1.
Table 1 - The UKESM atmospheric resolution is 1.25° lat vs 1.875° lon. I also had a question regarding the MIROC and E3SM atm. resolution - are they the same for both lat and lon?
Figure 2 - (c) is difficult to see all the ensemble means here, is it possible to have all the ensemble means on top of the individual ensemble members? Or reduce the alpha of the individual members?
Line 213 - “is the only G6-1.5K-SAI model to show…”
Line 235 - “In all four models,...” the authors should clarify that the first half of this sentence refers to the warming scenario to make it more clear for readers.
Figure 6 - It would be useful to include the G6sulfur models in the figure caption, similar to Figure 2, it would be useful for readers to easily find which models have been included.
Lines 302-303 - It would be worth naming the warming scenarios for G6sulfur and G6-1.5K-SAI in these sentences.