the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jun 2026
Temperature stabilization using Stratospheric Aerosol Injections minimises drying over Africa only when combined with a strong decarbonization effort
Abstract. Climate change poses severe risks to African agriculture, water resources, and ecosystems. Temperature overshoot scenarios, in which global warming temporarily exceeds target thresholds such as 1.5 or 2.0 °C before declining through mitigation and carbon removal later in the century, are plausible future trajectories. Yet, their regional impacts and the reversibility of changes during the overshoot remain poorly characterized. Stratospheric aerosol injection (SAI) has been proposed as a means to limit peak warming during overshoot; however, its effects on African climate extremes and water availability require careful assessment. This study analyses different CESM2-WACCM6 simulations to evaluate changes in temperature extremes, precipitation patterns, and surface moisture budget across Africa, using two baseline scenarios, the high GHG forcing scenario (SSP5-8.5) and the SSP5-3.4-OS overshoot scenario, which includes strong decarbonization and carbon removal efforts after 2040. In addition, three SAI intervention scenarios are assessed, targeting 1.5 and 2.0 °C (for the overshoot scenario, only) above pre-industrial levels. We compute selected ETCCDI-based climate indices, including Growing Degree Days, Warm Spell Duration Index, Consecutive Dry Days, and precipitation intensity metrics for baseline and overshoot (2060–2079) periods. Our results reveal near-universal, statistically significant changes (> 90 %) in temperature indices during overshoot, with 5–30 % increases depending on the metric. Precipitation indices exhibit more heterogeneous responses, with 40–80 % of the area showing significant changes. SAI interventions consistently reduce temperature-related indices across Africa, with the strongest cooling effects in tropical regions. However, precipitation responses to SAI display substantial spatial heterogeneity and scenario dependency: West Africa’s Sahel shows increased moisture availability under high SAI compared to SSP5-8.5, Central Africa exhibits mixed responses with regional drying in parts of the Congo Basin, and East Africa demonstrates a dipole pattern of coastal wetting and interior drying that intensifies at higher warming thresholds. All these changes are magnified under high-cooling scenarios (using the high forcing baseline) compared with cooling under overshoot, in which case many precipitation differences are reduced.
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RC1: 'Comment on egusphere-2026-3436', Anonymous Referee #1, 15 Jul 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3436/egusphere-2026-3436-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2026-3436-RC1 -
RC2: 'Comment on egusphere-2026-3436', Anonymous Referee #2, 08 Sep 2026
COMMENTS TO THE AUTHORS
Review of the manuscript entitled: “Temperature stabilization using Stratospheric Aerosol Injections minimises drying over Africa only when combined with a strong decarbonization effort” by Egbebiyi et al.
This study investigates the potential impacts of stratospheric aerosol injection (SAI) on temperature and precipitation-related agroclimatic indices as well as surface moisture availability over Africa. The authors use CESM2-WACCM6 simulations under SSP5-8.5 and SSP5-3.4-OS, together with SAI experiments targeting 1.5°C and 2.0°C warming levels. The study shows that temperature-related indices exhibit widespread and statistically significant changes under warming and that SAI substantially reduces these changes. In contrast, precipitation and surface moisture responses are more heterogeneous and strongly dependent on the region, scenario and SAI target. The study further highlights that the hydrological effects of SAI are generally smaller under the overshoot scenario associated with strong mitigation and carbon dioxide removal (CDR).
The topic is timely and highly relevant, particularly given the growing interest in understanding the regional implications of solar radiation modification over Africa. The combination of agroclimatic indices and surface moisture budget provides an interesting perspective that goes beyond temperature changes alone. The manuscript is generally well structured and presents potentially useful results for understanding the risks and benefits associated with SAI over the African continent.
However, several aspects of the manuscript require clarification and improvement before publication.
Below are some comments that may be beneficial for consideration:
General comments
1- The distinction between the SSP5-8.5 and SSP5-3.4-OS baselines and the different SAI scenarios is central to the manuscript. However, the comparison between the two pathways could be developed more systematically throughout the Results, Discussion and Conclusion sections. In particular, the manuscript should more explicitly highlight how and why the response to SAI differs between a sustained high-emission pathway and an overshoot pathway associated with strong mitigation and CDR.
2- The manuscript relies on a single Earth system model (CESM2-WACCM6). Although this limitation is acknowledged in the Discussion, I suggest that its implications be emphasized more clearly when interpreting the regional precipitation and moisture-budget responses. Such responses can be strongly model-dependent, and the conclusions should therefore be framed accordingly.
3- The statistical analysis deserves further clarification. The authors use a Welch's t-test with only three ensemble members for each scenario and consider p < 0.05 as statistically significant. Given the very small number of ensemble members, the authors should provide additional justification for the robustness and appropriateness of this statistical approach and discuss its implications for the reported significance levels.
4- The manuscript would benefit from a clearer distinction between statistical significance and magnitude of change. In several places, the reduction in the percentage of statistically significant grid cells under SAI is interpreted as evidence that SAI “effectively returns temperature conditions closer to the baseline state.” Statistical significance alone does not necessarily quantify the magnitude or practical importance of the change. Consider reporting and discussing the actual magnitude of changes alongside their statistical significance.
5- The title emphasizes that SAI minimizes drying only when combined with strong decarbonization. This is an interesting and potentially important conclusion, but the manuscript should make this central message more explicit and quantitatively demonstrate it throughout the Results and Discussion sections.
6- The authors should say a few more words about the uncertainties and limitations associated with the SAI experimental design itself, including the use of idealized feedback-controlled injection scenarios and prescribed global temperature targets. These simulations should not necessarily be interpreted as representing a realistic future deployment strategy.
Specific comments
Line 1: In the title, please consider whether “minimises drying” is fully supported by the results across the whole African continent. The results show important regional contrasts, including reduced moisture availability under some SAI scenarios in East and Southern Africa. The title may therefore need to better reflect the regional and scenario-dependent nature of the response.
Lines 24–32: The abstract is generally informative, but the main quantitative findings could be presented more consistently. For example, the abstract reports that precipitation indices exhibit significant changes over 40–80% of the area, whereas the Results section gives substantially lower percentages for several precipitation indices. Please check that all percentages reported in the abstract are consistent with the results presented in the main text and figures.
Line 41: All acronyms must be clearly defined from the first occurrence they are used. Therefore, please apply this instruction to "CMIP6" and all other acronyms used throughout the text
Lines 65-66: Please clarify the terminology used throughout the manuscript between “stratospheric aerosol injection (SAI)”, “stratospheric aerosol geoengineering (SAG)”, and “solar radiation modification (SRM)”. The manuscript uses these expressions somewhat interchangeably. A consistent terminology would improve readability.
Lines 111–114: There appears to be an inconsistency between the description of the regionalization in the text and the caption of Figure 1. The text mentions five sub-regions (North, West, Central, East and Southern Africa), whereas the Figure 1 caption additionally mentions Northeast Africa (NEAF). Please clarify the regional classification and apply the same terminology consistently throughout the manuscript.
Line 138: Please ensure consistency in the notation of the overshoot scenario. The manuscript uses SSP5-3.4-OS and SSP5-34-OS in different places. The scenario name should be standardized throughout the text, tables and figures.
Lines 138–157: The experimental design should be described more clearly.
Lines 165: Please clarify the citation and terminology associated with the ETCCDI framework. The text currently states “the Expert Team on Climate Change Detection and Indices (ETCCDI) Peterson (2002)”. Please check the corresponding reference and citation year, and revise the sentence.
Lines 199–208: The statistical methodology needs further explanation. It would be useful to specify exactly what constitutes the samples in the Welch's t-test: are the tests performed using annual values, ensemble-member means, or period means? Given that only three ensemble members are available, this point is particularly important for assessing the robustness of the significance analysis.
Line 214: Please revise the sentence “these indices plateau begin to decline” for grammatical clarity.
Lines 284–287: The physical mechanisms invoked to explain the precipitation response to SAI are interesting. However, the discussion could be strengthened by distinguishing more clearly between the mechanisms supported directly by the simulations and mechanisms inferred from previous studies.
Lines 295–305: The interpretation of the surface moisture budget deserves some caution. In particular, the statement that SAI “could help preserve surface water availability” in West Africa should be framed carefully because the analysis is based on P − ET, which represents a simplified surface moisture balance and does not directly quantify water availability in rivers, soils, reservoirs or groundwater.
Lines 314–329: This paragraph appears to substantially repeat the preceding description of Figure 5. Please consider removing the duplicated material and using this space to provide additional interpretation of the seasonal differences.
Lines 331–332: Please clarify the apparent contrast between the statement that East Africa is the greatest beneficiary, with an increase of approximately 0.5 mm/day, and the preceding results indicating that SAI produces lower moisture levels than the high-warming baseline in East Africa. The reference scenario used for each comparison should be explicitly stated.
Lines 396–398: The conclusion that excessive drying associated with SAI can be mitigated under a lower-emission scenario is one of the most important findings of the study. I suggest developing this point further and providing a more quantitative comparison of the hydrological response between the two emission pathways.
Lines 401–409: The limitations section is useful, but it could be expanded to include the idealized nature of the SAI experiments, the relatively coarse spatial resolution of the model for regional hydrological analysis, and the uncertainty associated with using a single model.
Tables
Table 1: The notation used for the SAI experiments should be standardized. In particular, please make clear whether “Geo SSP5-34-OS 1.5” and “Geo SSP5-34-OS 2.0” refer to temperature targets or experiments. It would also be useful to explicitly indicate which simulations are used as baselines and which are used as SAI experiments.
Table 2: Please carefully check the definitions and units of all indices. In particular, the units associated with R1MM, R20MM and PRCPTOT appear potentially inconsistent with their definitions. Please verify the table against the standard ETCCDI definitions.
Figures
Figure 2: Please clarify why only selected indices are presented in Figure 2 and explain whether the omitted indices exhibit similar temporal behaviour. This would help readers understand whether the selected indices are representative of the overall response.
Figure 3: The figure caption should clearly identify all scenarios represented in the figure. This is particularly important because the manuscript considers both 1.5°C and 2.0°C SAI targets, whereas the caption appears to focus only on the 1.5°C experiments. Please verify that the figure, caption and corresponding discussion are fully consistent.
Figure 4: The caption states that four scenarios are represented, but the manuscript considers several SAI experiments, including a 2.0°C target. Please clarify which scenarios are included in this figure and why some experiments may have been excluded.
References
Please carefully check the References section for consistency, completeness and formatting. Several references appear to contain incomplete bibliographic information or inconsistencies in formatting.
In particular:
The reference to Kravitz et al. (2017) appears incomplete and should be checked carefully.
The reference to Mupangwa et al. (2016) should be checked against the statement for which it is cited in the Introduction section, particularly the claim concerning the contribution of agriculture to the Gross Domestic Product.
The reference to Ofori & Asongu (2021) appears to concern foreign direct investment, governance and inclusive growth rather than the specific agricultural statement for which it is cited. Please verify whether this is the appropriate reference.
Please check the consistency between the citation of Emmons et al. in the text and the year given in the reference list.
Similarly, please verify the citation year and bibliographic details of Danabasoglu et al. in the text and reference list.
Citation: https://doi.org/10.5194/egusphere-2026-3436-RC2
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