Africa's rapid development, coupled with population growth and urbanization, necessitates a balanced approach to sustainable development and climate change mitigation. While the African Union's Agenda 2063 envisions sustainable livelihoods and clean environments, the perception is that local mitigation efforts have minimal impact on the African climate due to its relatively low greenhouse gas (GHG) emissions (less than 4% of global CO₂ emissions from fossil fuels and industry). The presence of cooling aerosols, masking some GHG warming, complicates the issue. Reducing these aerosols, whether for air quality or climate change mitigation, can lead to a 'climate penalty'—near-term warming as the masking effect diminishes. This penalty has been observed in regions like the US, Europe, and China. However, the existing literature predominantly focuses on the temperature aspect of this penalty, neglecting the significant influence aerosols have on precipitation patterns. While the impacts of aerosol changes in Europe, North America, and Asia have been extensively studied, research on the role of African aerosols is limited. Given Africa's large aerosol emissions relative to its GHG emissions, understanding the local climate impacts of reduced African aerosol emissions is crucial. This study addresses this gap by using climate modeling to investigate the impact of emission changes, particularly on African precipitation.

Numerous studies have examined the 'climate penalty' associated with aerosol reductions, highlighting near-term warming resulting from unmasked GHG warming. The IPCC reports also emphasize this penalty. However, most research overlooks the influence of aerosols on precipitation. While studies on aerosol impacts in other regions like Europe, North America, and Asia are abundant, there's a paucity of research focusing on African aerosols. Previous studies have highlighted the influence of remote aerosol changes on Sahel rainfall, particularly concerning historical droughts. However, the role of future local African aerosol impacts remains unclear. This study builds upon the Integrated Assessment of Air Pollution and Climate Change for Sustainable Development in Africa, examining the health, crop, and climate benefits of the Agenda 2063 scenario, focusing on the additional modeling experiments that isolate the effects of reduced cooling aerosols.

The study utilizes the NASA Goddard Institute for Space Studies (GISS) global climate model to simulate climate change scenarios. The simulations use an Africa-wide model with national-scale resolution constructed within the Low Emissions Analysis Platform (LEAP) to project emissions under different scenarios. Three key scenarios were examined: a baseline 'business-as-usual' scenario; a scenario representing the decarbonization and sustainable development goals of the African Union's Agenda 2063 plan; and a 'No Decarbonization Aerosol Reductions' (NDAR) scenario, which isolates the effect of aerosol reductions from decarbonization. Outside of Africa, emissions follow the Shared Socio-economic Pathway 3 (SSP3_7.0), a high-emissions scenario. The model includes both internally generated climate-sensitive and prescribed emissions. Ten simulations were run for each scenario from 2015 to 2064, starting from different initial conditions. The analysis focused on changes in surface air temperature and precipitation, comparing the different scenarios. The GISS-E2.1-G model, used in CMIP6, was employed. This model includes representations of various aerosols and their impact on cloud albedo. The monthly output of the simulations, including temperature and precipitation data, was used for analysis. The study specifically looks at the differences in precipitation and temperature between the Agenda 2063 and the NDAR scenarios to determine the contribution of aerosol reduction to precipitation changes.

The study reveals weak impacts of African emissions cuts on projected temperatures, with only modest warming under the Agenda 2063 scenario compared to the baseline. However, the impact on precipitation is substantial. The baseline scenario projects significant drying in tropical Northern Hemisphere Africa during summer (June-August). The Agenda 2063 scenario largely eliminates this projected drying, even leading to a modest increase in precipitation. The analysis shows that reduced cooling aerosols contribute significantly (approximately 33-90%) to this avoided drying. The NDAR scenario, separating out the effects of decarbonization aerosol reductions, shows that these reductions are responsible for roughly half of the overall precipitation increase and almost all of the avoided drying in West Africa. Aerosol optical depth (AOD) changes significantly across scenarios, with substantial reductions under Agenda 2063 and NDAR, primarily driven by decreased carbonaceous aerosols. These reductions lead to decreased atmospheric heating, increased atmospheric stability, and enhanced convective precipitation. While changes in scattering aerosols and cloud cover also play a role, the impact of decreased absorbing aerosols is more significant. Changes in the Atlantic meridional temperature gradient were also observed under the Agenda 2063 scenario but appear secondary to the local aerosol effects, except perhaps in far West Africa. This suggests that both local and remote aerosol mechanisms contribute to the observed precipitation changes. The magnitude of the precipitation changes simulated here could be at the higher end of possible outcomes due to the high baseline SO2 emissions in the model and should be considered with some caution.

The findings challenge the traditional 'climate penalty' paradigm, which focuses solely on temperature changes associated with aerosol reductions. The study highlights that reducing aerosol emissions, even if accompanied by modest near-term warming, offers substantial benefits by mitigating projected rainfall decreases. The avoidance of drying of a magnitude comparable to the 1970s and 1980s droughts is particularly significant. This result suggests that local aerosol effects on precipitation dominate over large portions of Africa. While some remote effects from changes in the Atlantic meridional temperature gradients may also play a role, especially in far West Africa, the impact is primarily driven by local aerosol changes. These findings have strong implications for African policy, suggesting that focusing solely on temperature changes as a metric for evaluating mitigation policies may be overly simplistic and potentially misleading. The substantial, potential benefits associated with increased precipitation and reduced drying outweigh the modest temperature increases associated with the near-term warming often associated with the climate penalty.

This study demonstrates the substantial influence of African aerosol emission reductions on projected rainfall patterns, challenging the simplistic view of the 'climate penalty'. Reducing aerosols is crucial for avoiding significant summer drying across much of the continent. This necessitates a more comprehensive approach to policy-making that considers the multifaceted effects of emission reductions, including both temperature and precipitation changes, alongside public health benefits. Future research could benefit from exploring this effect using multiple climate models to improve the robustness of the findings. The potential benefits warrant further quantitative investigation.

The study's conclusions are based on the GISS climate model. While this model realistically captures observed meteorological quantities, atmospheric composition, and associated trends, using multiple climate models would strengthen the robustness of the findings. The simulated precipitation response might be toward the upper end due to the comparatively high baseline SO2 emissions in the model, emphasizing the need for caution in interpreting the quantitative results. Model-to-model variability in precipitation response to regional aerosol changes needs further attention.