Climate models show significant uncertainty in projecting the effects of human activity on Sahel precipitation, with varying predictions of increases or decreases by the end of the 21st century. This uncertainty stems from differences in how atmospheric circulation models simulate responses to greenhouse gas (GHG) concentrations and uncertain changes in Sahel sea surface temperatures (SST), particularly regarding the differential warming of the northern hemisphere and its impact on the Sahel. Despite quantitative uncertainty, a consensus shows that summer precipitation related to the West African Monsoon is projected to increase in the central Sahel and decrease in the western Sahel. This zonal contrast intensifies with warming and is consistently reproduced across multiple climate models and emission scenarios. This pattern significantly affects local communities, impacting agriculture, precipitation extremes, and monsoon onset and withdrawal dates. The zonal contrast is attributed to two competing mechanisms: (i) the direct atmospheric effect of GHG increases and (ii) an indirect response mediated by the ocean through SST changes. Atmospheric warming strengthens low-level moisture flux, shifting the monsoon northward and increasing precipitation. However, these responses occur on different timescales, overlapping in transient emissions and obscuring their individual contributions to Sahel precipitation's future evolution. The study aims to unravel the timescale decomposition and drivers of Sahel precipitation changes, highlighting sources of uncertainty and exploring opportunities for effective mitigation action.

The introduction extensively reviews existing literature on climate change impacts on Sahel precipitation. It highlights the uncertainty in CMIP5 and CMIP6 model projections, attributing it to differences in atmospheric circulation model responses to GHGs and uncertainties in SST changes, particularly the differential warming of the northern hemisphere. The literature review summarizes the consensus on a future zonal precipitation contrast – increased rainfall in the central Sahel and decreased rainfall in the western Sahel – emphasizing the consistent findings across various models and emission scenarios. Existing research suggests two competing mechanisms responsible for this contrast: the direct atmospheric effect of GHG increases and the indirect ocean-mediated response via SST changes. The review sets the stage for the study by noting the lack of focus on the different timescales of these responses and their implications for mitigation strategies.

The study employs a timescale decomposition approach to analyze Sahel precipitation changes. Using idealized simulations (abrupt4xCO2 simulations) where CO2 concentration is abruptly quadrupled, the researchers define a "fast response" as the change occurring within the first 5-10 years after the CO2 quadrupling. This response is linked to anomalously strong inter-hemispheric temperature gradients and strong warming over northern Africa, leading to a northward monsoon shift and increased precipitation. A "slow response" is defined as the change occurring between years 5-10 and 120-140, associated with changes in ocean circulation, impacting the Pacific Ocean and the Atlantic Multidecadal Variability, resulting in decreased Sahel precipitation and a southward monsoon shift. The robustness of these patterns is verified using CMIP6 idealized atmosphere-only simulations (AMIP). The fast response is largely attributed to land warming, while the slow response is linked to ocean warming. To assess the combined contributions, the authors regress the fast and slow responses onto temperature anomalies under historical and SSP585 emission scenarios. This allows estimating the contributions of each response to global mean surface temperature (GMST) change and then scaling precipitation anomalies to reconstruct precipitation changes under different scenarios. A mitigation scenario (SSP534-over) is analyzed to assess the impact of reducing GHG emissions. The method also addresses uncertainties by considering internal climate variability and inter-model variance in fast and slow responses, utilizing a 21-year running mean to filter high-frequency variability. AMIP experiments further isolate the effects of CO2 forcing and uniform SST warming to clarify the role of land vs. ocean warming in the fast and slow responses. Abrupt4xCO2 experiments help define the fast and slow response timescales, analyzing the first 150 years of simulations. Historical and SSP585 simulations assess the effects of climate change on Sahel precipitation under varying emission scenarios. SSP534-over experiments model an aggressive mitigation strategy.

The study finds that the fast response to climate change dominates initial precipitation changes, particularly over the central Sahel. This response, linked to land warming and an enhanced inter-hemispheric temperature gradient, leads to increased rainfall. The slow response, associated with ocean warming and changes in ocean circulation, causes a decrease in rainfall, primarily affecting the western Sahel. The combination of these opposing responses creates the zonal contrast in precipitation change. Analyzing the SSP585 scenario, the fast response significantly contributes to the increase in central Sahel precipitation, while the slow response contributes to the decrease in western Sahel precipitation. A reconstruction method accurately reproduces the observed precipitation changes under SSP585 for the central Sahel. The study demonstrates that a sharp reduction in GHG emissions (as in the SSP534-over scenario) would rapidly reduce the impact of the fast response on central Sahel precipitation. However, the slow response would persist, and effects on western Sahel precipitation would be less pronounced in the short term. The analysis of uncertainty highlights that the primary source of uncertainty in precipitation change originates from the model-dependent fast response. The study successfully separates uncertainties related to the fast and slow responses, improving confidence in climate projections. A mitigation strategy leads to a significant reduction in climate change's impact on central Sahel precipitation, with a 30-year delay after the reduction of CO2 emission. The effectiveness is rationalized by the rapid development of the fast response and its primary influence on central Sahel precipitation.

The findings directly address the research question by demonstrating the dominance of the fast response in driving initial precipitation changes in the central Sahel and the opposing influence of the slow response in the western Sahel, explaining the observed zonal contrast. The results emphasize the potential effectiveness of mitigation strategies in rapidly reducing the impact of climate change on Sahel precipitation, specifically by targeting the fast response. This has significant implications for adaptation and mitigation efforts, offering a time-bound perspective on achieving tangible results. The analysis of uncertainties enhances confidence in climate projections by identifying the source of uncertainty as the fast response, which is model-dependent. The study's contribution lies in its novel timescale decomposition approach, offering a refined understanding of the complexities of climate change impacts on the Sahel and suggesting actionable strategies. The results underscore the urgency for concerted international action to mitigate climate change, emphasizing the achievable impact within a predictable timeframe.

This study demonstrates the significant role of the fast response to climate change in shaping Sahel precipitation, particularly in the central Sahel. The authors show that mitigation strategies focusing on reducing GHG emissions can effectively mitigate climate change's impact on this region within a few decades. A major contribution is the separation of uncertainties based on timescales, thereby increasing confidence in climate projections. Future research could investigate the detailed mechanisms of the fast and slow responses and further explore the regional variations in climate change impacts.

The study's methodology relies on the assumption that Sahel precipitation changes directly scale with changes in global mean surface temperature. While the reconstruction method shows good results, it might overestimate or underestimate the magnitude of precipitation change in certain regions. The impact of anthropogenic aerosols is not explicitly accounted for, which might influence the results. The analysis mainly focuses on monthly mean precipitation, and the influence of mitigation on precipitation extremes needs further investigation. The number of models available for some scenarios (e.g., SSP534-over) is relatively small, potentially limiting the generalizability of the findings.