Afforestation in West Africa: potential benefits and trade-offs on regional climate extremes

West Africa is currently experiencing extensive agricultural intensification associated with rapid population growth. Those anthropogenic land use and land-cover changes (LULCC) can have significant impacts at regional and seasonal scales but also for extreme weather events, posing high vulnerability to human, natural, and economic systems. However, the effects of LULCC on extreme events in West Africa remain largely unexplored at the regional scale, lacking consensus. Here, for the first time, we employ high-resolution LULCC experiments (at 3 km resolution, spanning 2012-2022) performed with the fully coupled atmosphere-hydrology WRF-Hydro system (i.e. Weather Research and Forecasting model fully coupled with version 5.2 of the WRF-Hydro hydrological module) to investigate the potential impacts of LULCC (deforestation and afforestation scenarios) on regional climate extremes in the West African Savannas region. Analyzing 18 extreme weather indices, we find that deforestation significantly affects temperature extremes, though with modest average impacts (<3%), consistently impacting regional rainfall extremes ~2 times more than mean rainfall conditions while significantly increasing drought duration. Our findings also reveal contrasting regional biophysical responses to afforestation concerning temperature extremes: converting grassland to evergreen forests tends to mitigate the biophysical warming effect, reducing extreme heat indices through enhanced plant transpiration from largely increased canopy foliage. Conversely, converting grassland to savanna may intensify extreme heat events due to the albedo-induced warming effect and increased downward longwave radiation, resulting in more absorption of shortwave radiation by the surface. This work emphasizes the necessity of fully coupled modeling frameworks that integrate all aspects of LULCC and the potential local positive feedback between the terrestrial hydrological system and the overlying atmosphere to improve the evaluation of land-based mitigation and adaptation strategies.