Potential impact of the pan-African Great Green Wall on Sahelian summer precipitation: A global modeling approach with MPAS

The pan-African Great GreenWall for the Sahara and the Sahel initiative (GGW) is a reforestation program to reverse the degradation of land. We investigate characteristics of mean precipitation due to proposed land use changes to woody savannah with three hypothetical courses of the GGW, with an area between 0.8 to 1.25 Mill. km2, and between the 100–400 mm isohyetes. The global Model for Prediction Across Scales (MPAS) was applied for this investigation, employing ensembles with 40 members for the rainy season from June to September and 50 members for August where precipitation has its peak. In comparison to observational reference, the results show that a wet bias on the order of 33% in the eastern Sahel and a moderate dry bias of −41% in the western part are present in the MPAS simulations. Our simulations do not provide any significant evidence for GGW induced changes in the characteristics of the summer precipitation, nor for positive changes within the Sahel supporting the forestation activities, nor for potentially adverse changes in the neighboring regions. At the regional scale, changes are present, but they are not significant at the 5% level. Also, changes simulated for further hydrometerological variables such as temperature, radiation fluxes or runoff are comparatively small.