Climatic impacts of irrigated afforestation of the Sahara in a complex Earth System Model
Authors: Tronje Kemena, Katja Matthes, Thomas Martin and Andreas Oschlies
Afforestation of the Sahara might potentially be effective to mitigate climate change but requires substantial amounts of irrigation to sustain the planted trees. Enhanced evapotranspiration would moisten the Saharan air. Its impact on the global atmospheric hydrological cycle and circulation is widely unexplored.
Here we use for the first time a high-top state-of-the-art Earth system model, (NCAR’s CESM-WACCM), to investigate in detail atmospheric feedbacks and their side effects to such a large-scale afforestation project.
Atmospheric feedbacks reduce the precipitation potential over an afforested Sahara and raise questions at the self-sustainability of such an intervention. Only 26% of the evapotranspirated water re-precipitates over the Saharan Desert, considerably large amounts are advected south- and westward to the Sahel zone and to the North Atlantic. In the Sahel zone, the West African Monsoon precipitation enhances and in the North Atlantic tropical cyclones (TCs) are intensified. Humidity induced changes in mid-tropospheric relative humidity, sea surface temperatures, vertical wind shear and maximum potential intensity provide substantial contributions to intensified TC development. The Saharan afforestation has a considerably larger impact on the TC development than a worst-case global warming scenario.