The Sahel transition to persistent drought in the early 1970s is an archetypal example of recent abrupt climate change. This workshop assessed the mechanisms for variability at interannual and interdecadal timescales, and discussed mechanisms of future climate change and sources of model disagreement. Participating scientists brought a diverse range of expertise: mesoscale and paleo observationalists; atmospheric dynamicists; dust and vegetation modelers.
There was strong agreement that the main driver of the Sahel drought was sea surface temperature (SST) variations and not land use or cover changes associated with human activity. This conclusion stems from general circulation model studies reporting successful simulations of multidecadal Sahel rainfall variations given the long-term history of observed SST. Attribution is uncertain, as possible ultimate causes of SST and rainfall variability include anthropogenic forcing by greenhouse gases and aerosols, variations to the Atlantic meridional overturning circulation, and dust forcing from the Sahara. Specific atmospheric mechanisms connecting SST anomalies to the Sahel region remain uncertain, as do the relative roles of the Indo-Pacific warming and North Atlantic SST anomalies. Atmospheric changes from aerosols may directly influence Sahel rainfall without mediation from SST.