Drought in the African Sahel: variability, change and agency

Tyndall working papers are "work in progress". Whilst they are commented on by Tyndall researchers, they have not been subject to a full peer review. The accuracy of this work and the conclusions reached are the responsibility of the author(s) alone and not the Tyndall Centre. Summary The African Sahel experienced a prolonged dry episode in the latter decades of the twentieth century, characterised by years in which annual rainfall totals were consistently below the long term mean for the century, and punctuated by years of severe drought. Since the late 1990s there has been some amelioration of the regional climate, and in 2003 there was abundant rainfall throughout much of the Sahel, and also in parts of the Sahara, prompting speculation that the region was experiencing a shift to a wetter climate. This paper examines the possibility that the Sahel is experiencing such a shift, examining observed rainfall variability in the context of historical and palaeo-environmental evidence and model-based studies of recent and potential future climate variability and change. A review of the literature leads to the conclusion that dry conditions in the late twentieth century were most probably driven by changes in ocean surface temperatures, and in particular a warming of the southern hemisphere oceans and the Indian Ocean, which led to changes in atmospheric circulation. Interactions between the Sahelian land surface and the regional atmosphere via the medium of vegetation dynamics (and possibly dust production) appear to have played a role in modulating interannual and decadal scale variations in rainfall. While the principal forcing mechanisms of Sahelian rainfall over the past few decades are consistent with human-induced global warming, a causal mechanism linking human activity and drought in the Sahel cannot be proven, and models of future climate change suggest that such warming may ultimately lead to a more humid regime in the Sahel and parts of the Sahara.