Linking Past, Present and Future Climate Change to Adaptation in the African Sahel

The semi-arid African Sahel has received unique attention in the climate science community and beyond since inception of persistent drought at the end of the 1960s. Initially, drought was attributed to rapid population growth leading to mismanagement of land resources. The hypothesis of a positive bio-geophysical feedback tied human-induced baring of the soils to a reduction of precipitation, which further exacerbated the loss in vegetation cover [Charney 1975, in Q J Roy Meteor Soc]. In contrast, the seminal work of Lamb dating back to the 1970s [e.g., 1978 in Mon Wea Rev, 1982 in Nature] tied Sahel drought to variations in the surface temperature of the tropical Atlantic, and attendant regional atmospheric circulation, paving the way for seasonal climate prediction.

In the more recent ~10 years the climate of the Sahel has again emerged as the focus of active research, this time as a possible "canary in the coal mine" for anthropogenic climate change. Advances in climate science have first conclusively tied persistent drought to subtle shifts in the surface temperature of the global oceans [Giannini et al. 2003], "freeing farmers of blame" in the drought, then partially attributed these shifts to the influence of greenhouse gases and aerosols [Held et al. 2005, in Proc Nat Acad Sci; Booth et al. 2012, in Nature]. In the meantime the region has partially recovered from drought, and is experiencing an increased frequency of flooding [Tall 2010, in Proc Env Sci], underlined, to the extent that it has been documented, by a subtle increase in the intensity of precipitation [Lodoun et al. 2013, in Env Develop; Alhassane et al. 2013, in Secheresse].

Here I present a novel interpretation for the role of the oceans in effecting precipitation change in this region, an interpretation that has benefited from collaborative research through sustained opportunities of engagement catalyzed by the regional Climate Outlook Fora organized by ACMAD, especially PRESAO, the West African COF. Sahel rainfall responds to the relative temperature of the North Atlantic, source of the moisture that converges in the region, with respect to the global tropical oceans. The temperature of the global tropical oceans, which is communicated first vertically through deep convection, then laterally by atmospheric waves [Chou and Neelin 2004, in J Climate; Held and Soden 2006, in J Climate; Sobel et al. 2001, in J Atmos Sci], broadly determines the threshold for convection. The temperature of the North Atlantic relative to that of the global tropical oceans measures the potential for the moist, but cool air that is converged onto the African continent from the adjacent ocean to trigger deep convection and precipitation.

This interpretation consistently explains past drought, partial recovery, and the current alternation of wet and dry states on time scales from daily to interannual [Lodoun et al. 2013, in Environmental Development; Salack et al. 2014, in Clim Dyn]. It also sheds light on the uncertainty in future projections, relating them to the uncertainty in patterns of sea surface temperature change [Giannini et al. 2013, in Env Res Lett]. This contribution aims to frame the physical context in which to discuss societal response to drought, and its applicability to adaptation to current variability and future change.