How Will Climate Change Affect West African Sahel Rainfall?
The Sahel region of West Africa is a semi-arid zone between the Sahara Desert and the humid Gulf of Guinea coast, roughly between 10-20°N. This region is irrigated by summer monsoon rains and rain-fed agriculture is the primary sustenance for Sahel populations. Severe droughts therefore have devastating negative societal impacts. The future frequency of Sahel droughts and future changes of its water balance are therefore of great interest.
Research addressing this question relies on global climate model simulations based on a range of anticipated greenhouse gas (GHG) emissions scenarios. Evidence is mounting that increasing concentrations of the GHG pollutants carbon dioxide and methane in the air cause corresponding increases in atmospheric temperatures.
Climate simulation models can also be used to research the impact that GHG will have on precipitation trends in a particular region. However, the challenge to correctly simulate Sahel rainfall trends is particularly daunting because multiple physical mechanisms compete to drive the trend upward or downward. In addition, a variety of climate model deficiencies, related to the simulation of one or more of these physical processes, can distort models' climate change projections.
To date, no consensus has emerged regarding the impact of anticipated GHG impacts on the water balance of the Sahel in the second half of the 21st century. Some studies project wetter conditions while others predict more frequent droughts, and each describes the atmospheric processes associated with its prediction. One study projects more frequent droughts before 2050 predicated on the assumption of continent-wide decline in vegetation cover.
Maynard et al. made climate projections based on increasing GHG concentrations using the ARPEGE global climate model of the French Meteorological Service. Figure 2 shows the simulated difference in July-September precipitation rates (mm per day) during 2070-2099 compared to 1980-2010. These results predict that the Sahel will be somewhat rainier in the last 30 years of the 21st century, although this contradicts the projection shown in Fig. 3 (discussed below). Maynard et al. relate their prediction of wetter conditions to projected large increases in surface air temperature over northern Africa, which in turn are expected to enhance atmospheric moisture transport to the Sahel by winds blowing from the Gulf of Guinea and from the tropical Atlantic.
Held et al. discuss experiments with the NOAA Geophysical Fluid Dynamics Laboratory's CM2 global climate model. An ensemble of six simulations, based on a range of assumed trends of increasing GHG concentrations, projects a progressively drier Sahel between 2040 and 2150. Figure 3 shows Sahel precipitation trends from Held et al., expressed as July-September proportional amounts relative to the 1901-2000 observed mean. The black line shows observed rainfall, the thick blue line shows the model ensemble mean for the 20th century and the thin lines show each of the six CM2 projections of Sahel rainfall for the 21st century and beyond.
Held et al. ascribe the drying trend to projections that South Atlantic ocean surface temperatures will warm faster than the North Atlantic, a consequence of increasing buildups of aerosol (particle) pollution in the Northern Hemisphere. Research shows that a relatively warmer ocean south of the Equator inhibits the northward seasonal movement of rain into the Sahel. In addition, GHG warming tends to lower relative humidities making it harder to rain.
In a recent review of the literature, we found that five of nine studies that address impacts during the second half of the 21st century project some drying, but results in only two of those are unambiguous. Two other studies project a distinctly rainier Sahel during the second half of the 21st century. More definitive projections await simulations by more accurate climate models, perhaps with higher densities of computational points to provide greater spatial details of simulated climate processes.
Druyan, L.M., 2010: Studies of 21st century precipitation trends over West Africa. Intl. J. Climatol., doi:10.1002/joc.2180, in press.
Held, I.M., T.L. Delworth, J. Lu, K.L. Findell, and T.R. Knutson, 2005. Simulation of Sahel drought in the 20th and twenty-first centuries. Proc. Natl. Acad. Sci. 102, 17891-17896, doi:10.1073/pnas.0509057102.
Maynard, K., J. Royer, and F. Chauvin, 2002. Impact of greenhouse warming on the West African summer monsoon. Clim. Dynam. 19, 499-514, doi:10.1007/s00382-002-0242-z.
Please address all inquiries about this research to Dr. Leonard Druyan.