Drought in the Sahel

A new paper by Jian Lu and Thomas Delworth argues tentatively for a link between greenhouse warming and drought in the Sahel.(Lu and Delworth, 2005)

This matters, of course, far more than my usual hobbyhorse of drought in the southwestern U.S. Here when our water supplies run low because of drought, we suffer economic dislocation. In the Sahel, the semiarid band across Africa on the southern border of the Sahara, famines set in, governments collapse, people die.(Glantz, 2003)

Lu and Delworth used the new AM2 atmospheric general circulation model at the Geophysical Fluid Dynamics Laboratory in Princeton to simulate African rainfall forced with observed sea surface temperatures and sea ice for 1950 to 2000.

There’s been a long argument over the relative role of external climate forcing versus overgrazing and the cutting of forests in the drought.(Zeng, 2003) Model runs done by Alessandra Giannini a couple of years ago seemed to favor an ocean link (i.e. external forcing).(Giannini et al., 2003) Now Lu and Delworth seem to be taking this the next step and suggesting that greenhouse-warmed oceans could be to blame:

Forcing from the tropical oceans is dominant in driving the Sahelian rainfall trend. The response of Sahel rainfall to a general warming of the tropical oceans suggests a possible link to greenhouse gas-induced climate change.

They’re not running around shrieking that they’ve got a smoking gun here. Their paper is very cautiously worded. But they discuss other results that use an ocean-atmosphere coupled model forced with CO2 (rather than the observed sea surface temperature model discussed above) that both reproduces drought in the Sahel over the second half of the 20th century and forecasts more to come:

For simulations of the 20th century, in which the model is forced with observational estimates of changes in greenhouse gases, aerosols, volcanic forcing, solar forcing, and land use, the models simulated a drying trend over the Sahel in the second half of the 20th century, with an amplitude approximately 50 percent of the observed trend. In addition, simulations of the 21st century using this model, as well as additional experiments using idealized increases of CO2, all simulate a further drying of the Sahel in response to general warming. Detailed analysis of these coupled model experiments will be reported in a separate paper (Held et al., submitted manuscript, 2005). These coupled model results suggest that a CO2 induced global SST increase does play a part in the Sahel drought, at least in the context of the GFDL coupled climate model.

Giannini, A., Saravanan, R. and Chang, P. (2003) Science, 302, 1027-1030.
Glantz, M. H. (2003) Climate affairs : a primer, Island Press, Washington, DC.
Lu, J. and Delworth, T. L. (2005) Geophys. Res. Lett., 32, 22
Zeng, N. (2003) Science, 302, 999-1000.