In spite of the fact that GCMs have failed to accurately reproduce observed patterns and totals of precipitation (Lebel et al., 2000), model predictions of imminent CO2-induced global warming often suggest that this phenomenon should lead to increases in rainfall amounts and intensities. Hence, many scientists are examining historical precipitation records in an effort to determine how temperature changes of the past millennium have impacted these aspects of earth's hydrologic cycle. In this summary, we review what some of them have learned about rainfall in Africa.
Beginning in southern Africa, Richard et al. (2001) analyzed summer (Jan-Mar) rainfall totals over the period 1900-1998, finding that interannual variability was higher for the periods 1900-1933 and 1970-1998, but lower for the period 1934-1969. The strongest rainfall anomalies (greater than two standard deviations) were observed at the beginning of the century. However, the authors conclude there were "no significant changes in the January-March rainfall totals," nor any evidence of "abrupt shifts during the 20th century," suggesting that rainfall trends in southern Africa do not appear to have been influenced by CO2-induced - or any other type of - global warming.
Moving to the equatorial region of East Africa, Nicholson and Yin (2001) report there have been "two starkly contrasting climatic episodes" since the late 1700s. The first, which began sometime prior to 1800, was characterized by "drought and desiccation." Extremely low lake levels were the norm, as drought reached its extreme during the 1820s and 1830s. In the mid to latter part of the 1800s, however, the drought began to weaken and floods became "continually high," but by the turn of the century lake levels began to fall as mild drought conditions returned. The drought did not last long, however, and the latter half of the 20th Century has seen an enhanced hydrologic cycle with a return of some lake levels to the high stands of the mid to late 1800s.
Verschuren et al. (2000) also examined hydrologic conditions in equatorial East Africa, but over a much longer time scale, i.e., a full thousand years. They report the region was significantly drier than it is today during the Medieval Warm Period from AD 1000 to 1270, while it was relatively wet during the Little Ice Age from AD 1270 to 1850. However, this latter period was interrupted by three episodes of prolonged dryness: 1390-1420, 1560-1625 and 1760-1840. These "episodes of persistent aridity," according to the authors, were "more severe than any recorded drought of the twentieth century."
The dry episode of the late 18th / early 19th centuries recorded in Eastern Africa has also been identified in Western Africa. In analyzing the climate of the past two centuries, Nicholson (2001) reports that the most significant climatic change that has occurred "has been a long-term reduction in rainfall in the semi-arid regions of West Africa," which has been "on the order of 20 to 40% in parts of the Sahel." There have been, she says, "three decades of protracted aridity," and "nearly all of Africa has been affected ... particularly since the 1980s." However, she goes on to note that "the rainfall conditions over Africa during the last 2 to 3 decades are not unprecedented," and that "a similar dry episode prevailed during most of the first half of the 19th century."
The importance of these findings is best summarized by Nicholson herself, when she states that "the 3 decades of dry conditions evidenced in the Sahel are not in themselves evidence of irreversible global change." And especially, we would add, are they not evidence of global warming-induced change. Why not (to both points)? Because a longer historical perspective of the type we are constantly striving to obtain clearly indicates, in the first instance, that an even longer period of similar dry conditions occurred between 1800 and 1850. And in the second instance, this remarkable dry period occurred when the earth was still in the clutches of the Little Ice Age, a period of cold that is without precedent in at least the last 6500 years ... even in Africa [see our Journal Review of the work of Lee-Thorp et al. (2001)]. Hence, there is no reason to think that the past two- to three-decade Sahelian drought is in any way unusual or that it was caused by the putative higher temperatures of that period. Simply put, like many other things, droughts happen.
References
Lebel, T., Delclaux, F., Le Barbé and Polcher, J. 2000. From GCM scales to hydrological scales: rainfall variability in West Africa. Stochastic Environmental Research and Risk Assessment 14: 275-295.
Lee-Thorp, J.A., Holmgren, K., Lauritzen, S.-E., Linge, H., Moberg, A., Partridge, T.C., Stevenson, C. and Tyson, P.D. 2001. Rapid climate shifts in the southern African interior throughout the mid to late Holocene. Geophysical Research Letters 28: 4507-4510.
Nicholson, S.E. 2001. Climatic and environmental change in Africa during the last two centuries. Climate Research 17: 123-144.
Nicholson, S.E. and Yin, X. 2001. Rainfall conditions in equatorial East Africa during the Nineteenth Century as inferred from the record of Lake Victoria. Climatic Change 48: 387-398.
Richard, Y., Fauchereau, N., Poccard, I., Rouault, M. and Trzaska, S. 2001. 20th century droughts in southern Africa: Spatial and temporal variability, teleconnections with oceanic and atmospheric conditions. International Journal of Climatology 21: 873-885.
Verschuren, D., Laird, K.R. and Cumming, B.F. 2000. Rainfall and drought in equatorial east Africa during the past 1,100 years. Nature 403: 410-414.