Nicholson and Yin (2001) described climatic and hydrologic conditions in equatorial East Africa from the late 1700s to close to the present, based on histories of the levels of ten major African lakes. They also used a water balance model to infer changes in rainfall associated with the different conditions, concentrating most heavily on Lake Victoria. This work revealed "two starkly contrasting climatic episodes." The first, which began sometime prior to 1800 and was characteristic of Little Ice Age conditions, was one of "drought and desiccation throughout Africa." This arid episode, which was most extreme during the 1820s and 30s, was accompanied by extremely low lake levels. As the two researchers describe it, "Lake Naivash was reduced to a puddle ... Lake Chad was desiccated ... Lake Malawi was so low that local inhabitants traversed dry land where a deep lake now resides ... Lake Rukwa [was] completely desiccated ... Lake Chilwa, at its southern end, was very low and nearby Lake Chiuta almost dried up." Throughout this unfortunate period, they report that "intense droughts were ubiquitous." Some, in fact, were "long and severe enough to force the migration of peoples and create warfare among various tribes."

As the Little Ice Age's grip on the world began to loosen in the mid to latter part of the 1800s, however, things began to tend towards the better for most of the continent. Nicholson and Yin report that "semi-arid regions of Mauritania and Mali experienced agricultural prosperity and abundant harvests ... the Niger and Senegal Rivers were continually high; and wheat was grown in and exported from the Niger Bend region." Across the length of the northern Sahel, in fact, maps and geographical reports described the presence of "forests." As the nineteenth century came to an end and the twentieth century began, however, there was a slight lowering of lake levels, but nothing like what had occurred a century earlier (i.e., variability was much reduced). And then, in the latter half of the twentieth century, things once again began to pick up for the Africans, with the levels of some of the lakes actually rivaling the high-stands characteristic of the years of transition to the Modern Warm Period.

Concentrating on the more recent past, Nicholson (2001) says the most significant climatic change 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."

Describing the situation in more detail, Nicholson says "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? Because a longer historical perspective of the type we are constantly striving to obtain clearly indicates, as noted by Nicholson, that an even longer period of similar dry conditions occurred between 1800 and 1850. In addition, 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 (Lee-Thorp et al., 2001). Hence, there is no reason to think that the most recent two- to three-decade Sahelian drought was in any way unusual or that it was caused by the higher temperatures of that period.

Also taking a longer view of the subject were Nguetsop et al. (2004), who developed a high-resolution proxy record of West African precipitation based on analyses of diatoms recovered from a sediment core retrieved from Lake Ossa, West Cameroon, which they describe as "the first paleohydrological record for the last 5500 years in the equatorial near-coastal area, east of the Guinean Gulf." They reported that this record provides evidence for alternating periods of increasing and decreasing precipitation "at a millennial time scale for the last 5500 years," which oscillatory behavior they interpret as being "a result of south/northward shifts of the Intertropical Convergence Zone," specifically noting that "a southward shift of the ITCZ, combined with strengthened northern trade winds, was marked by low and high precipitation at the northern subtropics and the subequatorial zone, respectively," and that "these events occurred in coincidence with cold spells in the northern Atlantic." As with so many other temperature and precipitation records from around the world, their data for West Africa add to the growing body of evidence for the existence of a millennial-scale oscillation of climate that alternately brings either warmer or cooler temperatures or wetter or drier conditions to each of the locations it influences as it reverberates down through the ages.

Most recently, Therrell et al. (2006) developed "the first tree-ring reconstruction of rainfall in tropical Africa using a 200-year regional chronology based on samples of Pterocarpus angolensis [a deciduous tropical hardwood known locally as Mukwa] from Zimbabwe." This record revealed that "a decadal-scale drought reconstructed from 1882 to 1896 matches the most severe sustained drought during the instrumental period (1989-1995)," and that "an even more severe drought is indicated from 1859 to 1868 in both the tree-ring and documentary data." They report, for example, that the year 1860, which exhibited the lowest reconstructed rainfall value during this period, was described in a contemporary account from Botswana (where part of their tree-ring chronology originated) as "a season of 'severe and universal drought' with 'food of every description' being 'exceedingly scarce' and the losses of cattle being 'very severe' (Nash and Endfield, 2002)." At the other end of the moisture spectrum, they report that "a 6-year wet period at the turn of the nineteenth century (1897-1902) exceeds any wet episode during the instrumental era."

Consequently, for a large part of central southern Africa, as well as the other parts of the continent described above, it is clear that the supposedly unprecedented global warming of the 20th century has not resulted in an intensification of extreme dry and wet periods. If anything, just the opposite appears to have occurred.

References
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.

Nash, D.J. and Endfield, G.H. 2002. A 19th-century climate chronology for the Kalahari region of central southern Africa derived from missionary correspondence. International Journal of Climatology 22: 821-841.

Nguetsop, V.F., Servant-Vildary, S. and Servant, M. 2004. Late Holocene climatic changes in west Africa, a high resolution diatom record from equatorial Cameroon. Quaternary Science Reviews 23: 591-609.

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.

Therrell, M.D., Stahle, D.W., Ries, L.P. and Shugart, H.H. 2006. Tree-ring reconstructed rainfall variability in Zimbabwe. Climate Dynamics 26: 677-685.