Droughts in Africa
From ClimateWiki
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
Lau et al., 2006 explored “the roles of sea surface temperature coupling and land surface processes in producing the Sahel drought” in the computer models used by the IPCC for its Fourth Assessment Report. These 19 computer models were “driven by combinations of realistic prescribed external forcing, including anthropogenic increase in greenhouse gases and sulfate aerosols, long-term variation in solar radiation, and volcanic eruptions.” This work revealed that “only eight models produce a reasonable Sahel drought signal, seven models produce excessive rainfall over [the] Sahel during the observed drought period, and four models show no significant deviation from normal.” In addition, they report that “even the model with the highest skill for the Sahel drought could only simulate the increasing trend of severe drought events but not the magnitude, nor the beginning time and duration of the events.”
All 19 of the models used in preparing the IPCC’s Fourth Assessment Report were unable to adequately simulate the basic characteristics of what Lau et al. call one of the past century’s “most pronounced signals of climate change.” This failure of what the authors call an “ideal test” for evaluating the models’ abilities to accurately simulate “long-term drought” and “coupled atmosphere-ocean-land processes and their interactions” vividly illustrates the fallibility of computer climate models.
In a review of information pertaining to the past two centuries, Nicholson (2001) reports there has been “a long-term reduction in rainfall in the semi-arid regions of West Africa” that has been “on the order of 20 to 40% in parts of the Sahel.” Describing the phenomenon as “three decades of protracted aridity,” she reports that “nearly all of Africa has been affected … particularly since the 1980s.” Nevertheless, Nicholson says that “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,” when much of the planet was still experiencing Little Ice Age conditions.
Therrell et al. (2006) developed what they describe as “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 project 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 was the most droughty year of the entire 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, Therrel et al. 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, it is clear that the supposedly unprecedented global warming of the twentieth century did not result in an intensification of either extreme dry or wet periods.
Looking further back in time, Verschuren et al. (2000) developed a decadal-scale history of rainfall and drought in equatorial east Africa for the past thousand years, based on level and salinity fluctuations of a small crater-lake in Kenya that were derived from diatom and midge assemblages retrieved from the lake’s sediments. Once again, they found that the Little Ice Age was generally wetter than the Current Warm Period; but they identified three intervals of prolonged dryness within the Little Ice Age (1390-1420, 1560-1625, and 1760-1840), and of these “episodes of persistent aridity,” as they refer to them, all were determined to have been “more severe than any recorded drought of the twentieth century.”
Probing some 1,500 years into the past was the study of Holmes et al. (1997), who wrote that since the late 1960s, the African Sahel had experienced “one of the most persistent droughts recorded by the entire global meteorological record.” However, in a high-resolution study of a sediment sequence extracted from an oasis in the Manga Grasslands of northeast Nigeria, they too determined that “the present drought is not unique and that drought has recurred on a centennial to interdecadal timescale during the last 1500 years.”
Last, and going back in time almost 5,500 years, Russell and Johnson (2005) analyzed sediment cores that had been retrieved from Lake Edward—the smallest of the great rift lakes of East Africa, located on the border that separates Uganda and the Democratic Republic of the Congo—to derive a detailed precipitation history for that region. In doing so, they discovered that from the start of the record until about 1,800 years ago, there was a long-term trend toward progressively more arid conditions, after which there followed what they term a “slight trend” toward wetter conditions that has persisted to the present. In addition, superimposed on these long-term trends were major droughts of “at least century-scale duration,” centered at approximately 850, 1,500, 2,000, and 4,100 years ago. Consequently, it would not be unnatural for another such drought to grip the region in the not-too-distant future.
In summation, real-world evidence from Africa suggests that the global warming of the past century or so has not led to a greater frequency or greater severity of drought in that part of the world. Indeed, even the continent’s worst drought in recorded meteorological history was much milder than droughts that occurred periodically during much colder times.
References
Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
Holmes, J.A., Street-Perrott, F.A., Allen, M.J., Fothergill, P.A., Harkness, D.D., Droon, D. and Perrott, R.A. 1997. Holocene palaeolimnology of Kajemarum Oasis, Northern Nigeria: An isotopic study of ostracodes, bulk carbonate and organic carbon. Journal of the Geological Society, London 154: 311-319.
Lau, K.M., Shen, S.S.P., Kim, K.-M. and Wang, H. 2006. A multimodel study of the twentieth-century simulations of Sahel drought from the 1970s to 1990s. Journal of Geophysical Research 111: 10.1029/2005JD006281.
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.
Nicholson, S.E. 2001. Climatic and environmental change in Africa during the last two centuries. Climate Research 17: 123-144.
Russell, J.M. and Johnson, T.C. 2005. A high-resolution geochemical record from Lake Edward, Uganda Congo and the timing and causes of tropical African drought during the late Holocene. Quaternary Science Reviews 24: 1375-1389.
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.
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.
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