Figure 10-2: Mean surface air temperature anomalies for the African continent, 1901-1998, expressed with respect to 1961-1990 average, annual and four seasons (DJF, MAM, JJA, SON). Smooth curves result from applying a 10-year Gaussian filter.
Paleoclimatology and paleoenvironmental changes in Africa have been reconstructed from several lines of sedimentary evidence, such as fossil strand lines, diatom and pollen analyses, evidence of glaciation and fossil moraines on high mountains, sediment lithology, geochemistry and biogeochemistry, and so forth. Most records do not extend beyond 30,000 years before the present (BP), but they capture the climatic extremes of the last glacial-interglacial cyclethe last glacial maximum (22,000 to 14,000 years BP) through to the Holocene period (10,000 years BP to present) (Olago, 2001).
Temperatures during the last glacial maximum are estimated to have been 4-7°C lower than today, and they were coupled with intensive aridity and regression of lakes throughout the African continent, resulting from reduced precipitation as a consequence of weaker monsoons, stronger dry trade winds, and lowered SST (Coetzee, 1967; Flenley, 1979; COHMAP Members, 1988; Bonnefille et al., 1990; Vincens et al., 1993). Highland vegetation was depressed to significantly lower altitudes relative to today, and mountain glaciers were at their maximum extent. Grasslands were more widespread, lowland forests became fragmented, and subtropical desert margins advanced latitudinally by 300-700 km relative to their present positions (Flohn and Nicholson, 1980).
During the Holocene period of the past 10,000 years there was a "warm" climatic optimum roughly 5,000 years ago. At that time, more humid conditions generally were widespread, and deserts were markedly contracted. Lakes existed even in parts of the central Sahara. The current state of climate was reached roughly 3,000 years ago.
Observational records show that the continent of Africa is warmer than it was 100 years ago (IPCC,1996). Warming through the 20th century has been at the rate of about 0.05°C per decade (see Figure 10-2), with slightly larger warming in the June, July, August (JJA) and September-November seasons than in December, January, February (DJF) and March-May (Hulme et al., 2001). The 5 warmest years in Africa have all occurred since 1988, with 1988 and 1995 the two warmest years. This rate of warming is not dissimilar to that experienced globally, and the periods of most rapid warmingthe 1910s to 1930s and the post-1970soccur simultaneously in Africa and the world.
The climate of Africa has experienced wetter and drier intervals during the past 2 centuries. The most pronounced periods were during the 20th century. A very intense dry period, much like the current one, also prevailed for 2 to 3 decades during the first half of the 19th century. Humid conditions reminiscent of the 1950s prevailed around the 1870s or 1880s, but another milder arid interval of roughly 20 years commenced around 1895.
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