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. |
10.1.3.3. Paleoclimate of Africa
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 cycle—the 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.
10.1.3.4. Recent Historical Record
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 warming—the
1910s to 1930s and the post-1970s—occur 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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