Geoscience Reference
In-Depth Information
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1960
1965
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1975
1980
1985
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1995
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2005
2010
Figure 3.11 The July through September precipitation (mm/day) averaged over
the western and central Sahel (10ºW to 10ºE and 11ºN to 16ºN) is shown by
the dashed line; the solid line is a smoothed version to show decadal variations.
Figure courtesy of Bing Pu.
climate of the 1930s combined with economic depression forced a massive
westward migration, and persistent warm, dry conditions returned in the
1950s and again in the late 1980s. On longer time scales, tree ring data analysis
reveals the occurrence of multidecadal
megadroughts
in past centuries.
Northern Africa is also known to be susceptible to decadal-scale drought.
Recent examples are the Sahelian drought of the 1970s-1980s and the recent
drought over Sudan at the beginning of the twenty-irst century.
Figure 3.11
shows the summer rainfall record from the Sahel (11°N-16°N, 10°W-10°E)
for 1950-2010. Values plotted are rainfall differences from the mean climatol-
ogy. It is clear that Sahel rainfall has significant decadal-scale variability. Dry
conditions persisted through the 1970s and 1980s, for example, following two
decades of relatively wet conditions in the early 1950s.
3.5 CLIMATE VARIATIONS ON CENTURY
TO BILLION-YEAR TIME SCALES
In addition to the relatively short time scales already reviewed, climate varia-
tions occur on time scales of centuries and millennia, and even millions and bil-
lions of years. The study of earth's
paleoclimates
informs the study of present
and future climate by demonstrating that the climate system is capable of great
change, for example, from a warm planet some 70 million years ago that was
hospitable to dinosaurs to the ice age world of only 20,000 years ago. And it is
clear from the paleoclimate record that climate change can occur abruptly and
lead to mass extinctions of flora and fauna.
The study of paleoclimates is founded on the availability of
proxy data
de-
rived from fossil pollen, ocean and lake sediment cores, tree rings, ice cores,
and coral. Various historical records such as diaries, agricultural logs, shipping
records, and newspaper articles are also used to reconstruct past climate.
One well-known example of climate variation on the centennial time scale
is the
Little Ice Age
. Average global temperatures are estimated to have been
