Abstract:
"Paleoclimatic estimates of mean annual rainfall in the equatorial
highlands of Central East Africa have been established for the last 40
kyr. The values are inferred from nine fossil pollen sequences,
collected from six peat bogs located between 2S and 4S latitude, in
the forest belt, from 1800 to 2240 m a.s.l. The transformation of
pollen data into climatic parameters is achieved by the best
... analogues
statistical method, using a modern pollen data set of East and Central
Africa and calibration by meteorological data. The climatic
reconstructions are first performed for each individual
sequences. They are transformed into time-series using 88 of the 125
available radiocarbon dates (including 41 AMS dates). The synthesis of
the results is presented as a single curve, illustrating the
precipitation values obtained for 682 dated stratigraphic layers
plotted on a radiocarbon timescale. The precipitation changes are
presented at about a century resolution during the Holocene, and about
a millennium for the glacial period.
The most characteristic feature of the synthetic curve is the changing
variability through time.
During the last glacial period (30 to 15 kyr BP interval), our results
indicate a ca. 450 mm/yr (32%) precipitation decrease relative to the
present. Two maxima decrease periods are registered at ca. 19 kyr BP
(700 mm/yr, ca. 45%) and between 18 and 16 kyr BP (608 mm/yr,
ca. 42%). Several abrupt positive peaks of higher precipitation are
noticed pre 35 kyr BP and ca. 22-20 kyr BP.
During the lower Holocene (10-7 kyr BP), the mean calculated
precipitation estimate is 30 mm/yr (2%) below the present-day value,
with several abrupt positive shifts; the maximum at ca. 8 kyr BP
reaches 600 mm/yr (42%). Great variations between low and high
precipitation values are expressed post ca. 4 kyr BP.
These results are discussed in light of other climatic indicators
extracted either from the same sedimentary record, isotopes,
microfauna, or from other geological palaeoclimatic proxy in the
concerned region, and pattern of the monsoon record in marine
cores. Anthropogenic disturbance on past vegetation, as well as
humidity signal as a better response of forest to climatic forcing,
and possible CO2 effect need more investigation in the future. We feel
confident that the greater variability at the century scale registered
in the equatorial highlands provides a good explanation for increased
aridity in the desertic region of Sudan and Egypt during the last 4
kyr. The possibility that such greater variability could be induced by
high variability or instability of the Atlantic Ocean or global sea
surface temperature instability should be explored." 
