91av

AFRICA鈥檚 lethal droughts of the past 30 years have been puny compared with
arid spells in previous centuries, according to a pioneering reconstruction of
the climate using lake sediments in East Africa. Those centuries saw huge swings
in African rainfall, says Dirk Verschuren of the University of Ghent in
Belgium.

鈥淭he fluctuations seem to be tied to variations in the radiative output of
the Sun,鈥� Verschuren told 91av. 鈥淗igher solar radiation is
linked to droughts in equatorial East Africa.鈥�

Regionally, Verschuren says that these recurring droughts may be more of a
problem than global warming. 鈥淚n arid places like East Africa, central Asia and
Australia, I think they should be much more worried about natural variability in
rainfall. They need to be less dependent on hydroelectric dams and irrigated
补驳谤颈肠耻濒迟耻谤别.鈥�

Verschuren probed a thousand years of the East African climate by analysing
the sediments on the bottom of Kenya鈥檚 Lake Naivasha in the Rift Valley. 鈥淭he
lake is very unusual in that its level responds strongly to changes in rainfall
over a few years, but it has not dried out for at least the last millennium,鈥�
says Verschuren.

The lake has laid down a record of water levels in its sediment. Periods of
high water have a greater concentration of algae on the lake floor, which shows
itself in a higher organic carbon content in the sediment left behind in annual
layers. Using this and other indicators, Verschuren and his colleagues have
calculated past lake levels going back 1000 years
(see Figure).

There was a long dry period from 1000 to 1270, and three shorter intense
droughts from 1380 to 1420, 1560 to 1620 and 1760 to 1840. Each coincided with
an era of famine, political unrest and mass migration related through the oral
histories of local communities. 鈥淭he dates are not precise because oral
histories simply count back the generations, but a synchronised pattern
definitely emerges,鈥� Verschuren says.

Verschuren鈥檚 study, probably the most complete study of the pre-colonial
African climate, also casts an intriguing light on the early days of European
colonisation. The continent-wide drought almost certainly contributed to the
immense political chaos and migrations that were partly reflected in the growth
of the slave trade, says anthropologist John Reader of University College
London.