Climate scientists look for evidence of historical conditions wherever they can. Annual growth rings in conifer trees in Algeria and Tunisia, for example, deliver centuries’ worth of data on wetness. Northwest Africa was once lush but now is dry.
Ramzi Touchan, of the University of Arizona, and his colleagues have methodically looked at tree-ring data for a 500 year period.This geologic slice of information shows that droughts occur in irregular patterns. But the severest drought over that time span, as judged by the narrowness of the tree rings, was the recent period 1999-2002.
This by itself does not prove that the Maghreb region of Africa is becoming more arid, says Touchan, but it does provide some baseline perspective from which to weigh later climate developments.
(Results published in Geophysical Research Letters, August 2008)