To assess whether drier weather conditions have started, Ramzi Touchan of the University of Arizona and colleagues studied newly developed multi-century tree ring records from Tunisia and Algeria for a longer-term perspective on northwestern African drought.

Using a set of 13 chronologies from Atlas cedars and Aleppo pines, the scientists analyzed the widths of individual tree rings that can indicate the years when water was relatively scarce.

The researchers said they developed an index of dryness based on precipitation and temperature for the years between 1456 and 2002. They discovered the most recent drought, occurring during 1999, appears to be the worst since at least the middle of the 15th century. That, they said, is consistent with early signatures of a transition to more arid mid-latitude conditions, as projected by several climate models.

The study that included scientists from Columbia University, the University of Ourgla in Algeria and the Institute of Sylvo-Pastoral in Tunisia appears in the journal Geophysical Research Letters.