Separate from global warming, natural climate change in eastern Africa over the past millennium has resulted in extended periods of drought followed by periods of heavy rainfall, a new study shows.

"We have shown that there have been large fluctuations in water availability on a decadal to centennial time scale and that these changes occurred due to natural oscillations in climate," said Brian Cumming, a biologist with the Paleoecological Environmental Assessment and Research Laboratory at Queen's University in Canada and co-author the study.

"Changes in precipitation were related to changes in solar intensity," he explained. "During times of lower solar activity over the last thousand years, rainfall was high in eastern Africa whereas in times of increased solar activity rainfall was low."

Cumming collaborated on the study with Queen's University biologist Kathleen Laird and University of Ghent biologist Dirk Verschuren. Their report was published in the Jan. 27 edition of Nature.

The researchers warn that a catastrophic dry period, possibly lasting decades, might visit eastern Africa again in the next 50 to 100 years. A drought of this magnitude could trigger large-scale migration, political upheaval and economic and social devastation.

"Long droughts have been implicated in the collapse of many ancient civilizations on virtually every continent, including our example in eastern Africa, the Mayan in Central America and many others," said Cumming.