For at least 3,000 years, a drumbeat of potent droughts, far longer and more severe than any experienced recently, have seared a belt of sub-Saharan Africa that is now home to tens of millions of the world’s poorest people, climate researchers report in a new study.

The last such drought, persisting more than three centuries, ended around 1750, the research team writes in the April 17 issue of the journal Science.

The scientists warned that more such mega-droughts are inevitable, although there is no way to predict when the next one could unfold.

That sobering prediction emerged from the first study of year-by-year climate conditions in the region over the millenniums, based on layered mud and dead trees in a crater lake in Ghana. Although the evidence was drawn from a single water body, Lake Bosumtwi, the researchers said there was evidence that the drought patterns etched in the lake bed extended across a broad swath of West Africa.