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For at least 3,000 years, a series 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. The lead authors of the report, Timothy M. Shanahan of the University of Texas at Austin and Jonathan T. Overpeck of the University of Arizona, warned that global warming was likely to exacerbate those droughts.

Kevin Watkins, director of the office of Human Development Reports of the United Nations, said: “Many of the 390 million people in Africa living on less than $1.25 a day are smallholder farmers that depend on two things: rain and land. Even small climate blips such as a delay in rains, a modest shortening of the drought cycle, can have catastrophic effects.”

Given the sub-Saharan region’s persistent vulnerability, Watkins added, the new findings and the prospect of further global warming could be “early warning signs for an unprecedented and catastrophic reversal in human development.” The study said that some of the past major droughts appeared to be linked to a distinctive pattern of increases and reductions in surface temperatures of the Atlantic Ocean, known as the Atlantic multi-decadal oscillation.

Typically over the last 3,000 years, a severe drought developed every 30 to 65 years, the researchers said. But several centuries-long droughts in the climate record, the most recent persisting from 1400 to around 1750, are harder to explain. While that drought occurred during a cool spell in the Northern Hemisphere called the “Little Ice Age,” other sustained droughts appear to have hit West Africa when the world was warm overall, they reported.