A 3,000 year record of African climate holds warnings of devastating droughts yet to come, say US researchers.
Using lake sediments from Lake Bosumtwi in Ghana, a team led by Jonathan Overpeck and Timothy Shanahan reconstructed the variability of the African monsoon for nearly every year of the past three millennia. Dire droughts appear to be unavoidable in West Africa, and there are worrying implications related to the Sahel drought in the 60s, 70s and 80s that killed thousands.
“What’s disconcerting about this record is that it suggests that the most recent drought was relatively minor in the context of the West African drought history,” says Shanahan, of the University of Arizona, Tucson (press release 1).
And while some droughts – like the one in Sahel – lasted for decades, the sediment record shows some lasted for centuries.
The team used differences in oxygen isotopes in the lake sediments to identify dry and wet periods. Higher concentrations of the heavier O18 isotope indicate dry periods, where more of the common, lighter O16 evaporates, they say.
“Support for our geochemical interpretations also came from evidence for past lake stands during drought periods, including a partially submerged forest, which grew during a century-long drought only a few hundred years ago when the lake was much lower,” says Shanahan. (The picture right shows a submerged forest tree in the lake.)
The sediment record also shows droughts seem to be linked to variations in sea surface temperatures called the Atlantic Multidecadal Oscillation.
“More and more, it’s starting to look like the AMO is a big player affecting climate change around the Northern Hemisphere, including drought variability over Western Africa and western North America,” says Overpeck (press release 2).
Shanahan and Overpeck both warn that global warming will only make things worse for the region.
Kevin Watkins, director of the office of Human Development Reports of the United Nations, told the NY Times the study was “a critical report”.
“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,” he says. “Even small climate blips such as a delay in rains, a modest shortening of the drought cycle, can have catastrophic effects.”
The Cleveland Plain Dealer notes that Lake Bosumtwi – which was formed from a meteorite impact crater – is an “ideal place” for climate research:
The bullseye lake catches whatever falls at this crucial atmospheric juncture, whether it’s precipitation or wind-blown dust. Sediments drift to the calm lake bottom, where they form distinct, visible layers year by year. The layers, called varves, stack up like pancakes.
Because the crater lake is only fed by rainfall, not by rivers or groundwater, it’s a direct, sensitive indicator of how the climate behaves. “Think of it as a gigantic rain bucket,” Peck said.
On his NY Times blog Andrew Revkin provokes a very interesting discussion by posing the following to leading scientists: “One clear-cut lesson [of this study] seems to be that human-driven warming, for this part of Africa, could be seen as a sideshow given the normal extremes. Tell me why that thought is misplaced if you feel it is.”
More
Research paper in Science
West Africa faces ‘megadroughts’ – BBC
Image: Photograph by J.T. Overpeck and W. Wheeler, University of Arizona.