A meteor helps reveal West Africa's troubling drought history

How do you help cities, towns, and villages build resilience to drought when droughts can last for more than a century, not "merely" decades?

That's a question scientists have posed through a new record of drought in western Africa. It stretches back some 3,000 years. And it reveals a couple of sobering features.

First, droughts similar to the Sahel drought of the 1970s and '80s, in which rainfall amounts fell by some 30 percent, pop up on average every 30 to 60 years.

Second, this drought, which aid and public-health officials say killed some 100,000 people and prompted millions to migrate, was a hydrological piker. So-called megadroughts lasting nearly a century or more have occurred six times in the last 2,700 years. Two of those took place in the last 1,000 years. The most recent occurred between 550 and 200 years ago.

For people living in the United States, especially out west, this is – or should be – a familiar message. Research into the US West's hydrological history has uncovered evidence of droughts over the past 1,000 years or more that were more intense, covered a wider swath of the country, and lasted far longer than anything the 20th century offered up.

For West Africa, the record is disconcerting, says Timothy Shanahan, an assistant professor of geosciences at the University of Texas at Austin. "It suggests that the region is vulnerable to larger and longer-lasting droughts than we have seen in our lifetime. Were we to shift into one of these century-scale patterns of drought, it would be a lot more severe and a lot more difficult for people to adjust to these changes."

The record "is going to be challenging for policymakers and planners," says Dr. Shanahan, who spearheaded the study and wrote up the results, published in the April 17 issue of the journal Science.

It's worth noting that no one is talking global warming regarding these results. Just "natural" variability.

Add human-triggered warming to the picture, however, and it makes the challenge of adaptation that much greater, notes Jonathan Overpeck, a geoscientist at the University of Arizona in Tucson and head of the University's Environmental Studies Laboratory.

Climate scientists "have strong confidence that continued warming will take place in the absence of reductions of greenhouse gases," says Dr. Overpeck, one of the paper's co-authors. "Even if we reduce greenhouse gases somewhat, we're likely to have an increase in temperature in this region over this century in excess of 5, even 10 degrees Fahrenheit. That could make the droughts, when they occur, much harder to adapt to."

Even without global warming in the picture, the study's implication for the region "is depressing," says Peter Bloch, an economist with the University of Wisconsin at Madison's forestry and wildlife ecology department who works in the region.

He notes that over the past 20 to 25 years,  West Africa has experienced very little starvation, in spite of rapid population growth. "It's a major achievement to be feeding three times as many people as there were 25 years ago," and with less rain.

But the challenge, he continues, is that most of the big-ticket food aid the region has received has been aimed at farming for export, instead of food production for local consumption. As a result, food imports have grown. And efforts to improve resilience to drought have been piecemeal – what some have dubbed "a thousand points of green" – rather than a coordinated regionwide effort.

For aid agencies, as well as for local and regional officials, the paper represents a shot across the bows, suggests Mohamed Bakarr, a senior environment specialist at the Global Environment Facility in Washington. GEF is a multinational organization that helps finance projects that help developing countries meet their obligations under several international environmental treaties.

"We're not talking about small decisionmaking here," in response to what the climate can dish out, he says. The consequences of a drought on the scales Shanahan and colleagues have identified "will be catastrophic."

The study owes its scientific success to a meteor, which smacked into the African continent 1.07 million years ago in what is now Ghana. The crater filled with water to become Lake Bosumtwi, with a depth of 250 feet. Its bottom is deprived of dissolved oxygen, so nothing lives down there to stir up the muck. And the crater rim provides some shelter from winds that would stir things up. The result: a lake bottom consisting of neatly layered silt, each layer deposited by each year's seasonal monsoon.

The hunt for signals of dry and wet years led the scientists to look for changes in the concentration of minerals and metals in the lake-bottom sediment layers. During dry periods, lake levels fall. That exposes more of the crater rim to the erosive effect of any rain that does fall. So dry periods are marked by sediments containing high concentrations of aluminum, silica, potassium, calcium, titanium, and iron. With more rain, the lake level rises, reducing the rim's exposure and reducing the mount of metals and minerals in the sediment that form a new layer.

"It's a fantastic archive," Shanahan says.

Countries in the region have made some progress in learning the lessons of the last major drought, Dr. Bakarr says. After the drought in the Sahel in the 1970s and '80s, nine countries in the region formed the Interstate Committee for the Fight Against Drought in the Sahel. It's based in Ouagadougou, Burkina Faso, and focuses on developing sustainable approaches to farming and water use, as well as drought forecasting.

Bakarr notes that projects in the region are encouraging farmers to feed livestock in pens, rather than allowing them to graze open land and destroy vegetation – a factor in the desertification of the Sahel during the last major drought. Improved water-storage techniques, as well as encouraging farmers to plant trees in their fields to shade crops, are other arrows in the drought-resilience quiver

But in the face of droughts lasting a century or more, the arrows are likely to fall far short of their target.

The scale and magnitude of the preparations needed to deal with the region's now-clear drought patterns "require the kinds of investment to match what is happening in the health sector or in the food-security sector," Bakarr  says.  "The development community needs to see these kinds of calls as requiring major commitments and major investments, and not just a mere policy response."