Richard Black, BBC News 16 Apr 09;
Severe droughts lasting centuries have happened often in West Africa's recent history, and another one is almost inevitable, researchers say.
Analysis of sediments in a Ghanaian lake shows the last of these "megadroughts" ended 250 years ago. Writing in the journal Science, the researchers suggest man-made climate change may make the situation worse.
But, they say, the droughts are going to happen again anyway, and societies should begin planning for them.
"It's disconcerting - it suggests we're vulnerable to a longer-lasting drought than we've seen in our lifetime," said Tim Shanahan from the University of Texas in Austin, who led the research team.
"If the region were to shift into one of these droughts it would be very difficult for people to adapt; and we need to develop an adaptation policy."
The region's most recent dry episode was the Sahel drought which claimed at least 100,000 lives, perhaps as many as one million, in the 1970s and 80s.
But the historical "megadroughts" were longer-lasting and even more devoid of precipitation, the researchers found.
Deep impact
The evidence comes from Lake Bosumtwi in southern Ghana, a deep lake formed in a meteorite impact crater.
Sediments laid down each year form neat, precise layers.
"Nothing lives at the bottom of the lake, so nothing disturbs these layers," said Professor Shanahan.
"Most lakes have this seasonal deposition, but it's rare in the tropics to find a lake where the bottom is undisturbed."
Wet and dry years are distinguished by the ratio of two oxygen isotopes in the sediment.
Droughts lasting a few decades occur regularly over the 3,000 years contained in this record.
They appear to be linked to the Atlantic Multidecadal Oscillation (AMO), a natural climatic cycle in which sea surface temperatures in the tropical Atlantic Ocean vary over time.
The Sahel drought coincided with a cool phase of the AMO. This changes wind patterns, and decreases the strength of the monsoon rains in this region.
However, the cause of the longer, multi-century droughts is not clear.
"That's one of the scary aspects - we have no idea what causes them," said Jonathan Overpeck from the University of Arizona, who oversaw the research effort.
"In Africa, we could cross the threshold, driving the system into one of these droughts, without even knowing why."
Money flows
Michael Schlesinger, who first characterised the AMO a decade ago but was not involved in the current study, suggested a similarity between the outlook for West Africa and the southwestern portion of the US.
There, research has also shown a history of shorter and longer droughts.
"There are two things that need to be done, one of which California and Arizona and so on have done - and that is put in the water collection and distribution infrastructure to deal with the short periods of not very intense water stress," the University of Illinois at Urbana-Champaign scientist told BBC News.
"What West Africa won't handle - and neither will California - is the 100-year-long, deep megadrought.
"The only way I can see of dealing with that is desalination; if push comes to shove and these megadroughts appear - and they will, and it'll probably be exacerbated by man-made global warming - that will be the only thing to do."
Whereas the southwestern US could afford desalination, it is not clear that West African countries could - nor do they all have the infrastructure to move water inland.
The possibility of man-made climate change causing worse droughts is an example of the impacts that many developing countries fear, and which causes them to seek money from richer countries to protect their societies and economies.
Professor Schlesinger is at one with Tim Shanahan's team in suggesting that human-induced climate change would be likely to make droughts more severe, although computer models of climate produce varying projections for rainfall change over the West African region.
But even without changing the chances of drought, rising temperatures worsen the region's outlook, suggested Professor Overpeck.
"Even if we were able to reduce greenhouse gas emissions somewhat, we would still probably have warming in this region of about 2-4C over the century, and that could make droughts much harder to adapt to when they occur," he said.
"What it's pointing to is the need to reduce greenhouse gas emissions; but you can't do it all with mitigation, just as you can't do it all with adaptation."
Study Finds a Pattern of Severe Droughts in Africa
Andew C. Revkin, The New York Times 16 Apr 09;
For at least 3,000 years, a regular 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 reported in a new study.
That sobering finding, published in the April 17th issue of Science, 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 lakebed extended across a broad swath of West Africa.
More such mega-droughts are inevitable, the research team that studied the patterns said, although there is no way to predict when the next may unfold.
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 resulting from human-generated greenhouse gases was likely to exacerbate those droughts and that there was an urgent need to bolster the resilience of African countries in harm’s way.
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 multidecadal oscillation.
Typically over the last 3,000 years, a severe drought developed every 30 to 65 years, they researchers said. But several centuries-long droughts in the climate record, the most recent persisting from 1400 to around 1750, are harder to explain, they said.
While that extraordinary drought occurred during a cool spell in the Northern Hemisphere called the “little ice age,” other extreme droughts appear to have hit West Africa at points when the world was relatively warm over all.
In interviews, a range of independent experts on African climate and poverty said that the study underlined that it was important for developed countries to curb greenhouse gases to keep climate shifts around the globe in as manageable a range as possible.
But many stressed that the most urgent concern arising from the study was for the welfare of tens of millions of people with little capacity to endure today’s vagaries in rainfall, let alone epic dry spells.
“It’s a critical report,” said Kevin Watkins, the director of the Human Development Report office of the United Nations.
“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 said. “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, Mr. 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.”
To gather the data, the research team extracted cylinders of mud from the lakebed. The bottom of the circular lake, formed when a crater was blasted into the region one million years ago, has unusually fine layers of mud. Each layer represents a year’s accumulation, yielding a trove of chemical and physical clues to past temperatures and other conditions.
The team also studied wood samples from ancient dead trees that still poke from the lake’s surface, in areas that were exposed and forested during dry spells several centuries ago but are now under 45 to 60 feet of water.
Recent climate data from the lake analysis were compared with weather records from across the region, providing confidence that the lake record was a reasonable reflection of conditions elsewhere, according to the paper.
Richard Seager, a climate scientist at the Lamont Doherty Earth Observatory of Columbia University who has studied past extreme droughts in other dry areas, including the American Southwest, described the century-scale droughts revealed in the lake mud as “startling.”
He said the study showed that much more work needed to be done to refine computer simulations of climate so they could replicate such phenomena. Only then is there a chance that scientists can move toward predicting climate shifts reliably in particular regions and within specific time frames, he noted.
“The most pressing problem we now face is to predict climate in the near-term future — years to decades,” Dr. Seager said.
Mr. Watkins of the United Nations said that the urgency was multiplied by high population growth rates in West Africa. Just in the last century, when its populations were far smaller, periodic droughts in sub-Saharan African claimed hundreds of thousands of lives.
In an interview, Dr. Shanahan of the University of Texas said that the growing population density around Lake Bosumtwi itself, which is 20 miles southeast of Ghana’s second-biggest city, Kumasi, suggested the potential human impact of a seismic drought. (From 1972 to 1974, when Ethiopia’s population was around 31 million people, one million died in a severe drought, for example. Today Ethiopia has more than 70 million residents.)
“There was nothing between the lake and Kumasi when we first went there,” he said. “But three years later it’s a traffic jam.”
Africa trapped in mega-drought cycle
Catherine Brahic, New Scientist 16 Apr 09;
The infamous 1970s drought of the African Sahel region, which lasted several decades and killed more than 100,000 people, was actually a "minor" event, say researchers who have uncovered evidence that such droughts occur cyclically in the region and can be much more severe.
Timothy Shanahan and colleagues at the University of Texas, Austin, analysed the first rainfall dataset that spans several millennia. "What's disconcerting about this record is that it suggests the most recent drought was relatively minor in the context of the West African drought history," he told New Scientist.
The researchers analysed a sediment core pulled from the bottom of Lake Bosumtwi, Ghana's only natural lake. The lake is an ancient meteorite impact crater, making its levels very dependent on rainfall.
By studying the relative amounts of different oxygen isotopes in the sediment core, the team could reconstruct rainfall dating back 3000 years. Higher concentrations of the slightly heavier – and therefore harder to evaporate – 18O indicate periods of drought.
Dry for decades
They found that the region's history was punctuated by droughts lasting several decades, every 30 to 60 years. Each was comparable to the drought of the 1970s, which killed more than 100,000 people, according to UN estimates.
Alessandra Giannini of the International Research Institute for Climate and Society at Columbia University says that historical accounts of how centres of political power moved throughout the region over the millennia are consistent with periodic periods of drought.
But the sediment cores also revealed a more alarming pattern. As well as the periodic droughts lasting decades, there was evidence that the Sahel region has undergone several droughts lasting a century or more.
The most recent mega-drought was just 500 years ago, spanning 1400 to 1750 and coinciding with Europe's Little Ice Age. At the time, Lake Bosumtwi dropped so low for so long that a forest sprouted on the crater's edges. Those trees now stand in 15 to 20 metres of water (see images, right).
Close to the edge
"Clearly much of West Africa is already on the edge of sustainability," says Jonathan Overpeck of the University of Arizona, Tuscon, who was Shanahan's doctoral supervisor while the Lake Bosumtwi study was carried out. He believes the situation could worsen with climate change.
Several studies have suggested that fluctuations in the surface temperature of the North Atlantic Ocean are partly responsible for shifts in the African monsoon. Shanahan and colleagues found more evidence in support of that when they compared sea temperature records with the patterns in their sediment samples and found a strong correlation.
Some models forecast that changes to North Atlantic temperatures caused by global warming will dry out the Sahel even more. "If we were to switch into one of these century-scale patterns of drought, it would be a lot more severe, and it would be very difficult for people to adjust to the change," says Shanahan.
But Reindert Haarsma, a meteorologist at the Royal Netherlands Meteorological Institute, points out that there is still disagreement among climate scientists on whether the Sahel will become wetter or drier with climate change. African weather is among the least studied globally, so forecasts are extremely uncertain.
Journal reference: Science (DOI: 10.1126/science.1166352)
Climate change could worsen African "megadroughts"
Deborah Zabarenko, Reuters Yahoo News 16 Apr 09;
WASHINGTON (Reuters) – The recent decades-long drought that killed 100,000 people in Africa's Sahel may be a small foretaste of monstrous "megadroughts" that could grip the region as global climate change worsens, scientists reported on Thursday.
Droughts, some lasting for centuries, are part of the normal pattern in sub-Saharan Africa. But the added stress of a warming world will make these dry periods more severe and more difficult for the people who live there, the scientists said.
"Clearly, much of West Africa is already on the edge of sustainability, and the situation could become much more dire in the future with increased global warming," said University of Arizona climatologist Jonathan Overpeck, a co-author of the study published in the journal Science.
The Sahel is an area between the Sahara desert and the wetter parts of equatorial Africa that stretches across the continent from the Atlantic Ocean in the west to the Red Sea in the east.
Overpeck and his colleagues studied sediments beneath Lake Bosumtwi in Ghana that gave an almost year-by-year record of droughts in the area going back 3,000 years. Until now, the instrumental climate record in this region stretched back only 100 years or so.
The researchers found a pattern of decades-long droughts like the one that began in the Sahel in the 1960s that killed at least 100,000 people, as well as centuries-long "megadroughts" throughout this long period, with the most recent lasting from 1400 to 1750.
The scientists also described signs of submerged forests that grew around the lake when it dried up for hundreds of years. The tops of some of these tropical trees can still be seen poking up from the lake water.
RISING TEMPERATURES, NASTIER DROUGHTS
During the recent Sahel drought, the lake's water level dropped perhaps 5 yards (meters). By contrast, during megadroughts the level fell by as much as 30 yards (meters).
"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," said Timothy Shanahan of the University of Texas, a co-author of the study.
The most recent decades of data culled from Lake Bosumtwi show that droughts there appear to be linked to fluctuations in sea surface temperatures, a pattern known as the Atlantic Multidecadal Oscillation, or AMO, the researchers said.
"One of the scary aspects of our record is how the Atlantic ... changes the water balance over West Africa on multidecadal time scales," Overpeck said in a telephone briefing.
The cause of centuries-long megadroughts is not known, but he said the added burden of climate change could make this kind of drought more devastating.
Temperatures in this region are expected to rise by 5 to 10 degrees F (2.77 to 5.55 degrees C) this century, the scientists said, even if there is some curbing of the greenhouse emissions that spur climate change.
"We might actually proceed into the future ... we could cross a threshold driving the (climate) system into one of those big droughts without even knowing it's coming," Overpeck said.
(Editing by Will Dunham)
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