A new study of lake sediments in Ghana suggests that severe droughts lasting several decades, even centuries, were the norm in West Africa over the past 3,000 years.
The earlier dry spells dwarfed the well-documented drought that plagued West Africa in the late-20th century, and as the planet warms, the study's authors believe the region's rainfall patterns will have an even greater impact.
The team of geoscientists and climate scientists, led by Jonathan Overpeck of the University of Arizona and his former doctoral student, lead author Timothy Shanahan, who is now at the University of Texas (UT) at Austin, announced their findings in the April 17, 2009, issue of Science.
Because of close agreement amongst several data sets, the scientists believe the droughts are driven in part by circulation of the ocean and atmosphere in and above the Atlantic--and possibly beyond. If climate models for such circulation patterns hold true, the study suggests global warming could create conditions that favor extreme droughts.
"Clearly, much of West Africa is already on the edge of sustainability," says Overpeck, "and the situation could become much more dire in the future with increased global warming."
The findings emerged from sediments that lie at the bottom of Lake Bosumtwi in Ghana, deposits of soil and organic matter that contain annual bands of light (winter) and dark (summer) layers that stretch back more than three millennia.
"Lake Bosumtwi is really unique in that its one of the few locations in tropical West Africa where varves, annual sediment layers, are preserved. This allows us to look at changes in climate at very high resolution," said Shanahan, now an assistant professor at UT.
Added Overpeck, "The instrumental record of climate is just too short to understand how climate changes in Africa, and the lake sediments provide a fantastic way to put the short instrumental record--including the iconic Sahel Drought of the late 20th century--into a much longer perspective."
Oxygen (O) isotopes in calcium carbonate from the sediment provided a detailed record of dry and wet periods. Higher concentrations of common 16O indicated greater rainfall, while higher concentrations of slightly heavier, and therefore harder to evaporate, 18O indicated periods of drier conditions and drought.
"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," added Shanahan.
The researchers correlated the oxygen record from their sediment cores with concentrations of elements such as aluminum, potassium, silicon and iron that come from broken-down minerals in soil. The elements reveal drought conditions because during dry periods, the lake became smaller, exposing more soil and enabling it to wash in, and increasing the concentrations of the soil elements.
The resulting data sets show periods of dryness, particularly droughts in the 30-40 year range, that correlate to fluctuations in sea surface temperatures, a pattern called the Atlantic Multidecadal Oscillation (AMO). The oscillation has never been confirmed over long time periods, but computer simulations and several data sets, including tree-ring variations from sites around the West Atlantic, have hinted at the scenario.
"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," noted Overpeck
No existing evidence for the AMO had the breadth of the 3,000-year column of lake sediments that the new study includes. While the new data also may reflect some patterns resulting from other sea surface temperature patterns, such as Atlantic Niños and even the familiar El Niño events in the Pacific, the AMO correlation is the strongest and holds promise as a leading cause of the West African droughts.
The lake's sediment record is also punctuated by less frequent, but much more severe, century-long drought events. Because of the size and duration of those events, their impact would have been much more severe than the multi-decade droughts linked to the AMO.
"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," said Shanahan. "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."
As global temperatures increase, the oceanic and atmospheric circulations that control the AMO may change. The new study suggests such changes could lead to conditions that in the past three millennia caused the most severe droughts, and because of global warming, the droughts could be even hotter when they return.
"To reduce uncertainties, current climate models need more data from both high- and low-latitude field work, and from periods that extend back well beyond the instrumental record," said Paul Filmer, the NSF program director who funded the recent study. "This project is a good example of how work in the tropics on sediment records provides more detailed insight into climate patterns that affect millions of people in a highly vulnerable area of the world."
Shanahan and Overpeck's University of Arizona co-authors are J. Warren Beck, Julia E. Cole and David Dettman. Researchers from Lamont-Doherty Earth Observatory in Palisades, N.Y., the University of Akron, Syracuse University, the University of Rhode Island, Nkrumah University of Science and Technology in Kumasi, Ghana, and the Ghanaian Geological Survey also participated.
This research was supported by NSF grants 0601998, 0401908, 0214525, 0096232 and IGERT award 0221594.
Joshua A. Chamot | EurekAlert!
Further reports about: > African elephant > Aluminum > Amo > Atlantic Multidecadal Oscillation > Bosumtwi > El Niño > Iron > Lake Baikal > Organic Matter > Science TV > Silicon > climate models > computer simulation > dry spells > global temperature > global warming > greater rainfall > higher concentrations of slightly heavier > potassium > sea surface > sea surface temperature > warming planet > well-documented drought
Europe’s Demographic Future. Where the Regions Are Heading after a Decade of Crises
10.08.2017 | Berlin-Institut für Bevölkerung und Entwicklung
Scientists reveal source of human heartbeat in 3-D
07.08.2017 | University of Manchester
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
18.08.2017 | Life Sciences
18.08.2017 | Physics and Astronomy
18.08.2017 | Materials Sciences