Featured Research

from universities, journals, and other organizations

Mega-droughts In Sub-Saharan Africa Normal For Region: Droughts Likely To Worsen With Climate Change

Date:
April 17, 2009
Source:
National Science Foundation
Summary:
Some droughts lasted centuries in the past, and a warming planet may make future droughts more devastating. 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.

Large, often barren, tropical trees stand where they once grew when the area was in severe drought and water levels in Lake Bosumtwi in Ghana had bottomed out. Submerged in 15-20 meters of water, the trees are stark reminders of severe, long lasting dry spells from just a few centuries ago. In this photo, a partially submerged tree is surrounded by boys from nearby villages who still practice traditional fishing methods on the lake.
Credit: Photograph by J.T. Overpeck and W. Wheeler, University of Arizona

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.

Related Articles


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.


Story Source:

The above story is based on materials provided by National Science Foundation. Note: Materials may be edited for content and length.


Journal Reference:

  1. Timothy M. Shanahan, Jonathan T., Kevin Anchukaitis, J. Warren Beck, Julia E. Cole, David Dettman, John A. Peck, Christopher A. Scholz, and John King. Atlantic forcing of persistent drought in West Africa. Science, April 17, 2009

Cite This Page:

National Science Foundation. "Mega-droughts In Sub-Saharan Africa Normal For Region: Droughts Likely To Worsen With Climate Change." ScienceDaily. ScienceDaily, 17 April 2009. <www.sciencedaily.com/releases/2009/04/090416144520.htm>.
National Science Foundation. (2009, April 17). Mega-droughts In Sub-Saharan Africa Normal For Region: Droughts Likely To Worsen With Climate Change. ScienceDaily. Retrieved December 19, 2014 from www.sciencedaily.com/releases/2009/04/090416144520.htm
National Science Foundation. "Mega-droughts In Sub-Saharan Africa Normal For Region: Droughts Likely To Worsen With Climate Change." ScienceDaily. www.sciencedaily.com/releases/2009/04/090416144520.htm (accessed December 19, 2014).

Share This


More From ScienceDaily



More Earth & Climate News

Friday, December 19, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Navy Unveils Robot Fish

Navy Unveils Robot Fish

Reuters - Light News Video Online (Dec. 18, 2014) — The U.S. Navy unveils an underwater device that mimics the movement of a fish. Tara Cleary reports. Video provided by Reuters
Powered by NewsLook.com
Arctic Warming Twice As Fast As Rest Of Planet

Arctic Warming Twice As Fast As Rest Of Planet

Newsy (Dec. 18, 2014) — The Arctic is warming twice as fast as the rest of the planet, thanks in part to something called feedback. Video provided by Newsy
Powered by NewsLook.com
Prenatal Exposure To Pollution Might Increase Autism Risk

Prenatal Exposure To Pollution Might Increase Autism Risk

Newsy (Dec. 18, 2014) — Harvard researchers found children whose mothers were exposed to high pollution levels in the third trimester were twice as likely to develop autism. Video provided by Newsy
Powered by NewsLook.com
Ivory Trade Boom Swamps Law Efforts

Ivory Trade Boom Swamps Law Efforts

Reuters - Business Video Online (Dec. 17, 2014) — Demand for ivory has claimed the lives of tens of thousands of African elephants and now a conservation report says the illegal trade is overwhelming efforts to enforce the law. Amy Pollock reports. Video provided by Reuters
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
 
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:  

Breaking News:

Strange & Offbeat Stories

 

Plants & Animals

Earth & Climate

Fossils & Ruins

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:  

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile iPhone Android Web
Follow Facebook Twitter Google+
Subscribe RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins