April 17, 2009

Evidence For Potential ‘Megadroughts’

The kinds of devastating droughts that claimed the lives of more than 100,000 people in Africa's Sahel region in the 1960's and 70's may represent a normal natural occurrence rather than an anomaly, according to new research.  What's more, they may be but a foretaste of the catastrophic "Ëœmegadroughts' that have periodically rocked the region for thousands of year.

The dry spells that have brought well-documented and well-publicized disaster to millions of Africans in the last 50 years pale in comparison to the droughts that have hit the region in the past, said a team of scientists in the latest issue of Science.

Their report also cautioned that increasing global temperatures could amplify the effects of future droughts, making life in the region scarcely sustainable.

"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 climatologist Jonathon Overpeck of the University of Arizona, one of the authors of the study.

The Sahel is the region of Africa located between the Sahara and the equator and stretching from the Atlantic Ocean in the west to the Red Sea in the east.

Overpeck and his colleague Timothy Shanahan of the University of Texas at Austin examined the annually deposited layers of sediment in Ghana's Lake Bosumtwi.  These well preserved layers of organic matter offer an almost year-by-year account of oscillating trends in rainfall, allowing the researchers to study drought patterns as far back as 3,000 years.

"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," explained Shanahan.

Prior to this study, climatologists and geoscientists had only been able to examine the climate record of the last 100 years with any real accuracy.

"The instrumental record of climate is just too short to understand how climate changes in Africa," added Overpeck. "[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."

In examining the sediment, the team looked at the varying levels of oxygen isotopes present in calcium carbonate deposits.  In years of heavy rainfall, the relative level of the lighter oxygen isotope 16O was found in higher concentrations.  In years of drought, the heavier 18O isotope is found in higher ratios because it evaporates more slowly than its lighter counterpart.

The group's data showed that the periodic recurrence of dry periods, generally lasting 30-40 years, corresponded to a pattern of changing surface sea temperatures known as the Atlantic Multidecadal Oscillation (AMO).

Though it has not yet been proven that the AMO has operated over longer time periods, other various data gathered in the area have corroborated the hypothesis that it likely does.

"This paper provides a long-term context suggesting that the Atlantic Multiodecadal Oscillation does actually exist," explained Shanahan. "Our rainfall records are strongly related to these really distant sea surface temperature reconstructions...It suggests that the rainfall patterns are being generated by the sea surface temperature patterns and not by some other influence."

"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," added Overpeck.


One of the more disturbing observations that the group was able to extract from the sediment samples was the periodic recurrence of less frequent, but much more severe century-long droughts.  During the shorter droughts, such as the one in the last century, water levels fell by some 5-10 meters; during the megadroughts measured from the sediment deposits, levels fell by as much as 30 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," commented Shanahan concernedly.  "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 temperatures around the planet steadily rise, the atmospheric and oceanic circulation patterns that cause the AMO could also change.  Researchers believe that such changes could cause a resurgence of the conditions that prevailed during the megadroughts of the last 3,000 years "“ perhaps even worse.

Scientists speculate that temperatures in the region could increase by as much as 5-10 degrees Fahrenheit in the next 100 years if global warming trends continue.

"We could cross a threshold driving the [climate] system into one of those big droughts without even knowing it's coming," Overpeck said.


Image 1: 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.

Image 2: Photograph of a sediment core taken from Lake Bosumtwi, Ghana, displaying annually deposited layers. These layers provide a high-resolution chronology for the sediments and a means of reconstructing past climate variations. Credit: Photograph by T.M. Shanahan, J.T. Overpeck, University of Arizona.

Image 3: Life emerges from an old tree stump growing out of the ancient lake bed at Lake Bosumtwi, Ghana. Now inundated with water, during the severe droughts of the last three millennia, the lake bed once hosted trees and other arid environment vegetation. Credit: Photograph by T.M. Shanahan, University of Arizona.


On the Net: