Plains Drought Cycle Has Long History, Ominous Future

Brian Handwerk
for National Geographic News
June 14, 2005
Cyclical droughts have ravaged the United States' northern Great Plains
for thousands of years, a new study says. Scientists expect the
potentially devastating events to continue—perhaps with a boost
from global warming.

"These drought cycles have gone on pretty consistently throughout the last 4,500 years," said Jim Clark, an ecologist at Duke University's Nicholas School of the Environment and Earth Sciences in Durham, North Carolina. "They are pretty severe, so they have a large impact on the full set of ecosystem processes."

Many climatologists believe that devastating droughts like the 1930s Dust Bowl are not abnormal when viewed in larger historic context.

The new study reports that northern Great Plains droughts have recurred at roughly 160-year intervals. As in forest ecosystems, fire was a key player in the drought cycle and an important factor in regenerating plant life the Plains.

"As it starts to get dry, grass cover is lost," Clark explained. "There's no more fuel for fires, and as a result some pretty dramatic erosion occurs, which lasts for decades."

When moist climates reappear, so do prairie grasses, which help stem erosion. The newly returned grasses also provide fuel for fires. Fires, in turn, encourage further grass growth—after fires new grasses generally spring up in greater abundance.

Physical evidence of the cycle was found in sediment deposits at the bottom of North Dakota's Kettle Lake. Core samples taken there revealed layers of charcoal, plant fragments, and seeds. Scientists used a form of radiocarbon dating to determine the ages of the layers.

Layers of pollen and seeds indicated types and amounts of historic vegetation, while charcoal fragments showed the impact and extent of fire. Annual layers preserved in the lake's sediments span some 10,000 years. The team focused on analyzing only layers from the past 4,000 to 5,000 years.

The findings may be representative of past weather conditions throughout the northern Great Plains, a mixed-grass prairie region now characterized by intensive agricultural grazing. The area includes the Dakotas, eastern Montana and Wyoming, western Minnesota, and the adjacent Canadian areas of southern Manitoba, Saskatchewan, and Alberta.

Clark and his co-authors present their research in the current issue of the journal PNAS (Proceedings of the National Academy of Sciences).

Preparation and Predictions

The National Drought Mitigation Center helps people prepare and plan for droughts. Despite the center's focus on the future, the staff finds ancient data a useful tool.

"Whenever we have events at the rancher or farmer level, we try to include a paleoclimatology [the study of ancient climates] talk, and without fail it generates the most interest and questions," said climatologist Mark Svoboda from the center's University of Nebraska-Lincoln headquarters.

"When people learn of droughts in the 1600s that [were] shown to have had a major impact on the places where they live, it makes them realize the natural variability of climate," he said.

"The frequency of these occurrences is shown to have been there if you look at tree rings, lake sediments, ice cores, whatever," he added. "We can't dismiss that, and it opens some eyes."

Though future droughts are inevitable, the business of predicting them—even if they do come in cycles—is tricky.

Peter Leavitt, a biology professor at the University of Regina in Saskatchewan is at work in the Canadian prairies. He's reconstructing ancient climates in an attempt to forecast coming droughts.

"We're trying to identify any pattern in the past that will help us forecast the future," he said. "For example, people talk about '100-year floods' or '150-year floods.' We don't tend to do that with droughts, but there is no real reason why we shouldn't."

Global Warming

Despite the striking patterns that have emerged, the Kettle Lake drought cycles were irregular or even absent at times—the droughts don't always show up when the model says they should. As a result, the data don't exactly encourage short-term forecasting. "The long period of these fluctuations makes this kind of variability a bit difficult to deal with," Clark, the Duke ecologist, said. "In some ways it's predictable. It does have an actual period, but it's hard to adapt to variability on such a long time scale."

Rapid regional changes add further challenges. In recent centuries humans have irreparably altered the Plains ecosystem with agriculture and fire suppression.

The impacts on drought cycles are not yet known. Clark, though, suspects that drought cycles will continue.

"Drought cycles of the past show that rising aridity appears to have been enough to really increase erosion," he said. "These fluctuations in aridity due to natural forces are probably alive and well."

Many atmospheric models of global warming forecast dryer days ahead for continental centers. Though no one knows for sure, the change may worsen drought conditions.

"When you try to superimpose the human-caused aridity that we can expect with this sort of natural variability—extreme droughts every century or so—[it provides] all the more reason to be concerned.

The University of Regina's Leavitt suggests that people must prepare for recurring droughts regardless of the impacts of global warming.

"We've seen historic droughts in some cases five to seven decades long, with severity along the lines of [the Dust Bowl of] the 1930s," he said, noting that he expects such droughts to return. "They have occurred for the last several thousand years, so there's no reason to expect anything different."

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