for National Geographic News
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 continueperhaps 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 growthafter 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.
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