Today the arid dust bowl years of the 1930s seem an anomaly. Now fields of grain cover the once parched plains of Oklahoma, Nebraska, the Dakotas, and other Plains states. But if Kathleen Laird is right, dust bowl conditions may be far from unusual for the Great Plains. Her research shows that the region has suffered repeated droughts for thousands of years, but the last 700 years have in fact been unusually wet.
Laird, an ecologist at Queen’s University in Ontario, reconstructed the Great Plains’ past climate by studying diatoms, water- dwelling algae marked by ornate microscopic silica cell walls. The researchers collected diatoms and water samples from 53 different lakes across the Great Plains. Since the saltiness of the lakes varied, Laird’s team could determine which species of diatom were most abundant at which levels of salinity.
They next looked for a small lake that had no streams running either into or out of it, which meant that any changes in its salinity over time would have been chiefly determined by rainfall and evaporation. A drier climate would mean more evaporation in a given year, increasing the lake’s saltiness; that in turn would determine the types of diatom most abundant in the lake.
Laird eventually picked Moon Lake in North Dakota for her study and began taking diatom samples from lake sediments that had accumulated since an ice age glacier gouged out the lake some 10,000 to 12,000 years ago. She then dated the sediment layers using radiocarbon methods and gauged the salinity by matching diatoms in the layers with their preferred salt levels.
She found that extreme dry spells not only persisted for centuries at a time but occurred much more frequently than they do today. The worst drought years were 200-370, 700-850, and 1000-1200.
Are the Great Plains overdue for another dry spell? I think that’s a little hard to say, says Laird. No one knows what caused the cycle of droughts in the past, and that humans are now altering the climate with greenhouse gases further complicates the issue for the future. We’re doing things to the climate today that never happened before, she says, so we don’t have a really good grasp of exactly what sort of climate patterns we’re going to get.