While anxious farmers and ranchers in the Prairies await the winter snows to discover whether they will emerge from a two-year drought, scientists warn that far worse dry conditions likely lurk in Canada's near future.
Scientists predict the frightening possibility of a "megadrought" -- dry and dusty conditions that could endure for decades.
A megadrought is not the spawn of human-induced global warming; it reflects natural climate flips -- "regime shifts" -- that periodically and inexplicably occur.
Peter Leavitt, a biologist with the University of Regina, said that over the past 2,000 years, droughts have lasted for 45 years in Alberta, 70 years in Saskatchewan and 20 years in Manitoba.
He arrived at this by studying the effects droughts have had on the layers of algae found deposited on lake bottoms. The droughts are determined by examining changes in populations of algae varieties that occur as dry periods make rivers and lakes saltier.
"We estimate that there is 5- to 10-per-cent likelihood of a megadrought in the next 30 years," said Prof. Leavitt, whose research study at the university is titled Prairie Drought Project.
The prediction is based on a simple probability model that looks at the distribution of the long, dry periods in the past, then computes the statistical likelihood of a megadrought in the future.
One Alberta megadrought unearthed by Prof. Leavitt and his group occurred from 1690 to 1730.
While the most recent Prairies drought was estimated to have cost farmers $5-billion, simulations show that a megadrought price tag could hit $450-billion.
The odds of a dry period that does not reach quite the intensity of a megadrought are greater, however. In a recent presentation, Prof. Leavitt's group used newly uncovered historical data to estimate that over the next 30 years there is a 45-per-cent chance of a drought lasting five to 12 years.
Brian Cumming, a biologist at Queen's University who also studies lake-bottom cores, said "megaregime shifts" show that megadrought existed in Africa during the time it existed in North America.
Some of the effects of megadroughts on human populations were outlined this week to a group of science writers in St. Louis by David Stahle, a University of Arkansas professor. He presented evidence of tree rings formed during a drought that ravaged much of Mexico and the Western United States from 736 to 765.
It is likely, Prof. Stahle believes, that it also affected Canada. To illustrate the drought's severity, rainfall was 83 per cent of normal for nearly 30 years. During the six dry years of the 1930s, rainfall was 80 per cent of normal.
Two probable results of that megadrought were the collapse of the Mayan civilization in Central America and the depopulation of cities in what is present-day Mexico, Prof. Stahle said.
Although evidence of regime shifts is clear, scientists do not understand their cause.
Prof. Stahle said of his analysis that megadroughts appear to begin in Mexico and Central America, then migrate north. This suggests there might be a connection between megadroughts and La Nina, a cooling current in the southern and eastern tropical Pacific Ocean. However, he acknowledged that the changes could be caused by "a chain of bad luck growing out of a variety of different reasons."
Prof. Leavitt and colleagues at the University of Calgary and Queen's University will publish a paper outlining the extent of such changes over two millenniums.