Twistin' by the Wheat Pool

It emerges from behind a shroud of heavy rain against a sky as black as ink—a whirling, roaring vortex that stretches to the earth from the belly of a huge thundercloud, toppling buildings, uprooting trees and scattering cars as it cuts a swath of destruction through the landscape.

More tornadoes are reported in Canada than anywhere else in the world except the United States. The 80-odd twisters spotted here each year do tens of millions of dollars in property damage, and pose a risk to human life. According to scientists with Environment Canada's Meteorological Service of Canada (MSC), the frequency of these violent events could increase in the future as a result of climate change.

Sixty per cent of Canada's reported tornadoes take place in the southern portions of the prairie provinces of Manitoba, Saskatchewan and Alberta—one of two "tornado alleys" in Canada that also include the southwestern region of Ontario. While analyzing meteorological data on the two regions to determine trends in tornado frequency, MSC scientists noticed that springtime temperatures in the West were significantly warmer in the 1980s than the previous three decades, and that tornado reports also increased during this period. They also noted that tornadoes in the region were occurring, on average, 11 days earlier—suggesting that tornado frequency is physically related to mean monthly temperatures.

In general, tornado frequency increases in the spring and early summer with rising temperatures, then decreases in the late summer and fall as temperatures drop. Circulation models have predicted that mean monthly temperatures in the southern prairies will increase by two or more degrees with the doubling of atmospheric carbon dioxide levels due to greenhouse gas emissions. If this happens, the inference is that more western tornadoes will occur in the "shoulder" months of May, June, August and September as they become more like July—the peak month for tornadoes—and that the number of tornadoes in July will also rise as that month becomes warmer.

The exact cause and effect of this relationship is not clear, as tornadoes result from a combination of numerous complicated meteorological factors—some related to temperature and moisture, known as thermodynamics, and others to wind strength and direction, or dynamics. What we do know is that tornadoes form in severe thunderstorms that contain strong updrafts of warm, moist air that rotate as they rise. If the rotation grows sufficiently strong, the storm can evolve into a tornado.

We also know that thunderstorms form in unstable air masses, and that air masses become more unstable the warmer and more humid the atmosphere. Therefore, as temperatures rise, the thermodynamics become more favourable for creating thunderstorms and other extreme weather events. This theory was supported by a study on the possible effects of climate change on global lightning frequencies, in which scientists using a general circulation model found that a more unstable atmosphere (caused by a doubling of atmospheric carbon dioxide) resulted in a 40-per-cent increase in lightning frequency in continental areas, with western Canada being highly affected.

The more scientists learn about the complex connections between climate change and severe weather, the better able Canadians will be to prepare for and respond to the impacts of these events in the future.

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