Many Canadians with ragweed allergies face nearly an extra month of suffering as the climate warms, a new study shows.

Between 1995 and 2009, the ragweed pollen season grew by 25 days in Winnipeg and 27 days in Saskatoon, according to a paper published this week in Proceedings of the National Academy of Sciences.

Over that 15-year period, other areas of North America above 44 degrees latitude (which runs just south of Madison, Wis.) saw their ragweed pollen seasons expand by at least 13 days, according to the research led by Lewis Ziska at the United States Department of Agriculture in Beltsville, Md.

In general, the ragweed season expanded more at sites farther north.

The study linked the extended season to a delay in the first frost and the longer frost-free period of the year.

Between 1995 and 2009, the ragweed pollen season lengthened by 25 days in Winnipeg and 27 days in Saskatoon. (iStock)Ragweed causes hay fever and asthma among people who are allergic to it.

Previous studies have shown that about 10 per cent of the population in the U.S. is sensitive to ragweed pollen and that allergies result in U.S. health-care costs reaching $21 billion each year.

Researchers had already become aware that milder winters cause many plants to flower earlier, but had not figured out whether that resulted in a longer pollen season.

Ziska's study relied on ragweed pollen counts taken by the National Allergy Bureau in the United States and Aerobiology Research Laboratories, based in Ottawa, at eight American sites and two Canadian sites between Texas and Saskatoon. That was linked to weather data from U.S. weather stations and from Environment Canada.