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Canadians afflicted with seasonal allergies have good reason to be concerned about climate change. A new study suggests that generally warmer fall weather has extended the life of ragweed plants in Canada. As a result, allergy-inducing ragweed pollen lingers in the air for a longer period of time each year. And the change is most noticeable the further north you go.

"For allergy sufferers, it means that people who live in areas where the ragweed season tended to be shorter and less intense, like Canada, are likely to experience a longer allergy season and worse symptoms," said one of the researchers, Jay Portnoy of Children's Mercy Hospitals & Clinics in Kansas City, Missouri.

Ragweed pollen provokes sneezing, watery eyes and a scratchy throat in about 10 per cent of the population and just makes people feel miserable.

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For their study, the researchers analyzed about 15 years of pollen data from 10 locations along a south-north transect from Georgetown, Texas, to Saskatoon, Saskatchewan. They also looked at the number of frost-free days and delays in the onset of the first fall frost - which kills ragweed plants and brings the allergy season to an abrupt halt.

During the study period, 1995 to 2009, the allergy season actually shrank by a few days in the most southern locations. (Ragweed plants pollinate best when it's warm, but not too warm.) However, the allergy season got progressively longer with each location further north, according to the findings published in the Proceedings of the National Academy of Sciences.

For instance, in Papillion, Nebraska, the season was 11 days longer in 2009, compared to 1995. In Minneapolis, Minnesota, it grew by 16 days. The most dramatic effects were seen in the two most northern study locations; in Winnipeg, Manitoba, the ragweed season became 25 days longer, and in Saskatoon it lengthened by 27 days.

"These results are consistent with the projections of the Intergovernmental Panel On Climate Change," said he lead researcher, Lewis Ziska, a plant physiologist with the U.S. Department of Agriculture in Beltsville, Maryland. He noted the United Nation's advisory group concluded that climate change will increase temperatures to a greater extent at higher latitudes. In other words, areas closer to the poles will experience a disproportionately larger impact of global warming.

In this case, that means areas in the far north used to have their ragweed season cut short by an early frost. Now those same areas have a longer allergy season because the first frost has been delayed by milder fall temperatures.

"This study sheds light on just how quickly the world is changing - and those changes have a lot of health consequences," said one of the study authors, Kim Knowlton, a senior scientist at the Natural Resources Defense Council in New York.

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