To test this hypothesis, Ziska and his colleagues reviewed 15 years of pollen data from eight locations in the United States and two in Canada.
Two out of three southern locations -- Georgetown, Texas, and Rogers, Ark. -- experienced shorter ragweed seasons by three or four days in 2009 compared to 1995. Oklahoma City saw a one-day increase in its ragweed season during the same time period.
As the researchers went up in latitude from there, however, the length of ragweed season increased dramatically from 1995 to 2009. Papillion, Neb., had an increase of 11 days, while two spots in Wisconsin -- Madison and LaCrosse -- saw an increase in their ragweed season of 12 and 13 days, respectively.
Minneapolis and Fargo, N.D., saw their ragweed season increase in length by 16 days. And, in Canada, Winnipeg's season lengthened by 25 days, and allergy sufferers in Saskatoon were sneezing for 27 extra days.
"If this trend continues, and we see greater warming to the poles, allergy seasons will be different and, for some, they'll be extended," said Ziska.
So, what does this potentially longer season mean for you if you live in a northern climate?
"For people who have experienced mild ragweed seasons, they may experience more problems than in the past," said Dr. Jay Portnoy, chief of the section of Allergy, Asthma and Immunology at Children's Mercy Hospitals and Clinics in Kansas City, Mo.
"It will be a longer season for those who had shorter seasons. And, pollen levels have increased, too. Not only is the total amount of pollen increased, but the pollen is more potent, too. Patients who have ragweed allergy will suffer more," he said.
"This may change the timing of preventive medications, and for physicians trying to diagnose allergic disease, they may need to change their assumptions about what's in the air," Portnoy explained.
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