Inconvenient truth: A warmer climate is creating longer, stronger allergy seasons.

RODALE NEWS, EMMAUS, PA—Given the millions of allergy sufferers held hostage by the drippy noses, burning, watery eyes, and continuous sneezing sessions it induces, ragweed may be one of the most hated plants on the planet. And a new the U.S. Department of Agriculture (USDA)-led study published in the Proceedings of the National Academy of Sciences confirms what many allergy sufferers and allergists have already been noticing—hay fever season caused by ragweed seems to be getting more intense and lasting longer.

The study is the latest to make the connection between climate change and a more potent allergy season. (Allergy-related issues cost the United States about $21 billion a year, so a warming planet affects economics, too.) "The main takeaway is that we are already seeing a significant increase in the season length of ragweed; and that this increase in season length is associated with a greater warming at northern latitudes, consistent with the Intergovernmental Panel on Climate Change projections regarding climate change," explains lead study author Lewis Ziska, PhD, research plant physiologist with USDA's Crop Systems and Global Change Lab.

THE DETAILS: Researchers used ragweed pollen and temperature data recorded between the late 1990s and 2005 in 10 different locations in the U.S. and Canada and found that in all but two of the areas analyzed, the ragweed pollen season increased—in some cases by nearly a month. The lengthening of the allergy season coincides with an increase in warmer, frost-free days. Researchers noticed a general trend—the ragweed allergy season grew longest in the higher latitudes of the northern United States and Canada. Winnipeg, Ontario, allergy sufferers endured a 27-day-longer ragweed pollen season in 2005 compared to just 16 years earlier. In the U.S., Fargo, ND, and Minneapolis, MN, experienced a more than two-week increase in ragweed allergy season, with LaCrosse and Madison, WI, not far behind.