Global warming may stretch allergy season

You’re not imagining that sneezy nose and those itchy eyes: Allergies have become increasingly prevalent in the last three decades. Researchers point to one possible factor (when it comes to hay fever, at least): climate change.

Recent increases in the length of the ragweed pollen season are associated with warming, according to a study released Monday by the Proceedings of the National Academy of Sciences.

The researchers looked at pollen counts and weather data from 10 locations spanning more than 1,300 miles of the central part of North America – from Georgetown, Texas, to Saskatoon, Canada.

They determined that from 1995 to 2009, as latitude increased, so did the frost-free period – and the number of days of the pollen season.

Five sites, extending north from LaCrosse, Wis., to Saskatoon had increases in the length of the ragweed pollen season of at least 13 days. Saskatoon’s increase was 27 days.

The trend tracks Intergovernmental Panel on Climate Change projections, as well as shifts in plant hardiness zones in the upper Midwest, the authors reported.

Weed season for most of the U.S. is in the summer and fall. At least 10 percent of the U.S. population is ragweed sensitive. The study cited one report that ragweed may cause more seasonal allergic rhinitis – hay fever – than all other plants combined.