A changing climate means allergy-causing ragweed pollen has a longer season that extends further north than it did just 16 years ago, U.S. scientists reported on Monday.

In research that gibes with projections by the U.N. Intergovernmental Panel on Climate Change, plant and allergy experts found that ragweed pollen season lasted as much as 27 days longer in 2009 than it did in 1995. The further north in the Western Hemisphere, the more dramatic the change in the length of pollen season.

Ragweed pollen can cause asthma flare-ups and hay fever, and costs about $21 billion a year in the United States, according to the study published in the Proceedings of the National Academy of Sciences.