Millions may be allergic, literally, to global warming.
Ragweed pollen season in North America has grown longer in the last 15 years, finds a new study, particularly at northern latitudes, in a pattern consistent with climate change projections.
Many studies show that spring is arriving earlier as global temperatures rise, but researchers have been uncertain whether the pollen season would just shift earlier as a consequence, or whether it would lengthen.
The new study shows that, at least for ragweed, the most common plant allergen, a longer pollen season is in the cards, with greater changes occurring further north. What's more, the changes are already happening.
"In looking at a number of the locations that collect pollen, some were showing an increasing in season length and some not," said study lead author Lewis Ziska of the United States Department of Agriculture's Agricultural Research Service in Beltsville, Md.
The team wanted to solve this puzzle, so they looked at a string of pollen-counting stations running through the North American Midwest from near Austin, Texas in the south to Saskatoon, Saskatchewan in Canada.
The team found that north of 44 degrees latitude -- near LaCrosse, Wisc., the pollen season was significantly longer in 2009 than in 1995. Ragweed pollen was in the air for 13 days more in LaCrosse and 27 days more in Saskatoon in 2009 versus 1995.
"I was surprised to see the signal as strong as it was. I thought we might see something fairly weak, but I was surprised that even in the last couple of decades we could see things," Ziska said.
"It's the first work that I'm aware of that looks at season lengthening as opposed to just earlier spring, getting at that the length of time that we're exposed to pollen," said Perry Sheffield of Mount Sinai School of Medicine.
"We know that exposure to pollen exacerbates disease and also can cause sensitization in people that aren't yet allergic, so more exposure is bad," said Patrick Kinney of Columbia University's Mailman School of Public Health. "This is suggesting that the length of the season has gone up and therefore that exposure has gone up."
The study finds that the longer pollen season is associated with greater numbers of frost-free days each year, and a later first fall frost. The findings are consistent with what the Intergovernmental Panel on Climate Change projects for climate change, the authors note, particularly increased warming at higher latitudes.
One in 10 Americans test positive for ragweed sensitivity, the authors note, and allergies have risen in the United States and elsewhere over the last 30 years.
While extra days of misery or potentially serious symptoms like asthma attacks are enough to make the new findings bad news for allergy sufferers, the authors also cite the Centers for Disease Control's estimate that "allergic disorders" cost Americans $21 billion annually.
While ragweed is the most common plant allergen, trees, grass and other weeds also contribute. Trees are difficult to track in the same way, but Ziska plans to look at other weeds and at grasses in future work.
"This is an outstanding piece of research," said Paul Beggs of Macquarie University in Australia. "This research adds to the mounting evidence that one of the most important impacts of climate change on human health will be the adverse impacts on allergic respiratory diseases such as allergic rhinitis (hayfever) and asthma.
"Indeed, the significant lengthening of the ragweed pollen season, particularly in the higher latitudes of North America, over the last 15 or so years, revealed in this research, adds to the likelihood that climate change has for some time now already been having an adverse impact on human health."