Bad news for those who suffer their way through ragweed season: A team of researchers has found that increased warming, particularly in the northern half of North America, has added weeks to the fall pollen season. A changing climate means allergy-causing ragweed pollen has not only a longer season but one that extends further north than it did just 16 years ago.
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.
Minneapolis has tacked 16 days to the ragweed pollen season since 1995; LaCrosse, Wisc. has added 13 days, Winnipeg and Saskatoon in Canada have added 25 and 27 days, respectively.
The new research, published Monday in the journal Proceedings of the National Academy of Sciences, finds the longer pollen seasons correlate with the disproportionate warming happening around the planet and attributed to greenhouse gas emissions.
Upper latitudes are warming faster than mid-latitudes, and the pollen season is lengthening in proportion. Scientists and health officials found no appreciable warming in Texas, Arkansas or Oklahoma.
The impact goes far beyond mere sniffles and inconvenience. Some 50 million Americans have allergies, according to the Asthma and Allergy Foundation of America. Of those, 35 million suffer nasal allergies, known broadly as hay fever, said Mike Tringale, the association's vice president.
For 75 percent of those 35 million, ragweed (right) is the primary allergen, he added. And in many cases, allergies can trigger a bout of asthma, or make it worse.
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.
"This is not something that's hypothesized, this is not something that's modeled, this is not something that may or may not occur depending on the math that you do," said study author Lewis Ziska of the U.S. Department of Agriculture crop system and global change laboratory. "This is something that we're actually seeing on the ground in recent years."
Even in places where ragweed season didn't lengthen or even shortened slightly -- such as Texas, Oklahoma and Arkansas -- there was lots more pollen, which caused more intense symptoms, said co-author Dr. Jay Portnoy of the Allergy, Asthma and Immunology Section at Children's Mercy Hospital, the University of Missouri-Kansas City School of Medicine.
Ragweed is probably not the only pollen likely to have a longer season as the planet warms, Portnoy said in a telephone interview.
"We used ragweed as a marker but it's probably true for other pollens too," he said, including tree pollen that causes allergy symptoms in the U.S. spring. (Right: pollen seasons. click to see enlarged image)
Ragweed pollen was a reasonable marker because its season is naturally easy to track.
It's what's known as a short-day plant, which begins blooming when the days start getting shorter, that is, after the Northern Hemisphere summer solstice around June 21. It stops flowering with the first frost.
As global average temperatures have warmed, the first frost has been delayed, especially at higher latitudes, which has meant a longer season for ragweed. Because warming is greater at these high latitudes, the length of the season has been more pronounced.
The findings correlate with analysis last year by the National Wildlife Federation that found ragweed growth rates and pollen counts increased with global warming. In one study, accelerating spring's arrival by 30 days prompted a 54 percent increase in ragweed pollen production.
The danger with a lengthening season - and perhaps a more intense one - is pollen's potential to overwhelm immune systems that, up till now, have withstood the onslaught, Tringale said.
Much as water in a bathtub is not a problem until it starts to overflow, pollen for many is not an irritant until it crosses a particular threshold, he said.
"With the longer season, with the creeping breadth of the geographic footprint of the season, and with more powerful plants producing more pollen, it's a triple threat," he added. "Now you've got yourself a much wider population that could potentially be affected that might not have been affected before.'
Daily Climate,"Allergy season is extending, scientists find",accessed February 21,2011
Reuters,"Climate change creates longer ragweed season", accessed February 21, 2011
Scientific American, "Climate Change Extends Allergy Season in North America", accessed February 21, 2011