LINDELL BEACH, B.C. – Scientists have confirmed that ragweed season is lasting longer and ending later in some parts of North America.
Ragweed pollen is lingering almost a month longer than it did in 1995, much to the frustration of allergy sufferers.
U.S. Department of Agriculture scientists found that these increases are correlated to seasonal warming shifts linked to the dynamics of climate change.
Research plant physiologist Lewis Ziska led a scientific team that looked at 10 locations with at least 15 years of local ragweed pollen count data from 1995 to 2009. The locations followed a north-south transect from Austin, Texas, to Saskatoon.
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The researchers, who published their results in the Feb. 22 issue of the Proceedings of the National Academy of Sciences,compared the pollen data from each site to data from the other sites, including latitude, the number of frost-free days and delays in the onset of the first fall frost.
The study did not look at actual pollen amounts between the sites because collection methods have changed through the years and so would not be comparable, said Ziska.
“However, the timing of pollen capture, i.e., the start and end of the season, is independent of instrumentation. Hence we could look at season length as a function of latitude.”
The ragweed plant lives for only one season, but one plant can produce millions of pollen grains.
The flowers mature and release pollen as summer wanes and nights grow longer. Warmth, humidity and local breezes after sunrise stimulate the release, which is usually the time of the highest pollen count.
While most pollen falls close to its source plant, winds can carry the pollen for hundreds of kilometres.
Ziska’s study found that the number of frost-free days at high-latitude study sites had increased from 1995 to 2009, as had the length of the ragweed pollen season.
The pollen season lasted 13 to 27 days longer than in 1995, and a longer ragweed pollen season was strongly correlated with a delay in the onset of the first fall frost. The shift in climate most likely affects other weeds that trigger allergic reactions as well.
He said longer growing seasons and warmer temperatures may be associated with more pollen production in amount and season length.
“We have just initiated a new project with the Environmental Protection Agency and Rutgers University to begin looking at other weedy species.”
He said plant-based allergens fall into three categories: trees in spring, grasses and weeds in summer and ragweed in fall.
“One of the biggest challenges in studying climate change is finding out how the plant kingdom is adapting to increases in air temperature and other meteorological phenome n a,” said Edward B. Knipling, administrator of the USDA’s Agricultural Research Service.
“Studies like this also show us that these ecological shifts don’t stop at crop production. They can also have a significant impact on public health.”
The research did not look at increasing levels of reported allergies and asthma conditions.
“We were very interested in trying to find whether reported allergies/ asthma (cases) were increasing with latitude as well but we couldn’t find the numbers,” said Ziska. “This is one of the issues we need to follow up on.”
The Asthma Society of Canada says respiratory allergies affect one in five Canadians and 80 percent of people with asthma also suffer from allergic rhinitis or sinusitis. They often overlap because several of the same allergens are known to trigger asthma.
Statistics Canada reported two years ago that eight percent of Canadians older than 12 said they had been diagnosed with asthma. That rate has changed little since 2001.
Asthma rates are highest in Atlantic Canada and lowest on the Prairies and in British Columbia.
Hospitalization of children suffering from asthma increased during the 1980s but started to decrease by the mid-1990s. However, the rate remained higher than it was in the 1970s.
Asthma kills more than 500 Canadians a year and 150,000 are hospitalized annually.
