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Ragweed and Our Changing Climate

Friday, September 07, 2018


Eyes itching, nose running, no energy? It could be ragweed overload.



A scholarly article published in Environmental Health Perspectives  reports that millions of people suffer from seasonal allergies triggered by airborne pollen—not just in the spring, but in the fall, too. Growing evidence suggests their number are dramatically increasing with the changing climate.


The evidence points to a confluence of factors favoring longer growing seasons for the noxious weeds and other plants that trigger seasonal allergies and asthma attacks.


The article reports, “carbon dioxide (CO2), the principal global warming gas and also thought of as plant food—is also the source of carbon needed to make sugars during photosynthesis. When exposed to warmer temperatures and higher levels of CO2, plants grow more vigorously and produce far more pollen than they otherwise would.”


Physicians worldwide who treat allergic airway diseases are reporting an uptick in symptoms that they attribute to climate change. The World Allergy Organization, comprising 97 medical societies from around the world, reported that climate change is affecting the start, duration, and intensity of the pollen season, exacerbating the synergistic effects of pollutants and respiratory infections on asthma.


The ragweed season comes last, and it’s affecting scores of people today, possibly including the immune system-compromised folks who have never suffered from seasonal allergies before. It starts in late summer and persists until the plants die with the first frost. 


According to Lewis Ziska, PhD, our favorite research plant physiologist with the US Department of Agriculture (USDA), “The intensity of an allergic reaction depends on three interrelated factors: how much pollen a given species emits into the air, the duration of exposure, and the allergenicity of the pollen. In ragweed these factors coalesce in a perfect storm of allergic misery.”


Ziska also says, “What’s unique about ragweed is that it produces so much pollen—roughly a billion grains per plant, and the protein contained in the ragweed pollen coat is also highly reactive with the immune system.”


CO2 levels and pollen counts


Dr. Ziska started exploring potential links between pollen production, rising CO2 levels, and warming temperatures in the 1990s. His research proved that ragweed plants and their pollen output increased in tandem with climate change and rising CO2 levels.


Interestingly, recent studies suggest that pollen seasons for allergenic species, including ragweed, are lengthening more in the north than in the south, and more in cities than in the country. Cities are warmer because there is more pavement to hold the heat, and cities produce higher CO2 levels because of traffic, which also increases temperature. Therefore total counts of daily airborne pollen counts, including ragweed, are increasing.


The findings actually go right to the heart of the geographic complexities underlying climate change and its influence on biological systems. Ziska also explains that CO2 and atmospheric water vapor exert competing influences on warming trends—water vapor suppresses warming in wetter, rainier southern latitudes, in part by boosting cloud cover, while CO2 accelerates warming in dryer regions farther from the equator.


Ellen Troyer, with Spencer Thornton, MD, David Amess and the Biosyntrx staff


PEARL: Climate change, rising CO2 levels, plant biology, and public health implications will be a subject frequently covered in our Friday Pearls.