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A Changing Climate Worsens Allergy Symptoms

Why Global Warming Is Like Miracle-Gro for Weeds

For some of us, the yellow blossom of a daffodil and opening day at the ballpark are the first signs of spring. For about 40 million Americans, spring also means sneezing and congestion, a box of tissues, and a trip to the doctor. Unfortunately, for these hay fever sufferers, the suffering will likely get worse.

Researchers have found that changes in climate impose additional strains on those with pollen allergies. (Think weeds on Miracle-Gro.)

Three main factors related to climate change fuel increases in allergens. Carbon dioxide, the heat-trapping gas that is the primary cause of our warming planet, increases the growth rate of many plants and increases the amount and potency of pollen. Rising temperatures extend the growing season and the duration of allergy season. And an extended spring season alters the amounts of blooms and fungal spores that are known to exacerbate allergy symptoms.

While this is bad news for allergy sufferers, there is a flip side, too. Weed growth may provide clues on how to improve the production of certain crops.

"One way we have been getting at this is to look at cities as surrogates for climate change. You already have higher temperatures and higher carbon dioxide levels [in cities]. That in itself is a harbinger of things to come," said Lewis Ziska, Ph.D., a weed ecologist at the Agriculture Research Service division of the U.S. Department of Agriculture. "Spring appears to be coming earlier, and this is affecting the tree pollen, which is a main source of spring hay fever."

Ziska is known for his study of weeds in future warming scenarios (with higher atmospheric concentrations of carbon dioxide) projected for Earth. Ziska is eager to point out that carbon dioxide is blind: it does not discriminate between the plants that humans generally like (rice, forest trees, wheat) and plants we hate (ragweed, poison ivy). In fact, weeds often outpace the growth of useful plants when extra carbon dioxide is available.

In more than 20 years of study, Ziska has examined plant traits under all sorts of circumstances. He has looked at growth rates, bloom times, pollen production, and other factors under varying air temperatures, carbon dioxide levels, and moisture levels. His experiments on plants in Baltimore, MD, mimic the climate Americans will likely encounter by mid-century if heat-trapping emissions continue unabated: a temperature increase of three to four degrees Fahrenheit and a concentration of heat-trapping gases of 450 parts per million. Baltimore already has higher local emissions from transportation and manufacturing, and higher temperatures due to the urban heat-island effect, compared to rural areas of Maryland. 

Ziska found that these conditions can have freakish results. Weeds that grew five and six feet tall in the country had counterparts in the city that were 10 and 20 feet tall. Ragweed, specifically, grew faster, flowered earlier, and produced significantly greater pollen.

In order to understand the importance of changes in carbon dioxide on plant growth, think back to your elementary school science classes. Carbon dioxide is among the four fundamentals needed for plant growth; the others include water, nutrients, and light. Carbon dioxide is so good at producing extra nutrients for some plants, including ones like ragweed that are most problematic for hay fever, that even a small increase will have a big effect.

So, what does this mean for public health? "The influence of climate change on plant behavior exacerbates or adds an additional factor to the number of people suffering from allergy and asthma," Ziska said. The intersection of climate change and health is something that epidemiologists are just beginning to analyze closely, he added.

Left unchecked, rising carbon dioxide levels and further warming pose serious risks for allergy sufferers.  While some further warming over the next few decades is locked in, swift and deep reductions in heat-trapping emissions will benefit not only allergy sufferers, but all who are impacted by a changing climate.


This is part of a series of articles meant to strengthen the communication of climate science, by scientists, to the American public.  The series includes short articles on scientific terminology, the impacts of climate change, and new discoveries by leading researchers.  Some are listed in "Related Articles" on the right side of this page.

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