Climate Change Is Driving Longer, More Severe Pollen Seasons, Study Finds

If it feels like your seasonal allergies have become more severe and longer-lasting in recent years, you may be right. And new research shows climate change is to blame.

In a study published last week in the Proceedings of the National Academy of Sciences, researchers found that on average, pollen seasons in North America lengthened by 20 days and contained 21% more pollen since 1990.

The culprit, they found, is increased atmospheric temperatures and levels of carbon dioxide. Researchers have studied the growing pollen season and the effects of warming temperatures on pollen production in controlled settings for decades. But this research makes clear that climate warming is responsible for these changes—scientists determined that human-forced changes in climate contributed to about half of the trend in pollen seasons and about 8% of the trend in pollen concentrations.

“A clearly detectable and attributable fingerprint of human-caused climate on North American pollen loads provides a powerful example of how climate change is contributing to deleterious health impacts through worsening pollen seasons,” the authors write in the study’s conclusion.

Why the Season Is Growing

The researchers tracked factors like temperature, precipitation levels, frost days, and atmospheric carbon dioxide concentrations to understand their impact on pollen production. Their models show that between 1990 and 2018, temperature had the strongest impact, accounting for 14% to 37% of the variance. Carbon dioxide concentrations had some detectable effect on pollen outcomes, but it doesn’t appear to be as strong of a driver for the change.

William Anderegg, PhD, assistant professor of biology at the University of Utah, tells Verywell that warming temperatures signal to plants that it’s time to produce and release pollen. With more warm days early on, pollen seasons simply grow longer.

Climate change “is kind of relaxing the constraints of plant physiology and really lengthening the growing season, which is also tightly coupled with the pollen season,” Anderegg says.

The changes in pollen season and concentration were most pronounced in Texas, the Midwest, and the Southeast U.S. Anderegg says he expected to see more pollen production in regions with the greatest rates of warming, such as the Great Lakes and New England areas. He hypothesizes that the particular plants that grow in the regions that saw the most change are those that are most sensitive to temperature change.

Keeping Tabs on a Warming Climate

The researchers collected data from 60 pollen monitoring stations managed by the National Allergy Bureau. Pollen counters collect and hand-count pollen samples, and researchers only included stations that have five or more years’ worth of data.

Aaron Bernstein, MD, MPH, interim director of the Center for Climate, Health and the Global Environment at the Harvard T.H. Chan School of Public Health, tells Verywell that unlike clinical or lab studies, in which scientists can manipulate a single variable, it’s difficult to parse out many interwoven factors when studying climate.

So, scientists turn to attribution models, which can indicate the degree to which natural events would be worse than they would have been in a world with no human-caused climate change. The researchers used these computer models to understand how trends in pollen seasons would differ without the effects of climate change.

As scientists continue to study changes in pollen seasonality, creating more robust pollen counting systems may grow increasingly important.

“I think it's really quite clear we need a lot more monitoring and measurement of pollen trends across the U.S., that if you compare how much we monitor pollen to any other airborne pollutants, we monitor it much, much less than other pollutants,” Anderegg says.

Impact on Health

The authors state that pollen allergies can affect the health of people with asthma, students’ ability to focus in school, and people’s susceptibility to respiratory illness. According to the Centers for Disease Control and Prevention (CDC), 19.2 million adults have been diagnosed with hay fever—an allergic reaction to pollen—in the last year.

Bernstein notes that there are available allergy medications that can provide rapid and effective relief to many people who suffer from pollen allergies. Still, for people who are unable to access medication or suffer severe asthma, greater exposure to pollen can be highly detrimental, especially when combined with other climate-driven factors.

“As with so many things at a larger scale with climate, this is one facet of a much broader suite of health concerns, and you can't look at them in isolation,” Bernstein says.

For instance, the same heat that’s prolonging the growing season is causing heatwaves and inducing air pollution. Each of these consequences has adverse health effects—heat can make it difficult to concentrate and may exacerbate medical conditions, and smog can take a toll on the lungs. Warming can even drive insect population changes, which then impacts outcomes like insect-transmitted disease and the population of certain species which can kill trees and induce wildfires. Altogether, these factors can compound the health effects of asthma and pollen allergies.

“I think this is a smoking gun of the health risk from climate change that is probably clearer than any other,” Bernstein says. “And yet it's just one signal among many that come from the same warming of the planet.”

Though this particular study doesn’t connect pollen production with health outcomes, the findings support years of research on asthma and allergy suffering due to pollen. Anderegg says he hopes to research how regional pollen trends are related to health outcomes, such as hospital admissions for asthma.

Bernstein says that for people with asthma and allergies, the effects of climate change can be immediate and dangerous. When crafting climate solutions, he says, people must envision short-term solutions to meet urgent health needs, in addition to long-term goals.

“We will always have challenges that are more immediate than what climate change will bring in decades to come,” Bernstein says. “And if we continue to do research about a world no one's living in yet, we will never, in my view, get the attention, engagement, and action needed to push forward the solutions to climate change that are there so urgently needed.”