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The arrival of spring brings with it allergy season for millions of people around the world as flowering trees and plants release allergy-inducing pollens. Now, thanks to climate change, allergy season is about to get worse: The warming planet is extending the growing season, and along with it allergy-caused risks to human health.

By 2100, the amount of pollen produced during the flowering season could rise by 40 percent, according to new research published on Tuesday in Nature Communications––raising an urgent need to better understand the factors driving that increase. Even as drought and heat damage forests and grasslands, some grasses, weeds, and trees that produce allergy-inducing pollens thrive on rising temperatures and higher carbon dioxide concentrations, growing larger and producing more leaves.

Previous work examining historical trends estimates that, on average, the pollen season in North America arrives 20 days earlier and lasts eight days longer, and releases 20 percent more pollen into the air than it did 30 years ago.

The new research extends this picture to the end of the century, attributing a 40 percent increase in pollen produced to a pollen season that will begin as much as 40 days earlier and stretch out 19 days longer.

“Pollen has a huge impact on public health,” says Allison Steiner, an atmospheric scientist at the University of Michigan and senior author of the paper. “So many people are affected by seasonal allergies, yet predictive models for pollen are really not that good.”

To fill this gap, Steiner’s team developed models, based on historical data, that predict pollen emissions in response to factors like temperature and precipitation for 15 of the most common allergy-inducing plant taxa, or varieties. The models also account for increased atmospheric carbon dioxide, caused by climate change, as well as changes to the distributions of plant types over time. The expansion of grasses or trees, for example, might come at the expense of ragweed—a major allergen that has been projected to decrease by up to 80 percent in the eastern United States in that scenario.

“Our simulation looks at pollen emissions day by day,” Steiner says. “And you can see the progression—it starts in the Southeast, and then, as temperatures warm, the line of pollen production moves to the North.”

While many pollen-producing plants are expected to flourish, some types of pollen-producing trees, such as birch, won’t fare as well in a hot and carbon dioxide-rich environment—an effect that’s captured by this study.

Using these models, Steiner and colleagues then compared the amount of pollen released in the continental United States during a historical period (1995-2014) and two prospective climate scenarios (2081-2100): a middle-of-the-road greenhouse gas (GHG) emission future and a more extreme, fossil-fuel-driven one, representing an increase of 2-3 °C and 4-6 °C, respectively. The more dire scenario, in which carbon dioxide concentrations reach more than 2.5 times their current level, results in a protracted and more intense pollen season, doubling the historical trends of the past 30 years.

Perhaps more important, however, is the future predicted by the more moderate scenario, says William Anderegg, an associate professor at the University of Utah who was not involved with the research. “Basically, the pollen impacts decreased by half compared to the high-emission scenario—so the researchers really highlight how much tackling climate change will have critical benefits to our respiratory health,” notes Anderegg, who studies the effects of climate change on forest ecosystems.

The new study aligns with what allergist John James has observed firsthand. When James moved to Colorado 25 years ago, allergy season was largely confined to March and April. In the years since, that pattern has changed. “Patients started coming in earlier and asking, ‘Why are my symptoms lasting so long? I can’t seem to catch a break,’” says James, who consults for the Asthma and Allergy Foundation of America.

Increasingly punishing pollen seasons are a threat to global public health, multiple studies show.  Students with allergies perform worse than their peers in school; adults’ productivity at work suffers when hay fever attacks. At the same time, days on which pollen concentrations are highest have been linked to an uptick in emergency room visits for asthma, with associated costs to both individuals and healthcare systems.

The World Health Organization estimates that by 2050, half of the planet’s population will fall victim to at least one allergic disorder. Currently, allergies affect 10 to 30 percent of adults and up to 40 percent of children. This increase is driven not just by the rise in pollen concentrations, but also by the many ways the chemicals in pollutants interact with pollen.

Pollutants destroy pollen’s cell walls, “breaking the relatively large pollen grains into sub-micron particles that can then go deeper into the lungs and are more dangerous for patients,” says Isabella Annesi-Maesano, an environmental epidemiologist at the University of Montpellier in France. And pollutants can make the pollen itself more capable of eliciting an allergic reaction. Studies conducted in the lab demonstrate that a bump in atmospheric carbon dioxide results in pollen that contains more allergenic proteins, which provoke the production of antibodies responsible for the physical symptoms of an allergic reaction.

Steiner says the new work “represents a first step towards developing better tools to understand how pollen might change in the future and help people better prepare for the health impacts.” But additional challenges and work remain. For example, data is sparse; there are fewer than 100 pollen counting stations across the U.S. Monitoring pollen is also labor intensive, requiring a trained person to manually count and identify pollen grains.

“We measure and monitor pollen so much less than almost every other air pollutant, but we need this data to be able to pick up pollen’s long-term trends,” Anderegg says. But there is also good news: Solutions may be on the horizon. Several companies are developing AI techniques to automate the counting, making it more efficient. Additionally, one possibility that may come out of this research could be weekly projections of pollen counts similar to air quality forecasts that are already the norm in many regions of the country.

That, Anderegg says, could “build out local and regional capacity to cope with changing pollen and minimize the harm to people’s health.”

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