Climate Change Is Making Allergy Season Even Worse

Temperatures are rising, and they’re taking pollen counts with them.

Flowers with ominous clouds in the background.
Dominika Zarzycka / Getty

This article was originally published by The Conversation.

Brace yourselves, allergy sufferers: New research shows that pollen season is going to get a lot longer and more intense with climate change.

Our latest study finds that the U.S. will face up to a 200 percent increase in total pollen this century if the world continues producing carbon-dioxide emissions from vehicles, power plants, and other sources at a high rate. Under that scenario, the spring pollen season will generally start up to 40 days earlier and last up to 19 days longer than it does today.

As atmospheric scientists, we study how the atmosphere and climate affect trees and plants. Although most studies focus on pollen overall, we zoomed in on more than a dozen types of grasses and trees and how their pollen will affect regions across the U.S. in different ways. For example, because of species such as oak and cypress, the Northeast will see the biggest pollen increase, but allergens will be on the rise just about everywhere, with consequences for human health and the economy.

If your head is pounding at just the thought of this, we also have some good news, at least in terms of knowing in advance when pollen waves are coming. We’re working on using the model from this study to develop more accurate local pollen forecasts.

Let’s start with the basics. Pollen—the dustlike grains produced by grasses and plants—contains the male genetic material for a plant’s reproduction.

How much pollen is produced depends on how the plant grows. Rising global temperatures will boost plant growth in many areas, and that, in turn, will affect pollen production. But temperature is only part of the equation. We found that the bigger driver of the pollen increase will be rising carbon-dioxide emissions.

The higher temperature will extend the growing season, giving plants more time to emit pollen and reproduce. Carbon dioxide, meanwhile, fuels photosynthesis, so plants may grow larger and produce more pollen. We found that carbon-dioxide levels may have a much greater impact than temperature on future pollen increases.

Rather than treating all pollen the same, as many past studies have, we looked at 15 different pollen types.

Typically, pollination starts with leafy deciduous trees in late winter and spring. Alder, birch, and oak are the three top allergy-causing deciduous trees, though there are others, such as mulberry. Then grasses come out in the summer, followed by ragweed in late summer. In the Southeast, evergreen trees such as mountain cedar and juniper (in the cypress family) start pollinating in January. In Texas, “cedar fever” is the equivalent of hay fever.

We found that in the Northeast, the pollen seasons for a lot of allergenic trees will overlap more and more as temperatures and carbon-dioxide emissions rise. For example, oak trees used to release pollen first, and then birch would pollinate. Now their pollen seasons blur together.

In general, pollen season will change more in northern states than in the southern ones, because of larger temperature increases in northern areas.

Southeastern regions, including Florida, Georgia, and South Carolina, can expect greater grass- and weed-pollen increases in the future. The Pacific Northwest is likely to see peak pollen season a month earlier because of alder’s early pollen season.

Most pollen forecasts right now provide only a very broad estimate. Part of the problem is that there aren’t many observing stations for pollen counts: Fewer than 100 of these stations are distributed across the country. Michigan, where we live, doesn’t have any.

Measuring different types of pollen is a very labor-intensive process. As a result, current forecasts have a lot of uncertainties. They are likely based in large part on what a station has observed in the past and the weather forecast.

Our model, if integrated into a forecasting framework, could provide more targeted pollen forecasts across the country.

We can estimate where trees are from satellite data and on-the-ground surveys. We also know how temperature influences when pollen is released— what we call its phenology. With that information, we can use meteorological factors such as wind, relative humidity, and precipitation to figure out how much pollen gets into the air, and atmospheric models can show how it is blown around, to create a real-time forecast.

All of that information allows us to look at where pollen might be in space and time so that people dealing with allergies will know what’s coming in their area.

We’re currently talking with a National Oceanic and Atmospheric Administration lab about ways to integrate this information into a tool for air-quality forecasting.

Some unknowns still exist when it comes to long-term pollen projections. For example, scientists don’t fully understand why plants produce more pollen in some years than others. There’s not a good way to include that variability in models. Also unclear is how plants will respond if carbon-dioxide levels go through the roof. Further, ragweed and residential trees are hard to capture. There are very few ragweed surveys showing where these plants are growing in the U.S., but that can be improved.

A 2021 study found that the overall pollen season was already about 20 days longer in North America than it had been in 1990 and that pollen concentrations were up about 21 percent.

Increased pollen levels will have a much broader impact than a few sniffles and headaches. Seasonal allergies affect about 30 percent of the U.S. population, and they have economic impacts, including health-care costs and missed working days. In the coming years, those impacts will only intensify.