KEY CONCEPTS
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This summer’s record-breaking heat is lingering into fall: bad news for the 50 million people in the U.S. with allergies to ragweed pollen in the late summer and early fall.
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Our warming climate results in more freeze-free days each year — giving plants more time to grow and release allergy-inducing pollen earlier in spring and later into fall.
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Climate Central analysis finds that fall warming since 1970 has extended ragweed’s freeze-free growing season in 164 U.S. cities — by 11 days on average.
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Cities in the Northeast, Upper Midwest, and Northwest saw some of the biggest increases.
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Climate Central’s report Seasonal Allergies: Pollen and Mold reviews weather and climate trends that affect both spring and fall allergy seasons and the related health risks.
Summer heat lingering into fall
This summer’s relentless record heat has stuck around into fall. The planet just had a record-shattering September — the seventh-warmest for the U.S.
This is bad news for the 50 million people in the U.S. with allergies to ragweed pollen in the late summer and early fall.
In most U.S. areas, ragweed pollen typically peaks in September and lasts through October. But warmer fall temperatures extend the ragweed growing season.
Ragweed, which is found in most U.S. states, is the main cause of fall allergies. A single ragweed plant can produce up to 1 billion pollen grains that are carried by wind and cause a range of symptoms.
Ragweed can also thrive in both rural and urban areas. A 2003 study suggests that the urban heat island effect can even help ragweed grow faster and produce more pollen in cities.
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See Urban Heat Hot Spots to find the strongest urban heat islands within your city.
How is fall warming affecting local fall allergy seasons? New Climate Central analysis explores this question.
Growing season lasting later into fall
Studies have found that the length of ragweed pollen season across the U.S. — from Texas to North Dakota — is strongly linked with the number of fall days until the first frost.
Climate Central therefore assessed how the number of consecutive freeze-free days (with minimum temperatures above 32°F) during the fall season (Sept-Nov) has changed since 1970 in 201 U.S. cities.
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The freeze-free fall season lengthened in 164 cities, or 82% of the 201 analyzed.
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Across these 164 cities, the freeze-free fall season lengthened by 11 days on average.
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The freeze-free fall season is now at least two weeks longer in 53 cities.
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Over half (58%) of these 53 cities were in the Northeast, Upper Midwest, and Northwest — consistent with research finding that ragweed pollen season has grown fastest at higher latitudes.
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The four cities where the fall freeze-free season has grown the most since 1970 are: Reno, Nev. (+39 days); Bend, Ore. (+33 days); Toledo, Ohio (+28 days); Boise, Idaho (+27 days).
The widespread increase in freeze-free fall days can prolong allergy-inducing pollen production by the 17 types of ragweed that grow across the U.S. during the late summer and fall.
Warming climate, longer pollen season, worse allergies
Seasonal allergies can already last from early spring through late fall. But warming from carbon pollution results in more freeze-free days each year, giving plants more time to grow and release allergy-inducing pollen.
A recent Climate Central analysis found that the total annual freeze-free season in 203 U.S. cities grew 15 days longer on average since 1970.
This is consistent with peer-reviewed studies finding that human-caused warming has already led North American pollen seasons to lengthen by 20 days on average from 1990 to 2018.
Longer annual growing seasons and pollen seasons are due both to: earlier spring warm-up (last spring frost happens earlier) and later fall cool-down (first fall frost happens later).
There are three different plant-based allergy seasons in North America: tree pollen in the spring, grass pollen in the early summer, and weed pollen such as ragweed in the summer and fall.
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A video from the University of Michigan shows when each pollen season peaks across the U.S.
Mold can cause fall allergies, too.
In addition to pollen, some molds (fungi that grow on soil and dead plants) can be allergenic. Different kinds of molds may release tiny spores throughout the year, but tend to peak in late summer and fall.
Molds often grow on soil, leaf litter, and decaying plant matter, and their life cycles can therefore be closely linked to plant decay during fall. Studies have observed a significant increase in mold spore concentrations when plants die or leaves fall during autumn.
For people who have both pollen and mold allergies, this means that allergies can last for much of the year. Although outdoor mold isn’t as well-studied as pollen, climate change is likely affecting how both allergens impact people with allergies and asthma.
Climate Central’s report, Seasonal Allergies: Pollen and Mold, reviews weather and climate trends that affect allergy season locally — including how increased carbon dioxide in the atmosphere boosts pollen production, and why thunderstorms can increase the risk of asthma attacks.
LOCAL STORY ANGLES
Find local pollen and mold counts.
There are pollen and mold spore monitoring stations across the U.S. Local allergen counts and forecasts can be found through resources such as the National Allergy Bureau. State or tribal agencies for environmental protection or public health may also have relevant air quality reports.
See where your city ranks.
The severity of the allergy season varies across the country. Check out how your city ranks in the Asthma and Allergy Foundation of America’s 2023 report on the Allergy Capitals in the U.S., which ranks cities based on pollen scores, over-the-counter medicine use, and the availability of board-certified allergists.
CONTACT EXPERTS
Allison L. Steiner, PhD
Professor
University of Michigan
Related expertise: Warming trends and growing season length
Contact: alsteine@umich.edu
Noah Scovronick, PhD
Assistant Professor
Emory University’s Rollins School of Public Health
Related expertise: Links between human health, climate change
Contact: rob.spahr@emory.edu
FIND EXPERTS
Find a local allergist or immunologist listed through the American Academy of Allergy, Asthma and Immunology.
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices.
Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area.
METHODOLOGY
Daily minimum temperature data from 1970-2022 were obtained from the Applied Climate Information System. The length of the fall freeze-free season was determined based on the annual count of consecutive fall (September, October, November) days with minimum temperatures above 32°F. Of 247 total stations assessed, 42 were on average frost-free for most of the fall and were therefore excluded from this analysis. Summary statistics were reported for 201 of the 205 remaining stations due to large data gaps in the period of record for: Dothan, Ala.; Hazard, Ky.; Terre Haute, Ind.; and Wheeling, W.Va.
