Climate Change and Pollen Seasons: What Studies Show
If allergy season feels like it starts earlier and lasts longer than it used to, the measurements agree with you. The link between climate change and pollen seasons is now one of the better-documented corners of climate-and-health research, built not on projections but on decades of pollen counts collected station by station. This post sticks to named, published findings — what the studies measured, over what years, and what they did and did not claim — so you can see exactly how climate change and pollen seasons connect in the data.
Key takeaways
- Anderegg and colleagues, in a 2021 Proceedings of the National Academy of Sciences study of 60 North American stations, found pollen seasons lengthened by about 20 days and pollen concentrations increased by about 21% between 1990 and 2018, with seasons starting about 20 days earlier.
- The largest and most consistent increases were recorded in Texas and the Midwestern United States, and tree pollen showed the biggest rise among the pollen types.
- The US EPA's ragweed indicator tells the same story at the fall end: its Missouri summary reports Kansas City's ragweed season growing 18 days longer since 1995.
- The PNAS attribution is partial by design: human-caused climate change accounted for roughly half of the pollen-season trend and roughly 8% of the pollen-concentration trend.
The headline study: PNAS, 2021
The central paper is Anderegg and colleagues, published in the Proceedings of the National Academy of Sciences in 2021 under the title "Anthropogenic climate change is worsening North American pollen seasons." The team assembled long-term records from 60 pollen monitoring stations across North America, covering 1990 to 2018 and 821 station-years of hand-counted data, and compared the trends against climate model simulations.
What Anderegg and colleagues found
Their findings, as published: across North America, pollen seasons lengthened by about 20 days and pollen concentrations increased by about 21% between 1990 and 2018, with seasons also starting about 20 days earlier than in 1990. The largest and most consistent increases were recorded in Texas and the Midwestern United States, and tree pollen showed the biggest rise among the pollen types. Using model comparisons, the authors attributed roughly half of the pollen-season trend and roughly 8% of the pollen-concentration trend to human-caused climate change, and found that contribution growing stronger in the later half of the study period. You can read the paper itself via its PNAS record (DOI: 10.1073/pnas.2013284118).
Where the measured increases were largest
Those numbers land close to home on Pollen Atlas: the regions with the largest measured increases include the Texas cities we track daily, such as Dallas, Texas, and the ragweed belt around Tulsa, Oklahoma.
What the EPA's indicators showed for ragweed
A second line of evidence comes from the US Environmental Protection Agency's Climate Change Indicators programme, which tracked the length of the ragweed pollen season at monitoring locations as a formal indicator. The EPA's technical documentation for the ragweed pollen season indicator describes the method: season start and end dates derived from daily ragweed counts at each station, bounded by the plant's biology — ragweed does not flower before the summer solstice and stops at the first fall frost.
| Study or source | Period covered | Season-length finding |
|---|---|---|
| Anderegg et al. (2021), PNAS — 60 North American stations, 821 station-years | 1990–2018 | Pollen seasons lengthened by about 20 days, started about 20 days earlier, and pollen concentrations increased by about 21% |
| US EPA Climate Change Indicators — ragweed pollen season | Since 1995 | Ragweed seasons in the northern Great Plains and Upper Midwest grew 10 to 21 days longer; Kansas City's season grew 18 days longer |
| Peer-reviewed analysis of ten US and Canadian locations | 1995–2015 | Ragweed seasons lengthened by six to 25 days at most sites, closely tied to the lengthening frost-free season |
A northern pattern, tied to later frosts
The pattern the EPA reported was geographical. In its state-level climate summaries, the agency noted that the ragweed season in the northern Great Plains and Upper Midwest had grown 10 to 21 days longer than in 1995 as the first fall frost arrived later — its EPA Missouri summary, for example, reports Kansas City's ragweed season growing 18 days longer since 1995. A separate peer-reviewed analysis covering 1995 to 2015 at ten US and Canadian locations found ragweed seasons lengthening by six to 25 days at most sites, closely tied to the lengthening frost-free season. Southern stations generally changed less, because their frost-free season was already long.
How warming changes a pollen season, mechanism by mechanism
The studies point to three main mechanisms, each observable without any modelling at all. First, timing: warmer springs advance plant development, so trees flower earlier, and later first frosts let fall weeds such as ragweed keep releasing later — the season stretches at both ends. Second, quantity: a longer frost-free season gives plants more time to grow, and bigger plants produce more pollen. Greenhouse experiments cited in this literature have also shown ragweed producing more pollen under higher carbon dioxide concentrations — the AAFA summarises this work in its ragweed resources, and a 2025 Austrian study of common ragweed, published in Applied Sciences, measured about 1.19 billion grains from a single plant, a reminder of how much output one healthy plant represents.
Range: warmth-loving plants move north
Third, range: warming lets warmth-loving plants survive further north and at higher elevations, which is part of why ragweed's European invasion — covered in our post on the ragweed plant — keeps advancing. Cities like Atlanta, Georgia and Minneapolis, Minnesota sit at different points on these gradients, which is why the measured trends differ by region rather than arriving as one uniform national shift. The daily city rankings show today's snapshot; the studies above are what the snapshot looks like accumulated over thirty years.
What this means practically — and what the studies do not say
Two honest caveats belong in any factual account. The PNAS attribution is partial by design: human-caused climate change accounted for about half the season-length trend, not all of it — weather variability, land use, and local plant communities also shape any given city's season. And a longer average season does not mean every year is worse in every place; individual seasons still swing with rainfall and weather, as any sufferer knows.
Treating the allergy calendar as a moving target
Practically, the findings argue for treating the allergy calendar as a moving target. Season start dates memorised a decade ago may now be weeks early; a "fall" weed season may run meaningfully later in northern cities than it once did. Our pollen season calendar lays out the current typical timing, and your city page shows what is actually happening this year. If your seasons have been getting harder to predict, you are not imagining it — and if you want help mapping your own pattern, you can find an allergist near you in the Pollen Atlas directory. This article is general information only and is not medical advice.
Frequently Asked Questions
Are pollen seasons really getting longer?
Yes — Anderegg and colleagues reported in PNAS in 2021 that, across 60 North American stations, pollen seasons lengthened by about 20 days between 1990 and 2018. Pollen concentrations also increased by about 21% over the same period.
How much earlier do pollen seasons start now?
The same PNAS study found seasons starting about 20 days earlier than in 1990. Individual seasons still swing with rainfall and weather, so a longer average season does not mean every year is worse in every place.
Is climate change responsible for all of the change?
No — the PNAS authors attributed roughly half of the pollen-season trend and roughly 8% of the pollen-concentration trend to human-caused climate change. Weather variability, land use, and local plant communities also shape any given city's season.
Has the ragweed pollen season changed?
Yes — the US EPA's Climate Change Indicators programme reported ragweed seasons in the northern Great Plains and Upper Midwest growing 10 to 21 days longer than in 1995, as the first fall frost arrived later. Southern stations generally changed less, because their frost-free season was already long.
Sources: Anderegg et al. (2021), PNAS, DOI 10.1073/pnas.2013284118; US EPA, Climate Change Indicators — Ragweed Pollen Season technical documentation; US EPA (2016), What Climate Change Means for Missouri; Applied Sciences (2025), Ragweed Finder citizen-science study, Austria.
Conclusion: climate change and pollen seasons, in the record
The connection between climate change and pollen seasons is not a forecast — it is a measurement: about 20 days longer, about 20 days earlier, and about 21% more pollen across North America between 1990 and 2018, with the EPA's ragweed indicator telling the same story at the fall end of the calendar. Seasons will keep varying year to year, but the baseline has moved. Track where your city sits against that baseline — check today's counts and the season ahead, free at PollenAtlas.com.