Anthropogenic climate change is worsening North American pollen seasons

One of the most quoted papers about pollen and climatechange is written by William Anderegg, School of Biological Sciences, University of Utah in Salt Lake City.


“Anthropogenic climate change is worsening North American pollen seasons”


The impact of climate-change for hayfever-patiënts is a very interesting topic. A few questions about his research.

1.What is your specialisation as a researcher?

I am a plant ecophysiologist. I study how temperature, water, and other environmental conditions affect plant physiology and growth.

2. Could you describe the setup of the research on the level of the main phases?

We analysed long-term pollen records for 60 cities across North America. We combined pollen data with historical climate data and information from computer models on the strength of climate change at these locations.

3. What is the main conclusion of your research?

Between the early 1990s and 2018, pollen seasons are starting earlier, have grown longer, and have about 20% more pollen in the air. Climate change – particularly rising temperatures – are a major cause of these trends.

4. Anthropogenic climate change is about the impact of human activity on climate. Did you also look at the human impact on land use? Did e.g., changes in forest management or agricultural development changed the impact of the pollen load?

We did. We used satellite data to analyse changes in land-use and vegetation around these long-term pollen stations. We saw no evidence of local land-use or vegetation changes in driving these long-term trends.

5. Question about Fig 1. The bigger the circle, the more years has been included, red is a more severe pollen season, blue is less pollen load. Are all various pollen types added up?

Yes, that figure shows all types of pollen (all plant taxa, including trees, grasses, and weeds) added together.


6. As it contains many variables figure 2 is difficult to interpretate by a broad audience, what is the graphic showing?

Big picture – temperature is a major driver of pollen. Warmer temperatures are associated with earlier pollen season start dates, longer pollen seasons, and more pollen in the air.


7. As it contains many variables figure 3 is difficult to interpretate by a broad audience, what is the graphic showing?

Climate change is the largest driver of pollen season changes since the 1990s and a more moderate driver of the amount of pollen in the air.


8. There are many different landscapes and climate zones in North America, land use can vary a lot. Can you give some examples of states with show the strongest correlations with climate change on pollen type level? Like Birch trees in New York, Ragweed in Kentucky or grasses in California?

We didn’t break our analysis down to specific species in this paper, but we saw the biggest increases in trees in general and much of the South (like Texas and Georgia) and Midwestern US has the largest pollen increases.

image: weatherstation

9. Warmer climate can also reduce growth and flowering of for example grasses. Did you find examples of states and pollen types which produce less pollen in warmer periods? Like grasses in Texas?

We didn’t see specific examples of this, but this is something that we plan to look more into for future analyses.

10. Did you find any clues that people’s health changed over the years as well? Did people get more or less sensitive to pollen?

Our study didn’t have the time to connect all of the dots to people’s health.

11. Average temperature was found to be the strongest correlation. Did you find any interesting correlations if there was a huge difference between highest day-temp and lowest night-temp?

This is a fascinating question, but unfortunately, we weren’t able to look at this in the current study.

12. Climate change is not only about warmer average temperature, but also about more and severe rainfall, it influences storm patterns

Indeed, one of the surprises in this work was that we didn’t see strong influences of precipitation, but that might have been because we were looking primarily at seasonal and annual pollen trends. More detailed timescales of analysis, like weekly pollen counts, would likely shine more light on how precipitation and storms is mediating these patterns.

13. Did you find any differences in pollen and mold load related to climate change?

Mold is a really excellent question – that’s something we plan to look into in the future too, but we didn’t analyse it in this study.

More information:

website of lab William Anderegg >>

website of School of Biological Sciences University of Utah >>

Original paper on PNAS: Anthropogenic climate change is worsening North American pollen seasons