Warming Temperatures Drive Up Pollen Production and Allergies

Dr. Lewis Ziska confirms the link between global warming and longer allergy seasons

We can all name things in life that we want more of—more dessert, more vacation time, and more laughter, for instance. But a longer pollen season? That’s not likely to be at the top of anyone’s list.

But in North America, tree pollen is emerging on average two weeks sooner in spring, and ragweed pollen is spewing into the air for two to four weeks longer in fall.

Global warming and allergies 

The lengthening pollen season may exacerbate hay-fever symptoms for some 40 million Americans with allergic disorders, which trigger health care costs of more than $21 billion annually.

Dr. Lewis Ziska, a plant physiologist with the U.S. Department of Agriculture’s Crop Systems and Global Change Laboratory, and colleagues at the National Allergy Bureau and Aerobiology Canada, analyzed ragweed pollen counts at ten monitoring stations along a south to north transect, from Texas north to Saskatchewan, Canada, from 1995 to 2009. 

The Intergovernmental Panel on Climate Change (IPCC) has projected that warming is occurring faster as you approach the poles.  If so, then ragweed season length, already quite long in southern latitudes, should be expanding as you move north.

Extending ragweed season

The study demonstrated that the number of frost-free days, and a delay in the first autumn frost, has occurred since 1995 as a function of latitude, extending the ragweed pollen season into the fall, particularly for the more northern locations. For example, for Winnipeg, Manitoba, and Saskatoon, Saskatchewan—ragweed season was a whopping 25 and 27 days longer, respectively in 2009 than in 1995.

In the northern U.S. cities of Minneapolis, Minnesota, and Fargo, North Dakota, ragweed season was 13 days longer in 2009. In contrast, southern locations showed little climate effect and had no significant change in the ragweed pollen season, which is already three months long.

Ziska has studied plants and pollen in test plots and carbon dioxide chambers, but this study was the first continental-scale measurement of the effects of climate warming on ragweed plants. 

His research is consistent with IPCC assessments. “We were able to confirm that climate change is having an effect on pollen, that it is happening in real time, and we are seeing a real signal,” said Ziska.

Urban conditions show increased pollen count

In more than 20 years of study, Ziska has examined plant traits in relation to climate change. He has looked at growth rates, bloom times, pollen production, and other factors under varying air temperatures, carbon dioxide levels, and moisture levels.

Initially these experiments centered on examining urban to rural differences in climate, using cities like Baltimore, Maryland, as a surrogate for future climate change.

Because cities are filled with dark asphalt streets and rooftops, they tend to absorb and hold more heat than the countryside. Additionally, compared to rural areas of Maryland, Baltimore already has higher emissions of carbon dioxide from transportation and manufacturing. 

Ziska found that these urban conditions can have freakish results. Weeds that grew five and six feet tall in the countryside had counterparts in Baltimore that were ten and 20 feet tall.

Ragweed, specifically, grew faster, flowered earlier, and produced significantly more pollen in these urban conditions.

The current study is an effort by Ziska and colleagues to scale up to the continental level to see if climate change had progressed to where a temperature signal could be seen for ragweed in North America.

More ragweed produces more ragweed seeds 

As someone who is also concerned about the impact of ragweed on crops, Ziska states, “If you have more pollen, you have more seed production, and it’s likely you will have even more seed the following year.”

He goes on to say, “This is a factor of seed production in agronomy as well as pollen production. If weather is getting warmer, you’re likely to have more ragweed in that area and potentially greater effects on crop production."



Ziska, L., et al. 2011. Recent warming by latitude associated with increased length of ragweed pollen season in central North America. Proceedings of the National Academy of Sciences, USA. doi:10.1073/pnas.1014107108.

Ziska L., et al., 2008. Climate change, aerobiology, and public health in the Northeast United States, Mitigation Adaptation Strategies Global Change, 13:607–613, DOI 10.1007/s11027-007-9134-1

Environmental Protection Agency: http://www.epa.gov/ord/gems/scinews_aeroallergens.htm

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