Global Warming, Carbon Dioxide Levels and Ragweed Allergies

Content Cover Image

Annual Ragweed. Source: USDA

Global Warming's High Carbon Dioxide Levels
May Exacerbate Ragweed Allergies

The U.S. Department of Agriculture has released research results indicating higher carbon dioxide (CO2) levels associated with global warming may have doubled the amount of pollen that ragweed produces--mostly over the past four or five decades. Another doubling could occur by the end of this century. The research may help us better understand the impacts of high carbon dioxide levels on our environment and our health.

caption Ragweed Pollen In scientific studies, pollen production rose almost 400% with a 200% increase in the amount of CO2. Findings show that high CO2 levels have increased the potential production of ragweed pollen and may produce pollen earlier. The ragweed pollen season is now underway.

Lewis H. Ziska, a plant physiologist with USDA's Agricultural Research Service, did the pollen counts on ragweed grown in indoor chambers at various levels of atmospheric CO2, from about the turn-of-the-century levels of 280 parts per million (ppm) to today's levels of 370 ppm to future predicted levels of 600 ppm. Pollen production went from 5.5 grams to 10 grams to 20 grams as CO2 moved through these three levels.

caption Distribution: Ambrosia L. (Source: USDA)
Click to see larger image.
In the Spring of 1999, Ziska moved the experiments outside, growing ragweed at three locations in the Baltimore, Md., area chosen for their range in temperatures: Baltimore, typical of urban areas thought to be both heat islands and zones of high CO2 concentrations; a suburb; and a rural area.

Ziska says this ongoing experiment should show how global warming and higher CO2 levels might already be increasing ragweed pollen counts, especially in cities. Although less ragweed grows in cities, exposure to air pollutants such as ground-level ozone can make people more sensitive to ragweed pollen.

The Abstract of Ziska's 2011 article, "Recent warming by latitude associated with increased length of ragweed pollen season in central North America", published in the Proceedings of the National Academy of Sciences, notes:

"A fundamental aspect of climate change is the potential shifts in flowering phenology and pollen initiation associated with milder winters and warmer seasonal air temperature. Earlier floral anthesis has been suggested, in turn, to have a role in human disease by increasing time of exposure to pollen that causes allergic rhinitis and related asthma. However, earlier floral initiation does not necessarily alter the temporal duration of the pollen season, and, to date, no consistent continental trend in pollen season length has been demonstrated. Here we report that duration of the ragweed (Ambrosia spp.) pollen season has been increasing in recent decades as a function of latitude in North America. Latitudinal effects on increasing season length were associated primarily with a delay in first frost of the fall season and lengthening of the frost free period. Overall, these data indicate a significant increase in the length of the ragweed pollen season by as much as 13–27 d at latitudes above ∼44°N since 1995. This is consistent with recent Intergovernmental Panel on Climate Change projections regarding enhanced warming as a function of latitude. If similar warming trends accompany long-term climate change, greater exposure times to seasonal allergens may occur with subsequent effects on public health."

Scientific contact: Lewis H. Ziska, ARS Climate Stress Laboratory, Beltsville, Md., phone (301) 504-6639, fax (301) 504-6626,

Further Reading

Editor's Note

  • Ragweed Pollen Image: Courtesy of American Academy of Allergy, Asthma and Immunology.




(2012). Global Warming, Carbon Dioxide Levels and Ragweed Allergies. Retrieved from


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