Skip to main content
ARS Home » News & Events » News Articles » Research News » 2000 » Global Warming's High Carbon Dioxide Levels May Exacerbate Ragweed Allergies

Archived Page

This page has been archived and is being provided for reference purposes only. The page is no longer being updated, and therefore, links on the page may be invalid.

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

By Don Comis
August 15, 2000

WASHINGTON, Aug. 15--The U.S. Department of Agriculture released research results today 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.

"This research may help us better understand the troubling impact of high carbon dioxide levels on our environment and our health," said Agriculture Secretary Dan Glickman.

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.

This past spring, 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, conducting his research in collaboration with Johns Hopkins University School of Public Health, Towson University and Multidata Corporation, 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.

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