WASHINGTON, DC-- Higher carbon dioxide levels linked with global warming may have doubled the amount of pollen produced by ragweed plants, according to new research from the United States Department of Agriculture.[1] Due in part to the Industrial Revolution, atmospheric carbon dioxide has increased in the last century from 280 to 370 parts per million (ppm), with most of this increase occurring since the late 1950s.

"For every 10 ppm increase in carbon dioxide during this period, the potential pollen production of a single ragweed plant has increased 0.7 grams. If this trend continues, the potential pollen production of ragweed will double again by the end of the 21st century, when carbon dioxide levels are expected to reach 600 ppm," stated lead study author Lewis H. Ziska, PhD, a plant physiologist in the Climate Stress Laboratory of the US Department of Agriculture's Agricultural Research Service, in Beltsville, Maryland.

"Rising atmospheric carbon dioxide is an important factor in determining pollen levels. Carbon dioxide stimulates the growth not only of ragweed, but 96% of all plant species capable of producing pollen," Dr. Ziska added. "This research may help us better understand the impact of high carbon dioxide levels on our environment and our health."

Dr. Ziska led a study to determine the effects of atmospheric carbon dioxide on ragweed. He collected pollen counts from ragweed grown in indoor chambers at three different levels of atmospheric carbon dioxide: 280 ppm, 370 ppm, and 600 ppm. These levels were chosen to simulate mid-19th century and current levels of atmospheric carbon dioxide, as well as those predicted for the mid-to-late 21st century.

CO2 PROMOTES POLLEN PROLIFERATION

The influence of rainfall, temperature, and light on pollen levels is well known. But until now, the impact of atmospheric carbon dioxide levels on pollen growth was less understood, according to Dr. Ziska. Exposure to carbon dioxide stimulates plant growth through the process of photosynthesis, he explained.

In the study, pollen production rose from about 4.9 to 11.3 to 21.3 g per plant as carbon dioxide levels increased. The increases in pollen production from preindustrial to current carbon dioxide levels and from current to future levels are 132% and 90%, respectively.

Rising atmospheric carbon dioxide levels may not only increase ragweed pollen production but may also make ragweed more difficult to control. In other studies, Dr. Ziska and coworkers[2] found that when the carbon dioxide level is 700 ppm, two other allergy-inducing weeds (lamb's-quarter and quack grass) become more resistant to glyphosate (one of the most commonly used herbicides).

Dr. Ziska is now investigating the growth and pollen production of ragweed at three locations in the Baltimore area: within the city itself, in a nearby suburb, and in a more rural community. The different locations were chosen for their range in temperatures and carbon dioxide concentrations.

"This ongoing experiment should show how global warming and higher carbon dioxide 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," he noted.

--Deborah L. O'Connor

References
1. Ziska LH, Caulfield F. The potential influence of rising atmospheric carbon dioxide (CO2) on public health: pollen production of common ragweed as a test case. World Resource Rev. 2000; 12:449-457.
2. Ziska LH, Teasdale JR, Bunce JA. Future atmospheric carbon dioxide may increase tolerance to glyphosate. Weed Sci. 1999;47:608-615.