Harvard researchers say that higher levels of the greenhouse gas will also boost pollen production, causing allergy sufferers to suffer even more in the future.
Just last week, Duke University researchers reported that rising atmospheric levels of carbon dioxide will likely fuel the growth of a more poisonous form of poison ivy.
The researchers studied the growth of ragweed under conditions mimicking both today's levels of atmospheric carbon dioxide and those projected for the future, assuming that climate change continues at its current pace.
They found that the ragweed plants grown under the futuristic conditions produced about 55 percent more pollen than the plants grown under conditions more closely approximating today's climate.
Researcher Christine A. Rogers, Ph.D., tells WebMD that global warming is already having an impact on the growing season, with the arrival of spring occurring earlier in most places. This, combined with an increase in carbon dioxide levels, will result in longer, more intense allergy seasons in the future, she says.
"The clearest signal of global climate change is the earlier onset of spring," she says. "Our goal was to examine the interaction between the lengthening of the growing season and the increase in carbon dioxide."
More Carbon Dioxide, More Pollen
In their effort to do this, Rogers and colleagues grew ragweed seeds in climate-controlled greenhouses and carefully measured pollen production.
The air we now breathe has about 350 parts of carbon dioxide per million parts of air. Experts expect carbon dioxide levels to double within the next three to six decades, so the researchers studied ragweed growth at both 350 parts and 700 parts carbon dioxide per million parts of air.
They also released the ragweed seeds at different times to simulate early and late growing seasons.
A longer growing season was associated with an increase in pollen production for seeds grown at atmospheric carbon dioxide levels that were similar to current conditions. But it seemed to have less impact when seeds were exposed to the highest carbon dioxide levels.
"In future climates with elevated [carbon dioxide], we predict pollen production will be just as robust in years with late springs as in years with early springs," the researchers wrote. "Overall, pollen production in ragweed can be expected to increase significantly under predicted future climate conditions."
The study was conducted by researchers from Harvard School of Public Health's Exposure, Epidemiology, and Risk Program, and Harvard Medical School's Center for Health and the Global Environment. It was funded through a grant from the National Oceanic and Atmospheric Administration.
Opportunistic Plants Thrive
Center for Health and the Global Environment co-director Paul Epstein, M.D., tells WebMD that while the political debate over global warming continues, it is clear that atmospheric carbon dioxide levels are rising, caused by the burning of fossil fuels.
"There is no question about this within the scientific community," he says. "It is a fact."
Plants grow larger and use water more efficiently when exposed to higher levels of carbon dioxide. Because of this, Epstein says, some people believe increasing levels may be a good thing, resulting in better crops and a greener planet.
"But we are beginning to see some health effects of carbon dioxide build up that we couldn't have foreseen even a few years ago," he says. "It is the opportunistic plants like poison ivy and ragweed that thrive. These species are gaining a foothold because of carbon dioxide."
SOURCES: Rogers, C.A. Environmental Health Perspectives, June 2006; Vol. 114: pp. 865-869. Christine A. Rogers, Ph.D., senior research scientist, Exposure Epidemiology and Risk Program, Harvard School of Public Health, Boston. Paul R. Epstein, M.D., associate director, Center for Health and Global Environment, Harvard Medical School, Boston. "Biomass and toxicity responses of poison ivy (Toxicodendron radicans) to elevated atmospheric CO2," Proceedings of the National Academy of Sciences, June 5, 2006.
By Salynn Boyles
Reviewed by Louise Chang, M.D.
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