The Climate of Man

As global warming affects plant growth, allergy and asthma sufferers' symptoms could worsen

Modern medicine, technology, and our urbanized lifestyle often allow us to forget how intertwined our health is with the natural environment. But global warming-through its impact on the plants all around us-has begun to change that.

As carbon dioxide levels rise, plants are among the first to respond, because they use carbon dioxide to photosynthesize their own food. But as temperature and CO2  levels climb together, affecting both the growth rate and range of many plants, researchers are asking what will happen to us as the plants that provide food and medicine-or make us sick, such as ragweed-respond to climate change.

"This may be the first piece of the global warming puzzle that tells us, yes, we see that increasing greenhouse gas emissions have effects that are both local and predictable," says Kim Knowlton, a science fellow with NRDC's health program and the lead author of a recent NRDC report, Sneezing and Wheezing: How Global Warming Could Increase Ragweed Allergies, Air Pollution, and Asthma.

Heavy ozone smog caused by urban pollution and ragweed pollen are each known to aggravate asthma, but Knowlton and her NRDC colleagues were concerned by studies showing that the combined effects of the two could be even worse. NRDC mapped for the first time the regions where ragweed and unhealthy ozone levels overlap. About 110 million people across the United States are subject to double exposure, with areas of higher risk in Los Angeles, New York City, the southern Mississippi River Valley, the Great Lakes region, the mid-Atlantic states, and New England. According to the Asthma and Allergy Foundation of America, those regions are already home to 13 of the nation's 15 worst cities for asthma sufferers.

NRDC's report drew on the work of Lewis Ziska, who conducts research in aerobiology-the science of airborne biological particles such as pollen and other allergens-at the U.S. Department of Agriculture. Ziska realized that urban environments could be used as surrogates for a warmer world, because cities are warmer than nearby rural areas by anything from 3.5 degrees to 8.0 degrees Fahrenheit. That's close to the global temperature increase projected by the Intergovernmental Panel on Climate Change. In Ziska's studies of ragweed growth and pollen production in Baltimore and rural Maryland, plants in the city, where temperatures and carbon dioxide concentrations are higher, grow larger and produce more pollen than their rural counterparts.

Ragweed is an opportunistic plant, moving into cracks and crevices in sidewalks, vacant lots, construction sites, or any other disturbed environment it finds. Cities and suburban sprawl offer those features in abundance.

 In high-traffic areas, including cities, ports, and bus depots, the disturbed environment provides the perfect setting for diesel pollution to combine with pollen, potentially causing spikes in asthma rates during the late-summer ragweed season. "Pollen can attach to diesel fumes, which then allows it to be drawn more deeply into the lungs," Ziska says.

As Knowlton and Ziska see it, these studies bring global warming home. But Knowlton believes they have the potential to do much more. California's efforts to obtain a waiver that would allow state officials to enforce stricter controls on global warming pollution from automobiles have been denied by the Environmental Protection Agency, partly on the grounds that, as of yet, climate change has had no local effects.

But, Knowlton says, "maybe these are the first local effects."