Get Out Your Handkerchiefs

When Lewis Ziska wanted to see how a warmer world with more carbon dioxide in the air would affect certain plants, he didn't set up his experiment in a greenhouse or boot up a computer model. He headed for Baltimore. Cities are typically 7 degrees warmer than the countryside, as well as big sources of CO2. Although global levels of this greenhouse gas have reached 380 parts per million compared with preindustrial levels of 280, cities have way more—450 in Baltimore, 550 in Phoenix, 700 on a bad day in New York. So Ziska, a plant physiologist at the U.S. Department of Agriculture, compared ragweed growing in vacant lots in Baltimore with ragweed in rural fields—and discovered the dark side of sunny claims that global warming will produce a "greening of planet Earth." Urban ragweed grows three to five times bigger than rural ragweed, starts spewing allergenic pollen weeks earlier each spring and produces 10 times more pollen. In as few as 20 years the whole world will have CO2levels at least as high as some cities do now. As climate changes due to the greenhouse effect, hay-fever sufferers would do well to lay in copious supplies of Kleenex.

From mosquitoes that carry tropical diseases such as malaria, to plants that produce allergenic pollen, scientists are finding that a warmer, CO2-rich world will be very, very good for plants, insects and microbes that make us sick. Although the most obvious threat to human health is more frequent and more intense heat waves, such as the one that killed thousands of people in Europe in 2003, that is only the beginning.

In the case of plants, it's not just that they grow faster and shed pollen earlier as the world warms. The carbon-enriched air also alters their physiology. In a six-year study at a pine forest managed by Duke University, where pipes and fans adjust the CO2concentration of the air, scientists found that elevated CO2increases the growth rate of poison ivy. More surprising, by increasing the air's ratio of carbon to nitrogen, elevated CO2also increases the toxicity of urushiol, the rash-causing oil. "Poison ivy will become not just more abundant in the future," says Ziska. "It will also be more toxic."

Plants interpret warmth and abundant CO2as: what a great climate for reproduction. Monitoring stations in Europe are recording higher pollen counts for allergenic grasses and trees, led by birch and hazel, notes a 2005 study by the Center for Health and the Global Environment at Harvard Medical School. Those counts are rising earlier each year: the warming already underway is shifting the pollen season by almost one day per year. By 2017, you'll be reaching for tissues nine days sooner than you do now. The indoors may provide little escape, for toxic molds and fungi both produce more spores when CO2levels rise. "There will be itchy eyes and wheezing in January," predicts Harvard's Paul Epstein. More good news: in a greenhouse world pollen will be not only more abundant but more allergenic, he and Ziska find.

Since cities already have the high CO2levels that the rest of the world can soon expect, "there is no question these [climate-related] changes have already begun," says Arlington, Texas, Mayor Dr. Robert Cluck. "Every summer we're seeing West Nile virus earlier and earlier, and the higher levels of ozone that come with higher temperatures are increasing the rates of asthma and causing heart and lung damage comparable to living with a cigarette smoker."

In a greenhouse world, tropical diseases will expand their range and their prevalence. For instance, alternating floods and droughts—the pattern that comes with climate change—provide perfect conditions for mosquitoes that carry malaria, West Nile and dengue fever. Warming makes mosquitoes bite more, says Epstein, apparently because it gets their reproductive juices flowing. It also lets disease-causing microbes, which the mosquito transmits with its bite, mature more quickly: at 68 degrees Fahrenheit the malaria parasite takes 26 days to mature, but at 77 degrees it takes 13 days. Faster maturation means mosquitoes will transmit malaria for more of their lifetimes. They'll face fewer predators, too. The frequent droughts expected in a greenhouse world are murder on damselflies and dragonflies.

Already, mountains aren't as cool as they used to be, so insect-borne diseases are being reported at higher elevations in Latin America, Asia and Africa, including previously spared highland cities such as Nairobi. The mosquito that carries yellow fever and dengue fever, which once could not survive the cold above 3,300 feet, can now be found at 5,600 feet in Mexico. As dengue fever, yellow fever and malaria extend their range to higher elevations and higher latitudes, those diseases could appear in the developed world, too. The southern tier of Western and Eastern Europe, as well as the Southern United States, are most at risk, says Harvard's Epstein. Dengue fever has already popped up on the Mexican side of the U.S. border, a worrisome expansion of its current range. Say this for the climate contrarians who insist that a warmer world will be a better, more productive world: if they're referring to allergens and pathogens, they're dead right.