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When Lewis Ziska wanted to see how a warmer wood 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 forBaltimore.Cities are typically 7 degrees warmer than the countryside, as well as big sources ofCO₂. So Ziska, a plant physiologist at the U.S.Department ofAgriculture, compa,ed ragweed growing in vacant lots inBaltimore with ragweed in rural fields—and discovered the dark side of sunny claims that global warming will produce a "greening of planetEarth". 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 haveCO₂ levels at least as high as some cities do now.As climate changes due to the greenhouse effect, hayfever 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 weaves, such as the one that killed thousands of people inEurope 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 woad warms. The carbon-enriched air also alters their physiology. In a six-year study at a pine forest managed byDuke University, where pipes and fans adjust theCO₂ concentration and the air, scientists found that elevatedCO₂ increases the growth rate of poison ivy. More surprising, by increasing the air’s ration of carbon to nitrogen, elevatedCO₂ also 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 toxi
C、"
Plants interpret warmth and abundantCO₂as: what a great climate for reproduction. Monitoring stations inEurope are recording higher pollen counts for allergenic grasses and trees, led by birch and hazel, notes a 2005 study by theCenter for Health and the GlobalEnvironment 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. More good news: in a greenhouse world po]len will be not only more abundant but more allergenic, he and Ziska finD、
Since cities already have the highCO₂ levels that the rest of the world can soon expect, "’there is no question these climate-related changes have already begun," saysArlington, Texas, Mayo,"Dr. RobertCluck. "Every summner 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 bit more. They’ll face fewer predators, too. The frequent droughts expected in a greenhouse world are murder on damselflies and dragonflies.
As dengue fever, yellow fever and malaria extend their range to higher elevations and higher latitudes, those diseases could appear in the developed woad, too. The southern tier of western and easternEurope, as well as the southern United States, are most at risk, says Harvard’sEpstein.Dengue fever has already popped up on the Mexican side of the U.S. bor