How To Beat The Heat

If the world warms by even a few degrees, tropical diseases could follow the rising mercury. Once again, it is a specific quirk of global warming, not the global average, that poses the greatest threat. Nighttime temperatures will increase more--perhaps 10 percent more--than daytime temperatures. Winter readings will rise more than summer ones. Greenhouse skeptics see this as another sign that global warming will be benign; if the nights, which are naturally cooler than the days, are a little warmer than they are now, who will notice? And if winters are a bit balmier, bring it on. Except for one thing. Because nighttime and wintertime cold kills insects and their eggs, temperature determines how far from the equator tropical pests can survive. Mosquitoes that carry malaria cannot stand more than a few days below 62, explains Paul Epstein of Harvard University. Today that thermal barrier keeps malaria out of regions where 58 percent of the world's population lives. If the world warms 5 to 9 degrees, estimates the IPCC, 60 percent of the population will live in the malaria zone.

Some disease microbes have already benefited from climate change. Beginning in the late 1980s, the American Southwest experienced six years of drought. Populations of predators such as owls, snakes and coyotes fell. Then, in 1993, the spring rains fell in buckets. Pine nuts and grasshoppers flourished. Deer mice, which eat pine nuts and grasshoppers, exploded in number. The deer mice harbored hantavirus, which makes victims feel like they have the flu but can cause hemorrhaging, kidney disease and death from respiratory failure. Floods drove the mice from their burrows. They bit people; five died. ""This pattern of drought and flood is what the climate models predict,'' says Epstein, ""so we could be in for more outbreaks like this.''

Agriculture is another crucible of climate change. Mark Wilcox's family has been farming the rolling plains of Saskatchewan since 1903. Winters are rough and long, with temperatures dipping below minus 40 for weeks at a stretch. ""Whoever can leave for the winter, does,'' says Wilcox. But the cold weather has been good to Wilcox's wheat, the hard, low-gluten durum variety that millers turn into high-quality pasta flour. Wilcox, if he can coax greater yields from his 2,240 acres as global warming brings him more frost-free days and a longer growing season, could pick up customers from farmers to the south who can no longer grow durum wheat. ""If they grow less,'' says Wilcox, ""what is produced will be at a premium, and that'll help us, price-wise.'' Put Wilcox in the potential winners' column. Similarly, the hard, red winter wheat of the southern plains requires persistent winter cold to set flowers. In a greenhouse world, farmers in Oklahoma and Texas might be able to grow only low-quality, low-price spring wheat. They would be agriculture's losers.

How a warmer world will affect food production is one of the most complicated--and important--questions in the debate over warming. Skeptics argue that a worldwide greenhouse will be as beneficial to crops as an artificial greenhouse. In a warmer world, for one thing, growing seasons will be longer. In the northern temperate region, they are already eight days longer than just 11 years ago. And in a world richer in carbon dioxide (which plants ""breathe'' in), crops should grow larger faster. The IPCC optimistically notes that, ""on the whole, global agricultural production could be maintained [if carbon dioxide levels in the atmosphere doubled, but] regional effects would vary widely.''

That ""but'' conceals a lot of potential pain. Warming will shift climate belts toward the poles, and toward higher elevations, where it is cooler. Plants are already ""climbing'' the Alps at roughly 12 feet per decade. (Plants don't actually pack up their roots and walk to cooler climes, of course: shifting climate zones just changes where seedlings are able to take hold, and where mature plants die off.) That's why Wilcox could win while some farmer in Oklahoma could lose. Also, crops differ in how they respond to extra carbon dioxide. Wheat, rice and soybeans suck it up, and grow faster. Corn and sugarcane don't respond much. Other things being equal, farmers of the first three crops will win; farmers of the others might manage only a draw.

Warmth may seem like natural fertilizer, but ""in fact all plants are adapted to an optimal temperature,'' explains Cynthia Rosenzweig of Columbia University. ""Even Florida citrus yields could fall 30 percent if mean temperatures rise several degrees.'' Corn dies if temperatures reach 95 while the plant is flowering. And in a warmer world with more evaporation, the frequency and severity of dry spells in drought-prone regions may well increase. In the Central Plains, droughts could strike every other year by 2050. Thirsty corn and soybeans might fail in the Southeast and Great Plains but thrive farther north. ""Some farmers will be net winners but others will be net losers,'' says UMass's Hillel, coauthor with Rosenzweig of the forthcoming book ""Climate Change and the Global Harvest.'' ""Florida's orange-crop loss may be South Carolina's gain.''