Crops in the tropics will do worse, since extra heat where it's already hot hurts more than it helps.
Whenever global warming began looking too bleak—with such threats as dengue fever spreading like kudzu out of the tropics, storm surges turning coasts into continental shelf and Katrinas coming almost as often as Mardi Gras—I consoled myself by turning to the Greening Earth Society. A creation of the coal industry, whose product emits more greenhouse gases than any other fossil fuel, it painted climate change in Edenic terms, promising that the atmosphere's rising levels of carbon dioxide would act like airborne fertilizer, boosting crop yields and turning marginal regions into breadbaskets.
Turns out they weren't noticing the Gangotri glacier.
One of the Himalayas' largest, it has been shrinking since the late 18th century. But over the last 25 years it has shrunk about half a mile, a rate three times the historical norm. The retreat threatens more than the loss of a panoramic background for tourist snapshots. The Gangotri supplies 70 percent of the flow of India's Ganges River during the dry season, when farmers depend on it for irrigation. Glaciers on the Tibet-Qinghai Plateau, which are also shrinking, feed major rivers in China, which are also crucial for irrigation. "Without the ice melt, the Ganges and the Yellow rivers could dry up in the dry season, shrinking harvests," says Lester Brown, president of the Earth Policy Institute. "If the Ganges flows only part of the year, double cropping [in which farmers plant rice and wheat in back-to-back growing seasons, and which underlies India's green revolution] breaks down."
There is nothing like food riots to concentrate the mind on how climate change will affect harvests. You'd think that, for agriculture, warmer is better. Peach harvests are more bountiful in Georgia than Vermont, and the nation's breadbasket isn't North Dakota. No surprise, then, that the international panel of scientists that analyzes climate change concluded last year that the mid- and high latitudes will enjoy higher crop yields when average temperatures rise 1 to 3 degrees above today's, something we're on track for by 2020. Crops in the tropics and subtropics will generally fare worse, though, because "most tropical plants are already very close to the maximum temperature they can thrive in," says William Easterling of Penn State, who oversaw the analysis of climate change and food production for the Intergovernmental Panel on Climate Change. "Any warming pushes them beyond the optimum for photosynthesis and yield." But then, global warming was always going to produce some winners and some losers. (Look for Siberian wheat in your favorite ciabatta any day now.) However, details such as how vanishing glaciers will affect rivers that provide irrigation water to India and China are only now being factored in, and the results are enough to make you hoard 20-pound bags of rice.
Rain is one of those details. Overall, global warming will bring more rainfall, partly because warmer air holds more moisture. But rainfall has been coming in fits and starts—dry spells interrupted by deluges. That is a prescription for withering crops and then washing them away. More such "precipitation extremes," as the IPCC delicately calls them, are likely in South and East Asia, parts of Australia (which recently suffered its worst drought since 1900) and Northern Europe. Globally, the amount of land experiencing very dry conditions rose from less than 15 percent in 1970 to 35 percent today. You have to plow through 279 pages of the IPCC report to discover that its projections of higher crop yields in a slightly warmer world "do not yet include these recent findings on projected patterns of change in precipitation."
Even if average temps rise only a little, a greenhouse world has more heat waves. During Europe's 2003 scorcher, which brought temperatures 10 degrees above normal, Italy's corn yields fell 36 percent (a record), France's fruit harvest fell 25 percent and wine production was the lowest in 10 years. Crop losses totaled $14.6 billion. Get used to it. "In the past, the negative effect of unusual weather events was always temporary; within a year or two, things returned to normal," says Brown. "With climate in flux, there is no normal to return to."
In the United States, diminishing snowpack, particularly in the North-west, means less water is stored for the growing season. But the greater risk posed by climate change is that the match between weather and soil conditions will break down. Just because climate zones shift north doesn't mean crops can easily shift with them, says Linda Mearns of the National Center for Atmospheric Research. Corn grows in Iowa and wheat in Kansas because those pairings provide the ideal climate, soil and—crucially—"photoperiod." Plants need a precise amount of sunlight and day length to thrive, but "the sun will not move with the climate," says Mearns; the Dakotas will always have less daylight than Kansas, and thus be less hospitable to crops. Higher temps also mean that fewer crops are pollinated: corn tassels dry up above about 70 degrees; rice pollination falls from 100 percent at current temperatures to 50 percent if the world gets 3 degrees warmer. Since governments are doing basically nothing to avert global warming, our agriculture policy amounts to this: pray for good weather.