The place where most of the world's people could first begin to feel the consequences of global warming may come as a surprise: in the stomach, via the supper plate.
That's the view of a small but influential group of agricultural experts who are increasingly worried that global warming will trigger food shortages long before it causes better known but more distant threats, such as rising sea levels that flood coastal cities.
The scale of agriculture's vulnerability to global warming was highlighted late last year when the Consultative Group on International Agricultural Research (CGIAR), an umbrella organization representing 15 of the world's top crop research centres, issued an astounding estimate of the impact of climate change on a single crop, wheat, in one of the world's major breadbaskets.
Researchers using computer models to simulate the weather patterns likely to exist around 2050 found that the best wheat-growing land in the wide arc of fertile farmland stretching from Pakistan through Northern India and Nepal to Bangladesh would be decimated. Much of the area would become too hot and dry for the crop, placing the food supply of 200 million people at risk.
"The impacts on agriculture in developing countries, and particularly on countries that depend on rain-fed agriculture, are likely to be devastating," says Dr. Louis Verchot, principal ecologist at the World Agroforestry Centre in Nairobi, Kenya.
Wheat, the source of one-fifth of the world's food, isn't the only crop that could be clobbered by climate change. Cereals and corn production in Africa are at risk, as is the rice crop in much of India and Southeast Asia, according to Dr. Verchot.
In a cruel twist of fate, most of the hunger resulting from global warming is likely to be felt by those who haven't caused the problem: the people in developing countries. At the same time, it may be a boon to agriculture in richer northern countries more responsible for the greenhouse gas emissions driving climate instability.
"With climate change, the agricultural areas in Canada, Russia and Europe will expand, while the areas suited for agriculture in the tropics will decline," Dr. Verchot says. "Basically, the situation is that those who are well off now will be better off in the future, and those who are in problems will have greater problems."
Agriculture is vulnerable to global warming because the world's most widely eaten grains corn, wheat, and rice are exquisitely sensitive to higher temperatures. In the tropics and subtropics, many crops are already being grown just under the maximum temperatures they can tolerate.
Over the 10,000 years that humans have farmed, temperatures have been remarkably stable, at current levels or slightly cooler, and plants are finely attuned to this climate regimen.
Although it doesn't work exactly the same for each crop, a rough rule of thumb developed by crop scientists is that, for every 1-degree Celsius increase in temperatures above the mid-30s during key stages in the growing season, such as pollination, yields fall about 10 per cent.
In the case of rice, researchers found the plants were most sensitive to higher nighttime temperatures. For crops in general, optimum growing conditions generally range from about 20 to 35 degrees, and then diminish sharply. At 40 degrees temperatures that are now starting to occur in many areas heat stress causes photosynthesis to shut down. Such high temperatures are starting to become more common, such as during the devastating heat wave in much of Europe in the summer of 2003 that killed tens of thousands.
Average global temperatures will likely rise between 1.1 to 6.4 degrees over the next century, according to the authoritative Intergovernmental Panel on Climate Change, suggesting that, over most of the range of future temperatures, crops will suffer problematic declines. The panel is also warning that global warming will alter rainfall patterns, causing increasing numbers of droughts and floods.
The threatened wheat-growing area around India is known as the Indo-Gangetic Plain. Summer temperatures already sometimes reach a sizzling 45 degrees there, even though global warming is in its early days.