THE GREAT AGRICULTURAL system that feeds the human race is in trouble.

The rapid growth in farm output of the late 20th century has slowed such that it is failing to keep up with demand, driven by population increases and rising affluence in oncepoor countries.

Consumption of the four staples that supply most human calories - wheat, rice, corn and soybeans - has outstripped production for much of the past decade. The imbalance has resulted in two huge spikes in international grain prices since 2007, with some grains more than doubling in cost.


Global population is expected to reach 10 billion by the end of the century, and food output will have to double. Mexican farmers face water shortages. Right, Tortilleria Alma, a wheat tortilla factory in Mexico.

Those price jumps have worsened hunger for tens of millions of poor people, destabilizing politics in scores of countries, from Mexico to Uzbekistan to Yemen. The Haitian government was ousted in 2008 amid food riots, and anger over prices has played a role in Arab uprisings. Now, research suggests that a previously discounted factor is helping to destabilize the food system: climate change.

Many of the failed harvests of the past decade were a consequence of weather disasters, like floods in the United States, drought in Australia and blistering heat waves in Europe and Russia. Scientists link some of those events to human-induced global warming.

Temperatures are rising rapidly during the growing season in some of the most important agricultural countries, and a recent paper found that this had shaved several percentage points off potential yields, adding to the price gyrations.

For nearly two decades, scientists had predicted that climate change would be relatively manageable for agriculture, suggesting that it would probably take until 2080 for food prices to double.

In part, they assumed that rising carbon dioxide levels, the primary contributor to global warming, would act as a powerful plant fertilizer and offset many of the ill effects of climate change.

But the destabilization of the food system and the soaring prices have rattled many scientists.

“The success of agriculture has been astounding,” said Cynthia Rosenzweig, a researcher at NASA who helped pioneer the study of climate change and agriculture. “But I think there’s starting to be premonitions that it may not continue forever.”

Some researchers who advise governments on future crop prospects are pointing out what they consider to be gaping holes in computer forecasts. These include a failure to consider the effects of extreme weather that are increasing as the earth warms.


A rising unease about the future of the world’s food supply came through during interviews this year with more than 50 agricultural experts working in nine countries. These experts say that in coming decades, farmers need to withstand climate shocks while doubling the amount of food they produce to meet demand. And they need to do it while reducing the environmental damage caused by agriculture.

The situation is far from hopeless. From Mexico to India, farmers are showing that it may be possible to make agriculture more productive and resilient in the face of climate change. They have achieved huge gains in output in the past, and rising prices are a powerful incentive to do so again.

But new crop varieties and new techniques are required, scientists said. Despite the urgent need, they added, promised financing has been slow to materialize, much of the necessary work has yet to begin and, once it does, it is likely to take decades to bear results.

“There’s just such a tremendous disconnect, with people not understanding the highly dangerous situation we are in,” said Marianne Banziger, deputy chief of the International Maize and Wheat Improvement Center, a leading research institute in Mexico that is part of a global network of centers that focus on the world’s major crops. Others are in China, Colombia, Turkey, Georgia and the Philippines. “What a horrible world it will be if food really becomes short,” said Matthew Reynolds, a wheat physiologist at the Mexican center. “What will that do to society?”


- Farmers See Patterns Change


In the Yaqui Valley, in the Sonoran Desert of northwestern Mexico, wheat farmers like Francisco Javier Ramos Bours believe that climate change could be responsible for water shortages.

“All the world is talking about it,” Mr. Ramos said.

Farmers everywhere are facing water shortages as well as flash floods. Their crops are afflicted by emerging pests and diseases and by record heat.

In northeastern India, a rice farmer named Ram Khatri Yadav offered his own complaint. “It will not rain in the rainy season, but it will rain in the nonrainy season,” he said. “The cold season is also shrinking.”

Decades ago, Yaqui Valley wheat farmers were the vanguard of the Green Revolution, which used improved crop varieties and more intensive farming methods to raise food production across much of the developing world.

Norman E. Borlaug, a n American agronomist, began working here in the 1940s. His successes as a breeder helped raise Mexico’s wheat output sixfold. In the 1960s, he took his approach to India and Pakistan, where mass starvation was feared. Output soared there, too.

Other countries joined the Green Revolution, and food production outstripped population growth through the latter half of the 20th century. In 1970, Dr. Borlaug became the only agronomist ever to win the Nobel Peace Prize.

But in Oslo he issued a stern warning. “We may be at high tide now,” he said, “but ebb tide could soon set in if we become complacent.”

Just as he had predicted, cutbacks in agricultural research and development began to show up in the world’s food system toward the end of the century.

That lull occurred just as food and feed demand was starting to take off, thanks in part to rising affluence in Asia. Millions of people added meat and dairy products to their diets, requiring grain to produce. A policy of converting much of the American corn crop into ethanol contributed to demand.

Erratic weather, like a 2003 heat wave in Europe and a long drought in Australia, both possibly linked to climate change, cut wheat and rice production.

In 2007-2008, with grain stockpiles low, prices doubled or tripled. Countries began hoarding food, and panic buying ensued. Food riots broke out in more than 30 countries.

Farmers responded by planting as much as possible, and healthy harvests in 2008 and 2009 helped rebuild stocks. That factor, plus the global recession, drove prices down in 2009. But by last year, more weather-related harvest failures sent them soaring again. This year, rice supplies are adequate, but bad weather is threatening the wheat and corn crops in some areas.

Experts fear that the era of cheap food may be over. “Our mindset was surpluses,” said Dan Glickman, a former United States secretary of agriculture. “That has just changed overnight.”

The recent price spikes have helped cause the largest increases in world hunger in decades. The Food and Agriculture Organization of the United Nations estimated the number of hungry people at 925 million last year, and the number is expected to be higher this year. The World Bank says it could be as high as 940 million.

Hans-Joachim Braun, the current head of the Mexican corn and wheat institute, said booming cities are chewing up agricultural land and competing with farmers for water. In some of the world’s breadbaskets, farmers are pumping groundwater much faster than nature can replenish it.

“This is in no way sustainable,” Dr. Braun said.

The farmers of the Yaqui Valley grow their wheat in a near-desert. Their water comes by aqueduct from nearby mountains, but for parts of the past decade, rainfall was below normal. And Northern Mexico lies within a global belt that is expected to dry further because of greenhouse gases.

Dr. Braun is leading efforts to produce new wheat varieties able to withstand many kinds of stress, including scant water. But budgets are extremely tight. “If we don’t get started now,” he said, “we are going to be in serious trouble.”


- Shaken Assumptions


Scientists long believed that dependence on fossil fuels, for all its problems, would offer one enormous benefit.

Carbon dioxide, the main gas released by combustion, is also the primary fuel for plant growth. Using the energy from sunlight, they convert carbon in the air into energy-dense compounds like glucose. All life runs on these compounds.

Humans have already raised the level of carbon dioxide in the atmosphere by 40 percent since the Industrial Revolution, and are on course to double or triple it this century. Studies have long suggested that the extra gas would supercharge the world’s food crops.

But many of those studies were done in artificial conditions. For the past decade, scientists at the University of Illinois have been putting the “CO2 fertilization effect” to a real-world test.

They planted soybeans in a field, then sprayed extra carbon dioxide from a giant tank. They hoped the gas might bump yields as much as 30 percent.

But at harvest, the bump was only half as large. Their tests on corn, America’s most valuable crop and the basis for its meat production and its biofuel industry, were even worse. There was no bump.

Their work and that of others suggests that extra carbon dioxide does act as plant fertilizer, but that the benefits are probably less than needed to avert food shortages.

Other recent evidence suggests that longstanding assumptions about food production on a warming planet may have been too optimistic.

Two economists, Wolfram Schlenker of Columbia University in New York and Michael J. Roberts of North Carolina State University, have compared crop yields and natural temperature variability at a fine scale. Their work shows that when crops are subjected to temperatures above a certain threshold - about 29 degrees Celsius for corn and 30 degrees Celsius for soybeans - yields fall sharply.

This suggests that in some climates, with more scorching days, some crop yields could fall by 30 percent or more.

A paper by David B. Lobell of Stanford University in California and Dr. Schlenker suggests that temperature increases in France, Russia, China and other countries are already suppressing crop yields.

“I think there’s been an under-recognition of just how sensitive crops are to heat ,” Dr. Lobell said.

Such research is controversial. The findings go somewhat beyond those of a 2007 report by the United Nations’ Intergovernmental Panel on Climate Change, which found that while climate change was likely to pose severe challenges for agriculture in the tropics, it would probably be beneficial in some of the chillier regions of the Northern Hemisphere, aided by the carbon dioxide effect.

At the University of Illinois, a leading scientist behind the work there, Stephen P. Long, sharply criticized the report.

“I felt it needed to be much more honest in saying this is our best guess at the moment, but there are probably huge errors in there,” he said.

Oxfam, the international relief group, projected recently that food prices would more than double by 2030, with climate change responsible for perhaps half the increase. Dr. Rosenzweig, the NASA climate scientist, played a leading role in forming the old consensus. But she is taking a fresh look.

She is pulling together a global consortium of researchers whose goal will be to produce more detailed and realistic computer forecasts; she won high-level endorsement from British and United States officials. “We absolutely have to get the science lined up to provide these answers,” Dr. Rosenzweig said.


- Need for Cash and Conviction

Last June, in the remote Indian village of Samhauta, Anand Kumar Singh, a farmer, planted a new variety of rice. On August 23, a severe flood submerged his field for 10 days. In the past, such a flood would have destroyed his crop. But the new variety yielded a robust harvest.

“That was a miracle,” Mr. Singh said.

The miracle illustrated how far scientists may go in helping farmers adapt, but some leading researchers are less certain that crops can be made to withstand withering heat, though genetic engineering may eventually succeed.

Decades of work were required to improve the new rice strain, and money was tight; distribution to farmers was not assured. Then the Bill & Melinda Gates Foundation stepped in with a $20 million grant to finance final development and distribution of the rice in India and other countries. It may get into a million farmers’ hands this year.

The Gates Foundation has awarded $1.7 billion for agricultural projects since 2006, but governments realize that more effort is needed on their part.

In 2008 and 2009, in the midst of the crises set off by food prices, the world’s governments outbid one another to offer support. At a conference in L’Aquila, Italy, they pledged about $22 billion. But the financing has not fully materialized.

“It’s a disappointment,” Mr. Gates said.

President Obama pledged $3.5 billion at L’Aquila, more than any other country, and the United States has begun an initiative to support agricultural development in 20 of the neediest countries.

But amid budget struggles in Washington, the administration has won $1.9 billion from Congress. Perhaps the most hopeful sign is that poor countries themselves are starting to invest in agriculture in a serious way, as many did not do in the years when food was cheap.

In Africa, a dozen countries are on the verge of fulfilling a promise to devote 10 percent of their budgets to farm development, up from 5 percent or less.

“In my country, every penny counts,” said Agnes Kalibata, the agriculture minister of Rwanda. With difficulty, Rwanda has met the 10 percent pledge, and she cited a terracing project in the country’s highlands that has raised potato yields by 600 percent.

The United Nations recently projected that global population would hit 10 billion by the end of the century, 3 billion more than today.

The projections mean that food production may need to double by later in the century.

Unlike in the past, that demand must somehow be met on a planet where little new land is available , where water supplies are tightening, where the temperature is rising, where the weather has become erratic and where the food system is showing serious signs of instability.

“We’ve doubled the world’s food production several times before in history, and now we have to do it one more time,” said Jonathan A. Foley, a researcher at the University of Minnesota. “The last doubling is the hardest. It is possible, but it’s not going to be easy.”


By JUSTIN GILLIS