China is at war. It is not invading armies but expanding deserts that threaten its territory.

As old deserts grow, as new ones form and as more and more irrigation wells go dry, Beijing is losing a long battle to feed its growing population on its own.

In years to come, China will almost certainly have to turn to the outside world for grain to avoid politically destabilizing price spikes.

Enter the United States -- by far the world's largest grain exporter. The United States exports about 90 million tons of grain annually.

China requires 80 million tons of grain each year to meet just one-fifth of its needs.

China is America's banker; America could become China's farmer. To be dependent on imported grain, much of it from the United States, is China's worst nightmare. It could create nightmares for U.S. consumers, as well.

The evidence of China's plight is clear. Since 1950, some 24,000 villages in the northwestern part of the country have been totally or partially abandoned as sand dunes encroach on cropland.

And with millions of Chinese farmers drilling wells to expand their harvests, water tables are falling under much of the North China Plain, which produces half of the nation's wheat and a third of its corn.

Chinese agriculture is also losing irrigation water to cities and factories. Cropland is being sacrificed for residential and industrial construction, including highways and parking lots that accommodate China's voracious demand for automobiles.

In 2009, automobile sales in China totaled just under 14 million, surpassing those in the United States for the first time. For every 1 million cars added to this fleet, at least 50,000 acres are paved over.

China's food supply is already tightening. In November, its food price index was up 12 percent from 2009. The price of vegetables was up 62 percent.

In these conditions, how do you feed more than 1 billion people? This question vexes China's leaders, many of whom are survivors of the Great Famine, in which 30 million people starved to death between 1959 and 1961.

Last year, to halt rising food prices, the government auctioned corn, wheat, rice and soybeans from state reserves.

And in recent years, China has bought or leased land in other countries from Sudan to Indonesia to produce food and biofuels. But there is little to show in production from these lands so far.

If China, which imported about 2 million tons of U.S. corn and wheat combined in 2010, charges into the U.S. grain market, American consumers will find themselves competing with nearly 1.4 billion foreign consumers for the U.S. grain harvest.

That would raise the prices not only of products made directly from grain, such as bread, pasta and breakfast cereals, but also of meat, milk and eggs, which take large quantities of grain to produce. Corn futures have already hit $7 a bushel, up from $2 a bushel five years ago.

China has been here before -- with soybeans. In 1995, around the time the Communist Party prioritized grain production, China produced and consumed 14 million tons of soybeans.

By 2010, China was still producing 14 million tons of soy annually, but consuming 69 million tons. The change was dramatic, and it resulted in the restructuring of agriculture in the Western Hemisphere.

To meet overseas demand, the United States now has more land in soybeans than wheat. Brazil has more land in soybeans than in all grains combined. And Argentina is fast becoming a soybean monoculture. Today, nearly 60 percent of world soybean exports -- almost all from these three countries -- go to China.

Of course, when selling food to China, the United States is dealing with both an economic competitor and a creditor holding $900 billion worth of U.S. Treasury securities.

If China pushes U.S. food prices higher, tensions between the two countries may escalate. An even greater stress may develop between Washington and U.S. consumers, as Americans -- who think cheap food is a birthright -- are likely to press for restrictions on exports to China.

There is precedent for this: In the 1970s, the United States banned exports of soybeans to countries such as Japan to quash domestic food price inflation.

Though withholding food from an emerging superpower could lower domestic food prices, it would be bad diplomacy.

Even during the Cold War, the United States exported 10 million tons of wheat -- nearly a quarter of the U.S. harvest -- to the Soviet Union in 1972 after a crop failure there. Well-fed enemies are more predictable.

Would this work today? The Obama administration -- or any future administration -- faces a choice. If we limit grain sales to China, might the Chinese limit their monthly purchases at Treasury securities auctions?

What would happen to farmers who can't sell to the world's largest food market? We can't know how this tension will play out politically, but we do know that our huge deficits of the past 30 years restrict our bargaining power.

The United States has been the world's breadbasket for more than half a century. Our country has never known food shortages or spiraling food prices. But we will probably have to share our harvest with the Chinese, no matter how much that raises our prices.

Our world is about to change. It's hard to imagine that it will be for the better.

Lester R. Brown is president of the Earth Policy Institute and the author of "World on the Edge: How to Prevent Environmental and Economic Collapse."

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