As the new year begins, the price of wheat is setting an all-time high in the United Kingdom. Food riots are spreading across Algeria. Russia is importing grain to sustain its cattle herds until spring grazing begins. India is wrestling with an 18-percent annual food inflation rate, sparking protests. China is looking abroad for potentially massive quantities of wheat and corn. The Mexican government is buying corn futures to avoid unmanageable tortilla price rises. And on January 5, the U.N. Food and Agriculture Organization announced that its food price index for December hit an all-time high.

But whereas in years past, it's been weather that has caused a spike in commodities prices, now it's trends on both sides of the food supply/demand equation that are driving up prices. On the demand side, the culprits are population growth, rising affluence, and the use of grain to fuel cars. On the supply side: soil erosion, aquifer depletion, the loss of cropland to nonfarm uses, the diversion of irrigation water to cities, the plateauing of crop yields in agriculturally advanced countries, and—due to climate change —crop-withering heat waves and melting mountain glaciers and ice sheets. These climate-related trends seem destined to take a far greater toll in the future.

There's at least a glimmer of good news on the demand side: World population growth, which peaked at 2 percent per year around 1970, dropped below 1.2 percent per year in 2010. But because the world population has nearly doubled since 1970, we are still adding 80 million people each year. Tonight, there will be 219,000 additional mouths to feed at the dinner table, and many of them will be greeted with empty plates. Another 219,000 will join us tomorrow night. At some point, this relentless growth begins to tax both the skills of farmers and the limits of the earth's land and water resources.

Beyond population growth, there are now some 3 billion people moving up the food chain, eating greater quantities of grain-intensive livestock and poultry products. The rise in meat, milk, and egg consumption in fast-growing developing countries has no precedent. Total meat consumption in China today is already nearly double that in the United States.

The third major source of demand growth is the use of crops to produce fuel for cars. In the United States, which harvested 416 million tons of grain in 2009, 119 million tons went to ethanol distilleries to produce fuel for cars. That's enough to feed 350 million people for a year. The massive U.S. investment in ethanol distilleries sets the stage for direct competition between cars and people for the world grain harvest. In Europe, where much of the auto fleet runs on diesel fuel, there is growing demand for plant-based diesel oil, principally from rapeseed and palm oil. This demand for oil-bearing crops is not only reducing the land available to produce food crops in Europe, it is also driving the clearing of rainforests in Indonesia and Malaysia for palm oil plantations.

The combined effect of these three growing demands is stunning: a doubling in the annual growth in world grain consumption from an average of 21 million tons per year in 1990-2005 to 41 million tons per year in 2005-2010. Most of this huge jump is attributable to the orgy of investment in ethanol distilleries in the United States in 2006-2008.

While the annual demand growth for grain was doubling, new constraints were emerging on the supply side, even as longstanding ones such as soil erosion intensified. An estimated one third of the world's cropland is losing topsoil faster than new soil is forming through natural processes—and thus is losing its inherent productivity. Two huge dust bowls are forming, one across northwest China, western Mongolia, and central Asia; the other in central Africa. Each of these dwarfs the U.S. dust bowl of the 1930s.

Satellite images show a steady flow of dust storms leaving these regions, each one typically carrying millions of tons of precious topsoil. In North China, some 24,000 rural villages have been abandoned or partly depopulated as grasslands have been destroyed by overgrazing and as croplands have been inundated by migrating sand dunes.

In countries with severe soil erosion, such as Mongolia and Lesotho, grain harvests are shrinking as erosion lowers yields and eventually leads to cropland abandonment. The result is spreading hunger and growing dependence on imports. Haiti and North Korea, two countries with severely eroded soils, are chronically dependent on food aid from abroad.

Meanwhile aquifer depletion is fast shrinking the amount of irrigated area in many parts of the world; this relatively recent phenomenon is driven by the large-scale use of mechanical pumps to exploit underground water. Today, half the world's people live in countries where water tables are falling as overpumping depletes aquifers. Once an aquifer is depleted, pumping is necessarily reduced to the rate of recharge unless it is a fossil (nonreplenishable) aquifer, in which case pumping ends altogether. But sooner or later, falling water tables translate into rising food prices.

Irrigated area is shrinking in the Middle East, notably in Saudi Arabia, Syria, Iraq, and possibly Yemen. In Saudi Arabia, which was totally dependent on a now-depleted fossil aquifer for its wheat self-sufficiency, production is in a freefall. From 2007 to 2010, Saudi wheat production fell by more than two thirds. By 2012, wheat production will likely end entirely, leaving the country totally dependent on imported grain.

The Arab Middle East is the first geographic region where spreading water shortages are shrinking the grain harvest. But the really big water deficits are in India, where the World Bank numbers indicate that 175 million people are being fed with grain that is produced by overpumping. In China, overpumping provides food for some 130 million people. In the United States, the world's other leading grain producer, irrigated area is shrinking in key agricultural states such as California and Texas.

The last decade has witnessed the emergence of yet another constraint on growth in global agricultural productivity: the shrinking backlog of untapped technologies. In some agriculturally advanced countries, farmers are using all available technologies to raise yields. In Japan, the first country to see a sustained rise in grain yield per acre, rice yields have been flat now for 14 years. Rice yields in South Korea and China are now approaching those in Japan. Assuming that farmers in these two countries will face the same constraints as those in Japan, more than a third of the world rice harvest will soon be produced in countries with little potential for further raising rice yields.

A similar situation is emerging with wheat yields in Europe. In France, Germany, and the United Kingdom, wheat yields are no longer rising at all. These three countries together account for roughly one-eighth of the world wheat harvest. Another trend slowing the growth in the world grain harvest is the conversion of cropland to nonfarm uses. Suburban sprawl, industrial construction, and the paving of land for roads, highways, and parking lots are claiming cropland in the Central Valley of California, the Nile River basin in Egypt, and in densely populated countries that are rapidly industrializing, such as China and India. In 2011, new car sales in China are projected to reach 20 million—a record for any country. The U.S. rule of thumb is that for every 5 million cars added to a country's fleet, roughly 1 million acres must be paved to accommodate them. And cropland is often the loser.

Fast-growing cities are also competing with farmers for irrigation water. In areas where all water is being spoken for, such as most countries in the Middle East, northern China, the southwestern United States, and most of India, diverting water to cities means less irrigation water available for food production. California has lost perhaps a million acres of irrigated land in recent years as farmers have sold huge amounts of water to the thirsty millions in Los Angeles and San Diego.

The rising temperature is also making it more difficult to expand the world grain harvest fast enough to keep up with the record pace of demand. Crop ecologists have their own rule of thumb: For each 1 degree Celsius rise in temperature above the optimum during the growing season, we can expect a 10 percent decline in grain yields. This temperature effect on yields was all too visible in western Russia during the summer of 2010 as the harvest was decimated when temperatures soared far above the norm.

Another emerging trend that threatens food security is the melting of mountain glaciers. This is of particular concern in the Himalayas and on the Tibetan plateau, where the ice melt from glaciers helps sustain not only the major rivers of Asia during the dry season, such as the Indus, Ganges, Mekong, Yangtze, and Yellow rivers, but also the irrigation systems dependent on these rivers. Without this ice melt, the grain harvest would drop precipitously and prices would rise accordingly.

And finally, over the longer term, melting ice sheets in Greenland and West Antarctica, combined with thermal expansion of the oceans, threaten to raise the sea level by up to six feet during this century. Even a three-foot rise would inundate half of the riceland in Bangladesh. It would also put under water much of the Mekong Delta that produces half the rice in Vietnam, the world's number two rice exporter. Altogether there are some 19 other rice-growing river deltas in Asia where harvests would be substantially reduced by a rising sea level.

The current surge in world grain and soybean prices, and in food prices more broadly, is not a temporary phenomenon. We can no longer expect that things will soon return to normal, because in a world with a rapidly changing climate system there is no norm to return to.

The unrest of these past few weeks is just the beginning. It is no longer conflict between heavily armed superpowers, but rather spreading food shortages and rising food prices—and the political turmoil this would lead to—that threatens our global future. Unless governments quickly redefine security and shift expenditures from military uses to investing in climate change mitigation, water efficiency, soil conservation, and population stabilization, the world will in all likelihood be facing a future with both more climate instability and food price volatility. If business as usual continues, food prices will only trend upward.

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

Copyright © 2011 Earth Policy Institute

*NOTE: This piece originally appeared in Foreign Policy on Tuesday, January 10, 2011.