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As the UN climate change conference in Copenhagen approaches, we are in a race between political tipping points and natural ones. Can we cut carbon emissions fast enough to keep the melting of the Greenland ice sheet from becoming irreversible? Can we close coal-fired power plants in time to save at least the larger glaciers in the Himalayas and on the Tibetan plateau? Can we head off ever more intense crop-withering heat waves before they create chaos in world grain markets?

These are all climate change issues, but they have something else in common: food.

We need not go beyond ice melting to see that the world is in trouble on the food front. As the Greenland and West Antarctic ice sheets continue to shrink, sea levels will rise, threatening rice harvests around the globe. Recent projections show that the sea could rise up to six feet this century (if the Greenland ice sheet were to melt entirely, it would rise by 23 feet).

According to the World Bank, it would take only a three-foot rise in sea level to cover half the rice fields in Bangladesh, a country of nearly 160 million people. Such an increase would also inundate much of the Mekong Delta, which produces half the rice crop in Vietnam, the world’s No. 2 rice exporter. And it would submerge parts of the 20 or so other rice-growing river deltas in Asia.

Melting mountain glaciers are even more worrisome. The World Glacier Monitoring Service in Switzerland recently reported the 18th consecutive year of shrinking mountain glaciers around the world, from the Andes to the Rockies, from the Alps to the mountain ranges of Asia. Of these, the disappearance of glaciers in the Himalayas and on the Tibetan plateau threatens to shrink food supplies most sharply. Their annual ice melt sustains the major rivers of India and China — the Indus, Ganges, Yangtze and Yellow — during the dry season. And this water in turn supplies irrigation systems.

Yao Tandong, one of China’s leading glaciologists, warned last year in the journal Nature that two-thirds of the country’s glaciers could be gone by 2050, and he has said that “the full-scale glacier shrinkage in the plateau regions will eventually lead to an ecological catastrophe.”

It will also lead to a humanitarian catastrophe. China is the world’s leading producer of wheat. India is No. 2. These two countries also dominate the world’s rice harvest. But unlike in the United States (the third-largest wheat producer), where wheat is watered largely by rainfall, most crops in China and India are irrigated. The vanishing of mountain glaciers in Asia therefore represents the biggest threat to the world food supply that we have ever seen.

If China can no longer grow enough wheat and rice to feed its 1.3 billion people and goes shopping for massive quantities of grain, global food costs will rise dramatically. When domestic food prices skyrocketed in the 1970s, the United States restricted exports of grain and soybeans. This time around, with China holding $800 billion in US Treasury securities, we won’t be in any position to limit exports. China is our banker. China’s food shortage will be our shortage, too.

Sea levels and melting glaciers aside, rising temperatures will also have a direct impact on crops around the world. Agriculture as it exists today evolved over an 11,000-year period of remarkable climate stability. The crops we grow were bred to flourish in this climate. But they will become increasingly out of sync with their environment.

Even a relatively minor increase in temperature will dramatically shrink crop yields. A 2004 study published by the US National Academy of Sciences showed that for each 1 degree Celsius rise in temperature during the growing season, we can expect a 10 percent decline in rice yields. This appears to hold true for wheat and corn as well. In a world with limited grain stocks, a crop-shrinking heat wave in a major grain-producing region could lead to food shortages and political instability.

While delegates from around the world prepare to gather in Copenhagen, hunger is already spreading. For much of the late 20th century, the number of hungry people declined, bottoming out in the mid-1990s at 825 million. It then turned upward, reaching 873 million around 2005 and passing one billion in 2009, according to the UN Food and Agriculture Organization. The combination of rising seas, melting glaciers and crop-decimating heat waves could push these numbers up even faster.

As the number of hungry people has risen, so too has the number of failing states. How much hunger will the world be able to absorb before we have not just failing nations, but a failing global civilization?

Lester R Brown is president of the Earth Policy Institute and the author of “Plan B 4.0: Mobilizing to Save Civilization. ”

The Washington Post