The Cold War shaped world politics for half a century. But global warming may shape the patterns of global conflict for much longer than that — and help spark clashes that will be, in every sense of the word, hot wars.

We're used to thinking of climate change as an environmental problem, not a military one, but it's long past time to alter that mindset. Climate change may mean changes in Western lifestyles, but in some parts of the world, it will mean far more. Living in Washington, I may respond to global warming by buying a Prius, planting a tree or lowering my thermostat. But elsewhere, people will respond to climate change by building bomb shelters and buying guns.

"There is every reason to believe that as the 21st century unfolds, the security story will be bound together with climate change," warns John Ashton, a veteran diplomat who is now the United Kingdom's first special envoy on climate change. "The last time the world faced a challenge this complex was during the Cold War. Yet the stakes this time are even higher because the enemy now is ourselves, the choices we make."

Defense experts have also started to see the link between climate change and conflict. A 2007 CNA Corp. report, supervised by a dozen retired admirals and generals, warned that climate change could lead to political unrest in numerous badly hit countries, then perhaps to outright bloodshed and battle. One key factor that could stoke these tensions is massive migration as people flee increasingly uninhabitable areas, which would lead to border tensions, greater demands for rescue and evacuation services and disputes over essential resources.

With these threats looming, the U.N. Security Council held a precedent-setting debate on climate change in April 2007 — explicitly casting global warming as a national security issue.

Global warming could lead to warfare in three different ways.

The first is conflict arising from scarcity. As the world gets hotter and drier, glaciers will melt, and the amount of arable land will shrink. In turn, fresh water, plants, crops and cattle and other domestic animals will be harder to come by, thereby spurring competition and conflict over what's left. In extreme examples, a truly desiccated ecosystem could mean a complete evacuation of a hard-hit region. And the more people move, the more they will jostle with their new neighbors.

Such displacement can arise either suddenly or slowly. The growth of the Sahara, for instance, took many millenniums; many thousands of years ago, people were slowly nudged out of the inland region of northern Africa and into such great river valleys as the Nile and the Niger. Over time, incremental but prolonged rises in sea levels will also gradually uproot hundreds of millions of people.

But sometimes the displacement happens with shocking speed: Just think of the deadly hurricanes Katrina and Rita, which together drove millions of people to suddenly leave Louisiana, Mississippi and Texas.

As global warming and population growth increase, we could see far deadlier storms than Katrina. In 1991, a cyclone in Bangladesh displaced 2 million people and killed 138,000.

All this can lead to warfare when it's time for the displaced to find a new home. For most of human history, they could at least theoretically do so in unclaimed lands — a sort of territorial pressure valve whose existence tamped down conflict. But today, this reservoir of vacant turf no longer exists, except in the least hospitable parts of the planet. So when the displaced start eyeing currently inhabited areas, expect trouble — and the bigger the displacement, the bigger the fight.

The second cause of the coming climate wars is the flip side of scarcity: the problems of an increase in abundance. Suppose that global warming makes a precious resource easier to get at — say, rising temperatures in northern Canada, Alaska and Siberia make it easier to get at oil and gas resources in regions that had previously been too bone-chilling to tap. But what happens if some tempting new field pops up in international waters contested by two great powers? Or if smaller countries with murky borders start arguing over newly arable land?

James R. Lee runs American University's Inventory of Conflict and Environment project. He is at work on a book on climate change and conflict. Author's e-mail: jlee@american.edu.