In their wake come millions who must flee house and home. Unprecedented mass movement defines our global age. But increasingly, among the displaced is a population whose status only in recent years has gained some legitimacy: environmental refugees. It categorizes people who suffer from a wide spectrum of environmental disasters, man-made or natural. Their homes have become inhabitable, veritable wastelands.

Until recently, what made a refugee in the world's eyes was largely defined in political terms. The 1951 U.N. Convention relating to the Status of Refugees defined a refugee as a person with a genuine fear of being persecuted for membership in a particular social group or class. These days, the environmental refugee -- not necessarily persecuted, yet nevertheless forced to flee -- is gaining center stage. There are, according to the United Nations High Commission on Refugees, approximately 18 million political, religious or ethnic refugees in the world today. By comparison, there are "only" an estimated 10 million environmental refugees worldwide. But the term "environmental refugee" has not been officially recognized, and many countries have not bothered to count them, especially if the population is internally displaced. The International Red Cross put the number as high as 25 million in 1999.

A decade ago, ecologist Norman Myers predicted that humanity was slowly heading toward a "hidden crisis" in which ecosystems fail to sustain their inhabitants and people are forced off their land to seek shelter elsewhere. After Hurricanes Katrina and Rita, however, as the world watched in awe and horror while hundreds of thousands of displaced Americans scurried across the richest nation on Earth searching for new homes, Myers' "hidden crisis" is suddenly not so hidden. We can, with certainty, now add a million more to this growing population of environmental refugees.

Being displaced by natural disasters may very well become the central epic of our time. Many nongovernmental organizations and the United Nations itself now estimate that the number of environmental refugees will surpass 150 million by mid-century -- due to agricultural disruption, deforestation, coastal flooding, shoreline erosion, industrial accidents and pollution. In an increasingly interconnected world, a series of environmental catastrophes could bring nations to their knees in a blink of an eye. A huge earthquake in California, for instance, could undermine the U.S. economy, and along with it, the global economy. Entire nations could disappear. The World Bank estimates that with a 50-centimeter rise in sea level, two-thirds of Bangladesh, with a population 140 million, would be underwater, resulting in countless deaths and millions of environmental refugees.

But one need not evoke dramatic risks, such as tsunamis or the impact of large asteroids, to recognize the disasters that are taking place now. China, for example, remains a hot spot of environmental disaster. It is buckling under unsustainable development, rapid air pollution and toxic rivers. Desertification threatens the country's future. The result of these man-made catastrophes has been the displacement of millions.

John Liu, director of the Environmental Education Media Project who spent 25 years in China and witnessed the disasters there, has an unapologetic, four-alarm warning: "Every ecosystem on the planet is under threat of catastrophic collapse, and if we don't begin to acknowledge and solve them, then we will go down."

"One of the marks of a global civilization is the extent to which we begin to conceive of whole-system problems and whole-system responses to those problems," notes political scientist Walt Anderson in his book "All Connected Now." "Events occurring in one part of the world are viewed as a matter of concern for the whole world in general and lead to an attempt at collective solutions."

Whether humanity can move toward a global civilization will depend by and large on how it can act collectively deal with what's arguably the central issue of our time: global warming. There's an old saying, "A rising tide lifts all boats." But in the age of melting glaciers, that tide is an ominous threat. The global age will not be as golden as some had predicted unless this dire challenge is met by whatever means necessary. For rising tides will not just send more refugees fleeing but, if ignored, could swallow humanity itself.