In China the Gobi desert expands more than 10,000 square miles (26,000 square kilometers) each year, forcing many farmers there to leave in search of greener pastures, the UNU-EHS reports.
Thousands of miles away in Alaska about 200 villages are in serious danger of coastal erosion and flooding due to rising sea levels.
The entire Inupiat village of Shishmaref, population 600, is planning to move 13.5 miles (22 kilometers) inland in the next four years to avoid rising waters.
"Human migrations are expected to increase as average global temperatures continue to rise and we experience rising sea levels, more severe weather disasters, and other impacts as a result," said Janet Sawin, a climate change expert at the Worldwatch Institute in Washington, D.C.
Prolonged droughts, another possible effect of global warming, could also force people to leave increasingly arid lands. Some people, who traditionally migrate only temporarily, may not come back.
"There are indications in northeastern Brazil, for example, that some of these adaptations to cyclical events are becoming permanent," Oliver-Smith said.
Because many environmental changes are ultimately caused by humans, some experts, including Oliver-Smith, say the term "environmental refugee" is misleading.
"It tends to suggest that nature is at fault, when in fact humans are deeply implicated in the environmental changes that make life impossible in certain circumstances," Oliver-Smith said.
"To call Hurricane Katrina a natural disaster is a bit of misnomer," he added. "The damage would not have been nearly as bad had it not been for a whole host of human interventions."
What's In a Name?
While victims of political conflict are entitled to food and shelter through government and aid groups, environmental refugees are not yet recognized by international law.
Some skeptics of the idea have pointed out that environmental change may be just one of many factors that cause people to leave their homes. But Oliver-Smith disagrees.
"An environmental issue may not be the only thing driving people away, but it may be the big one," he said. "To wipe out the validity of the entire category because there may be other causes involved is absurd."
The UN University experts are now working to establish terms to make it easier to define those who have been displaced by environmental change.
"We have to recognize the complexity of this issue," said Oliver-Smith. "We can't simply draw a line and say, You're an economic migrant and you're a political refugee. This is what we've done, and I think it's created a great injustice."
Once this new class of refugee is defined, the UN experts say, work can begin on providing them with aid.
"Despite the seriousness of these trends, the issue of environmental refugees has received scant attention at the highest level," said Rhoda Margesson, foreign affairs analyst with the U.S. Congressional Research Service in Washington, D.C.
"The focus has been more on the impact that mass displacement has on the environment rather than on the role the environment itself plays in creating refugees."
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