Global warming is uprooting people from their homes and, left unchecked, could lead to the greatest human migration in history, said a report released Wednesday.
Estimates vary on how many people are on the move because of climate change, but the report cites predictions from the International Organization for Migration that 200 million people will be displaced by environmental pressures by 2050. Some estimates go as high as 700 million, said the report, released at U.N negotiations for a new climate treaty.
Researchers questioned more than 2,000 migrants in 23 countries about why they moved, said Koko Warner of the U.N. University, which conducted the study with CARE International.
The results were "a clear signal" that environmental stress already is causing population shifts, she said, and it could be "a mega-trend of the future."
The potential for masses of humanity fleeing disaster zones or gradually being driven out by increasingly harsh conditions is likely to be part of a global warming agreement under negotiation among 192 countries.
A draft text calls on nations to prepare plans to adapt to climate change by accounting for possible migrations.
At U.S. insistence, however, the term "climate refugees" will be stricken from the draft text because refugees have rights under international law, and climate migrants do not fill the description of "persecuted" people, said Warner.
The report, "In Search of Shelter: Mapping the Effects of Climate Change on Human Migration and Displacement," studies people in some of the world's great river deltas who could be subject to glacial melt, desert dwellers who are vulnerable to increasing drought, and islanders whose entire nations could be submerged by rising sea levels.
It did not try to assess conflicts caused by climate change. The war in Sudan's desert Darfur region has partly been blamed on contested water supplies and grazing lands, and concern over future water wars has mounted in other areas of the world.
The report said 40 island states could disappear, in whole or in part, if seas rise by six feet. The Maldives, a chain of 1,200 atolls in the Indian Ocean has a plan to abandon some islands and build defenses on others, and has raised the possibility of moving the entire population of 300,000 to another country.
Melting glaciers in the Himalayas threaten repeated flooding in the Ganges, Mekong, Yangtze and Yellow river basins, which support 1.4 billion people, or nearly one-fourth of humanity, in India, southeast Asia and China. After the floods will come drought when seasonal glacier runoff no longer feeds the rivers, it said.
In Mexico and Central America drought and hurricanes have led to migrations since the 1980s and they will get worse, it said.
Homes are not always abandoned forever, the researchers said. "Disasters contribute to short-term migration," especially in countries that failed to take precautions or lack adequate responses, said Charles Ehrhart of CARE.
Most migration will be internal, from the country to the city, it said.