A new report — “In Search of Shelter” — by the United Nations University, the charity CARE and Columbia University in New York lists the eco-migration “hot spots”: dry bits of Africa; river systems in Asia; the interior and coast of Mexico and the Caribbean; and low islands in the Indian and Pacific Oceans.

A one-metre rise in sea levels could displace 24m people along the Ganges, Brahmaputra, Irrawaddy, Salween, Mekong, Yangtze and Yellow rivers - which together support a quarter of humanity. A two-metre rise could uproot 14m people on the Mekong alone and swamp much of its farmland. Meanwhile, the melting of the Himalayan glacier will cause floods and erosion upstream, boosting the price of rice and other staples. And many regional conflicts could be exacerbated.

The scale of the likely population shift raises big questions. Will climate-change migrants be recognised? The classic definition of refugees — tossed between states by war or tyranny — is outdated. Eco-migrants will be paperless paupers, whose multiple woes are hard to disentangle.

Poverty campaigners want a revised legal regime to protect the new migrants. However, this looks tricky. America resists calling them “environmental refugees”: the word “refugee” implies guarantees that cannot realistically be given to the coming torrent of migrants. As American diplomats quietly admit, their rich country is still reeling from Hurricane Katrina in 2005, which killed 1,800 people and displaced hundreds of thousands.

Can the United Nations High Commissioner for Refugees (UNHCR) expand to cope with eco-migrants? It has already struggled to widen its remit to include the internally displaced (26m at the end of 2008) as well as strictly-defined refugees (10m, excluding the Palestinians who come under another agency). A tenfold surge in the numbers within its orbit would push the agency out of control, says James Milner, a professor at Ottawa’s Carleton University. Meanwhile some aid workers see signs of a competition between institutions to take ownership of the eco-migration issue, perhaps by oversimplifying it.

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