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Somali migrants in a disabled skiff wait for assistance from sailors aboard the guided-missile cruiser USS Lake Champlain

The world should brace itself for millions of climate refugees in coming decades, a mass migration that will be larger than any in human history, says a new report.

Although it's too early to estimate exactly how many people might be on the move, the report, compiled by researchers at Columbia University, developmental aid agency Care International and the United Nations University, cites other studies that suggest the number could be as high as 700 million by 2050.

"In coming decades, climate change will motivate or force millions of people to leave their homes… Although the precise number of migrants and displaced people may elude science for some time, the mass of people on the move will likely be staggering and surpass any historical antecedent," says the report, which is being released Wednesday.

The new warning adds to those by many scientists and environmentalists who say the impacts of global warming are starting to occur earlier, and are much larger than estimates made only a few years ago.

Chief among the worries is that sea-level rise, due to melting ice sheets on Greenland and Antarctica, is speeding up, putting coastal communities at risk of flooding.

Although the Intergovernmental Panel on Climate Change said in 2007 that sea levels would likely increase by a manageable quarter to half a metre in the coming century, more recent research on the two ice sheets has suggested that range may be too conservative.

The new report says a number of places would be hard hit if the rise reached one metre, including the densely populated Ganges and Mekong River deltas in Asia, and Egypt's Nile River delta. In these areas, now subject to intense farming, 23.5 million people could be affected and land under cultivation could fall by at least 1.5 million hectares.

"We're looking at large swaths of land that people live on and rely on for their livelihoods that are going to become partially or entirely uninhabitable," Kieran Green, a spokesman for Care Canada, said.

Although the report didn't look at the countries that might be receiving climate refugees, Mr. Green said Canada will likely be on the list.

"Realistically, we are a refuge for people fleeing in the world. Canada has to realize that it's going to effect us if we can't find ways" to prevent global warming or help people in hard-hit areas adapt to a changing climate, he said.

The report, titled In Search of Shelter, looked at a number of areas of the world that would likely be the source of refugees in a warmer climate.

Among them was Mexico and other parts of Central America, where models predict increasing drought as global warming proceeds. The region is particularly vulnerable because it already has many semi-arid zones and suffers from serious soil erosion.

In the Ganges River delta in Bangladesh more than five million people already live in areas vulnerable to cyclones. The report said that in 20 or 30 years, the country "may see mass movement of people from flood-prone areas." About 9.4 million people currently live in places that would be underwater by a two-metre sea-level rise.