At an impromptu meeting in Moura village on the south-eastern coast of Bangladesh last week, 30 families said that their only hope of survival was to become climate refugees.

"The tides come into the village every two weeks. Twenty years ago the sea was far away. Now it's a few yards and we fear that our children will die. We have lost our farmland and more than 50 people have already lost their homes to the rising sea. The drinking water is salty and there are no fish in the river. We all want to leave but where? We have no money, " said Hayaun Nesa Khatong.

At the rate that this stretch of the Bangladesh coast is being submerged there is little or no chance of Moura or many other nearby villages being habitable in five years. Unless the local government finds increasingly scarce land to evacuate them to, the villagers will have no option but to migrate. Most will go to the slums of Chittagong, 20 miles away, or to the capital Dhaka.

Five hundred miles away in India, on the other side of the Bay of Bengal, researchers in the oceanography department at the University of Jadavpur in Kolkata say dozens of islands in the Indian Sunderban region are being regularly flooded, threatening thousands. Unexpectedly fast sea level rises and storms are forcing the Indian government to consider evacuating nearly 70,000 people in the next five years.

These are people which developing countries have been saying privately that rich countries must take responsibility for. Now, ahead of the Copenhagen climate change summit, Bangladeshi finance minister Abul Maal Abdul Muhith has raised the stakes by stating the point publicly. "We are asking our development partners to honour the natural right of persons to migrate. We can't accommodate all these people."

Rich nations could help by providing cash to relocate refugees in their own countries, or by accepting them as refugees. Wealthy countries are terrified by the thought of climate refugees being given legal access. Worldwide, nearly 10m people from Africa, south Asia and elsewhere are thought to have migrated or been displaced by environmental degradation, weather-related disasters and desertification in the last 20 years. The UN expects a further 150m people will have to move in the next 50 years.

Climate change cannot be unequivocally linked to individual weather events and specific mass migrations. But experts, including the UN's climate change science body and the Intergovernmental Panel on Climate Change say warming is increasing the number and severity of humanitarian disasters. The International Red Cross estimates there are now more than 400 weather-related disasters a year and almost 90m people need immediate assistance. This figure could be as high as 350m by 2050.

The majority of people displaced by more severe climates will be the extreme poor, whose meagre resources will mean moving within borders, often only short distances from home.

But the UN says 28 countries are now at extreme risk from climate change, of which 22 are in Africa. Eleven countries have more than 10% of their land within 5m of mean sea level and a similar number have all of their territory below 5m. Five would be entirely threatened by just a 1m rise in sea level.

Because most borders between both African and Asian countries are relatively porous, there is a distinct possibility that tens of thousands of people will try to migrate between countries.

Bangladesh, which is expecting its 165m population to increase by nearly 100m in the next 60 years, is the most vulnerable large country, with 60% of its land less than 5m above sea level.