"Bangladesh is already facing consequences of a sea level rise, including salinity and unusual height of tidal water," said Mizanur Rahman, a research fellow with the London-based International Institute for Environmental Development.

"In the future, millions of people will lose their land and houses. Their survival will be threatened," Rahman told Reuters.

Experts say a third of Bangladesh's coastline could be flooded if the sea rises one metre in the next 50 years, creating an additional 20 million Bangladeshis displaced from their homes and farms. This is about the same as Australia's population.

Saline water will creep deeper inland, fouling water supplies and crops and livestock will also suffer, experts say.

Government officials and NGOs estimate about 10 million people are already threatened by annual floods and storms damaging riverine and coastal islands.

It is unclear how the government could feed, house or find enough clean water for vast numbers of climate refugees in a country of 140 million people crammed into an area of 55,500 sq miles (142,080 sq km).

"We are taking steps to face the threats of climate change. Bangladesh needs $4 billion to build embankments, cyclone shelters, roads and other infrastructure in the next 15 years to mitigate the threats," Mohammad Aminul Islam Bhuiyan, the top bureaucrat in the government's Economic Relations Division, told Reuters.

"These are big challenges and only time will say how efficiently we address them, including finding accommodation for the displaced millions," he said.

GLOBAL WARMING, NATURAL DISASTERS

In a taste of what the future might look like, Bangladesh suffered two massive floods and a cyclone last year that together killed about 4,500 people, made at least two million homeless and destroyed 1.8 million tonnes of rice, the country's main staple.

Even without the additional threat of global warming, the country's future is under pressure from a rising population and shrinking farmland.

The country lost a third of its agricultural land to accommodate more people as the population rose from 75 million in 1971.

Bangladesh has been able to increase food grain production to nearly 30 million tonnes from less than half that in the early 1980s because of better farming practices and high-yielding varieties of rice.

But many believe Bangladesh has reached saturation point in producing grains, while the population is still growing at nearly 2 percent annually.

The World Bank thinks Bangladesh should change cultivation practices to boost food security, plant large areas of forest in flood-prone areas along rivers and the coast and build embankments.

"We are conducting various studies to find options to save future environmental refugees," said Sakil Ahmed Ferdausi, a World Bank executive in Dhaka.

"The environmental refugee situation will turn into a dangerous problem in the future and the Bangladeshi government may find it difficult to face the challenge. So we asked donors to help the country," Ziaul Haque Mukta, of Oxfam International in Dhaka, said.

For Majid, the issues are more immediate.

He lives on Batikamari island on the Januma river, 300 km (180 miles) north of Dhaka and fears his remaining days will be spent on the run from the river, which is constantly creating and retaking land, depending on the season.

There are millions like him. Some have found temporary shelter, mostly on other islands in the rivers that emerge when water levels drop during the summer.

Government and non-government organisations (NGOs) are trying to help Majid and others.

Friendship, a Bangladeshi NGO, is providing houses, latrines, capital for agriculture, pumps for irrigation among the poor people in the river islands.

"Migration rate is very high among the islanders," Runa Khan, executive director of Friendship, told Reuters. "We have covered 3.5 million people in Bangladesh's riverine islands but many more are still left."

Friendship operates a floating hospital to provide health care to the islanders. It has treated 600,000 people since 2001.

But climate change could wipe out their nomadic lifestyle altogether.

"Where will all these people go?" asked Mohammad Nurul Islam, a resident of Cox's Bazar on the shore of the Bay of Bengal.