Official records list 102 islands on the Indian side of the vast Sunderbans, where the Ganges and Brahmaputra empty into the Bay of Bengal.

But scientists have been able to map only 100 islands and found the other two have been swallowed up, says Sugata Hazra, director of Kolkata's School of Oceanography Studies at Jadavpur University.

Fifty-two of the islands are inhabited with a population of more than 1.8 million people.

"Two islands, Suparibhanga and Lohacharra, which have gone under water could not be sighted in satellite imagery. The (disappearance of the) two islands have rendered over 10,000 people homeless," says Hazra.

"A dozen others on the western end of the inner estuary delta are threatened.

"As the islands sink, nearly 100,000 people will have to be evacuated from the islands in the next decade," Hazra tells AFP at his office on the university campus.

He blames global warming and the depletion of mangrove areas for the rising sea levels in the world's biggest delta.

The Sunderbans -- or beautiful forest in Bengali - covers a total of nearly 6,000 square kilometres (2,300 square miles).

The islands, separated by a complex network of hundreds of tidal rivers and creeks, form an important buffer shielding millions from cyclonic storms and tidal waves in the Bay of Bengal.

The report by the oceanography scientists has recently been sent to the federal government as part of India's input for the United Nations Framework Convention on Climate Changes.

"The study shows several changes in physical, biological and social components and the temperature of the group of islands has risen by over one degree centigrade," since 1965, Hazra said.

The annual number of cyclones has fallen, but they are more intense now due to global warming and this means more coastal flooding, erosion and more saline water moving in on the islands, he adds.

While rainfall has risen only marginally over the years, most of the rain no longer falls during the traditional monsoon periods.

"Rainfall has shifted to the post-monsoon period and this shifting is a definite indicator of climate change," he said.

The study shows that the temperature in this area is expected to rise by one degree centigrade by 2050.

Hazra says the relative mean sea level in the Bay of Bengal is rising at a rate of 3.14 millimetres a year due to global warming.

"And if this trend continues, the rising sea will devour nearly 15 per cent of the islands in the Sunderbans," he adds.

Sunderbans Biosphere Reserve director Atanu Raha, who also studied satellite images of the last 20 years, agreed two of the islands have sunk and a dozen more are under threat of submergence.

"Things like a rise in temperature, in sea level is a highly alarming trend and it needs more study to tackle the situation," he said.

To add to the fears, a study published earlier this month in the journal Science found that global warming may lift sea levels faster than previously expected.

The study by Stefan Rahmstorf, professor of ocean physics at Potsdam University, said rising temperatures could boost sea levels by as much as 1.4 meters (4.6 feet) by 2100 -- almost twice the rate previously forecast.

Climatologists so far agree that sea levels will increase 9-88 centimeters (4-35 inches) over 1990 levels by the end of the century.

But Rahmstorf suggests the range could be much higher, 50-140 centimeters (20-55 inches).

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