By Roger Harrabin BBC environment analyst, Sundarbans

Climate change could eradicate villages in the Sundarbans area Enlarge Image

At the end of a ramshackle jetty where the Ganges flows from India into the sea, a small family group pays homage to the holy waters.

The chief celebrant, head swathed in a shawl, rings a bell and chants prayers as he floats an offering of flowers on to the muddy waves.

A small girl hammers at a gong as an older man - unshaven with a white vest and loincloth - sounds a conch shell.

Down the jetty someone hurls a deafening home-made firework to create a fountain of stinking mud.

The waters bring life - but here in the Sundarbans, the world's largest mangrove forest, the waters are also taking life away.

Land underwater

As sea level rises - partly as a response to climate change - two islands have vanished from the map.

Professor Sugata Hazra, a stocky dynamo of a man, discovered their disappearance when he compared maps from the Raj with satellite images. He says 6,000 people have had to be relocated here because their land is underwater.

People like Bashunto Janna. He is 81 now and says he has not got long to live.

His family used to farm 85 acres on the vanished island of Lochachara. Now they have one acre in a village for displaced people on a nearby island, which itself is under threat from the waves.

The Sundarbans straddle India and Bangladesh.

Here on the Indian side, the Indian government is just about coping with the slowly unfolding crisis. Bashunto's adult children may hanker for life on the farm but at least they have homes and paid work.

But Professor Hazra warns that the way the sea is rising, by the end of the century there will not be thousands on the move along this coastline - there will be millions.

The problem is compounded by rapid population growth in the Sundarbans islands at the great delta in the Bay of Bengal.

Legendary tiger

The islands are remote and hard to access - making family planning and education all the harder - while more and more families are flocking into the most vulnerable areas to make a living from the sea.

As the waters rise it is expected that they will submerge the entire delta region, home to the legendary Bengal tiger.

The effects of climate change will be felt in cities too

And scientists on the Intergovernmental Panel on Climate Change (IPCC) are forecasting that problems here will be made much worse by more severe storm surges, droughts and floods.

From Calcutta you can follow the Ganges upstream to witness climate change from the other end.

I climbed 18 months ago to the glacier that feeds the Ganges - Gau Mukh, or cow's mouth. A great rot