The islands in India's Hugli River, in the Ganges estuary on the western edge of the Sundarbans region, illustrate advanced stages of the decay. At least three islands that a century ago were covered in mangroves -- Lohachahara, Suparibhanga, and Bedford -- have vanished. Others are eroding fast: Sagar Island has shrunk by about 20 square miles since the mid-20th century, even as its population has swollen with new arrivals from its disappearing neighbors. Crop-growing conditions on Sagar have deteriorated so much that residents now survive largely off seasonal labor elsewhere.

In some parts of the Sundarbans, the sea is advancing about 200 yards a year. "The people around the Sundarbans will lose a lot," said Tuhin Ghosh, an associate professor at Jadavpur University in Kolkata. "This is happening now." But even cities like Kolkata and Dhaka that lie some distance from the vanishing mangroves, he added, will find themselves "extremely exposed to cyclones and storm surges."