d half-broken canoe, a coconut palm teetering on a cliff, the gouged-out remnants of a family’s fish pond.

All that stands between Mr. Mandal’s home and the water is a rudimentary mud embankment, and there is no telling, he confessed, when it, too, may fall away. “What will happen next, we don’t know,” he said, summing up his only certainty.

The sinking of Ghoramara can be attributed to a confluence of disasters, natural and human, not least the rising sea. The rivers that pour down from the Himalayas and empty into the bay have swelled and shifted in recent decades, placing this and the rest of the delicate islands known as the Sundarbans in the mouth of daily danger.

Certainly nature would have forced these islands to shift size and shape, drowning some, giving rise to others. But there is little doubt, scientists say, that human-induced climate change has made them particularly vulnerable.

A recent study by Sugata Hazra, an oceanographer at Jadavpur University in nearby Calcutta, found that in the last 30 years, nearly 31 square miles of the Sundarbans have vanished entirely.

More than 600 families have been displaced, according to local government authorities. Fields and ponds have been submerged. Ghoramara alone has shrunk to less than two square miles, about half of its size in 1969, Mr. Hazra’s study concluded. Two other islands have vanished entirely.

The Sundarbans are among the world’s largest collection of river delta islands. In geological terms they are young and still under formation, cut by an intricate network of streams and tributaries that straddle the border between India and Bangladesh. Ever since the British settled them 150 years ago in pursuit of timber, the mangroves have been steadily depleted — half of the islands have lost their forest cover — and the population has grown.

Today the rising sea and destruction of forests threaten the Sundarbans’ most storied inhabitant, the royal Bengal tiger, which drinks these salty waters and has an appetite for human flesh. Environmental degradation also threatens the unsung human residents: four million people live here on the Indian side of the border alone.

The Intergovernmental Panel on Climate Change predicts that global warming, spurred by the buildup of heat-trapping gases in the atmosphere, could raise the ocean’s surface as much as 23 inches by 2100. [According to the panel’s latest report, released in early April, the ecology and people of this river delta system are among the most vulnerable in the world.]

It hardly seems to matter that Mr. Mandal and his neighbors — farmers and fishermen — are far too poor to produce much in the way of carbon emissions. They feel the assault already.

Mr. Mandal stood in his yard and pointed to the water. In his mind’s eye, he could still see the two islands that have already sunk into the sea. And there, he said, on what was then the outer edge of Ghoramara, was his old house, and the paddies and the vegetable patch he had cultivated with his own hands. Now there is only water.

Over the years, as the earth cracked and fell into the water, people on Ghoramara began moving farther inland. Mr. Mandal was among them. He built himself a new house in what was then the safe midsection of the island. He wonders where he will move now.

Hundreds of families have already been forced into a displaced people’s camp on Sagar, a neighboring island, which itself has shrunk by one and a half square miles in the last five years, according to the Jadavpur University study.

Even as India prospers, the Sundarbans have been left with little to no protection, and certainly none of the measures that wealthy low-lying countries like the Netherlands have undertaken to deal with the ravages of the sea.

Every year at least two cyclones pound the islands; scientists say the storms have grown increasingly intense, though less frequent. The mud embankments built over the years around these young, fragile islands are too feeble to keep away the tide. One storm, and one breach, can destroy a lifetime’s labor on the land.

“Nature didn’t create this place for humans to cut the forests and chase out the tigers and wildlife,” said Tushar Kanjilal, founder of the Tagore Society for Rural Development. “We are killing the Sundarbans. Our government, the people themselves, we are all together killing it. After 50 years, will they exist?”

Yudisthir Bhuiyan fled to Sagar one day during the equinoctial high tide, when the river surged and broke through the mud embankment that was expected to shield his village. His house collapsed. Cows and chickens were washed away. The family had no time to gather its belongings.

For a time they lived like refugees on the roadside, until the government settled them on a small patch of land on Sagar, far from the water. His island has since vanished entirely. He makes his living now as a day laborer.

At low tide it is possible to see the mud flats that represent Ghoramara’s vanishing past. At one tip of the island is a palm tree that drowns during high tide. There are embankments behind embankments, the futile efforts of islanders to hold the water back. Where once there was Mr. Bhuiyan’s bustling island, Lohachara, there is only water, with a ship passing by.

A new mud flat is visible now, just ahead of Mr. Mandal’s house, where mangroves are beginning to sprout. Mr. Mandal regards it as his only possible salvation, a would-be buffer against the tide.

To nurse hope like this is also to know despair.

Sheikh Suleman, now nearing 60, remembers when life was good. He was a young man then, and the labor of his great-grandfather, who settled in the Sundarbans, bore a life of bounty for the family. “There was fish,” he said. “There was land. We lacked for nothing.”

He was a young man when he noticed people beginning to leave the shores of Ghoramara. The water had begun to eat the land, he said. In a matter of two years, several dozen mud houses collapsed and washed away.

Mr. Suleman stared at the water each day. He looked for cracks in his land, the first sign of breakage. He built a new boundary wall. Then, when danger seemed imminent with only 25 feet of land left in front of his doorstep, he decided to move. “One day it can just wash away,” he said.

On Sagar, the government gave him enough land for a one-room hut and a fish pond. He survives largely on the earnings of two sons, who work in nearby Calcutta.

There was a time, he recalled, when his own harvest of coconuts was so plentiful that his wife would give them freely to their neighbors. Now, he said, she has to beg for a coconut. His eyes welled up with tears.