At a climate conference in Warsaw in November, there was an emotional outpouring from countries that face existential threats, among them Bangladesh, which produces just 0.3 percent of the emissions driving climate change. Some leaders have demanded that rich countries compensate poor countries for polluting the atmosphere. A few have even said that developed countries should open their borders to climate migrants.

“It’s a matter of global justice,” said Atiq Rahman, executive director of the Bangladesh Center for Advanced Studies and the nation’s leading climate scientist. “These migrants should have the right to move to the countries from which all these greenhouse gases are coming. Millions should be able to go to the United States.”

River deltas around the globe are particularly vulnerable to the effects of rising seas, and wealthier cities like London, Venice and New Orleans also face uncertain futures. But it is the poorest countries with the biggest populations that will be hit hardest, and none more so than Bangladesh, one of the most densely populated nations in the world. In the Ganges Delta, made up of 230 major rivers and streams, 160 million people live in a place one-fifth the size of France and as flat as chapati, the bread served at almost every meal.

A Perilous Position

Though Bangladesh has contributed little to industrial air pollution, other kinds of environmental degradation have left it especially vulnerable.

Bangladesh relies almost entirely on groundwater for drinking supplies because the rivers are so polluted. The resultant pumping causes the land to settle. So as sea levels are rising, Bangladesh’s cities are sinking, increasing the risks of flooding. Poorly constructed seawalls compound the problem.

The country’s climate scientists and politicians have come to agree that by 2050, rising sea levels will inundate some 17 percent of the land and displace about 18 million people, Dr. Rahman said.

Bangladeshis have already started to move away from the lowest-lying villages in the river deltas of the Bay of Bengal, scientists in Bangladesh say. People move for many reasons, and urbanization is increasing across South Asia, but rising tides are a big factor. Dr. Rahman’s research group has made a rough estimate from small surveys that as many as 1.5 million of the five million slum inhabitants in Dhaka, the capital, moved from villages near the Bay of Bengal.

The slums that greet them in Dhaka are also built on low-lying land, making them almost as vulnerable to being inundated as the land villagers left behind.

Ms. Khatun and her neighbors have lived through deadly cyclones — a synonym here for hurricane — and have seen the salty rivers chew through villages and poison fields. Rising seas are increasingly intruding into rivers, turning fresh water brackish. Even routine flooding then leaves behind salt deposits that can render land barren.