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 this 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.

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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 sea walls 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.

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Rising seas are increasingly intruding into rivers, turning fresh water brackish. Even routine flooding then leaves behind salt deposits that can render land barren.

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Mr. Karim estimated that as many as 50 million Bangladeshis would flee the country by 2050 if sea levels rose as expected.

Fascinating story in The New York Times about the devastation climate change, rising sea levels and coastal erosion have on a low-lying and poor country like Bangladesh.