But experts estimate that as many as 250 million people—a population almost that of the entire United States—could be on the move by 2050. They will go because temperatures are rising and desertification has set in where rainfall is needed most. They will go because more potent monsoons are making flood-prone areas worse. They will go because of other water events caused by melting glaciers, rising seas and the slow and deadly seepage of saline water into their wells and fields.

The worst migration cases will be nations like the Maldives and small islands in the Pacific. Their inhabitants will go because their homelands will likely sink beneath the rising sea.

According to the Intergovernmental Panel on Climate Change, a minimum of 207 million people in Latin America, Asia and Africa will not have enough water inside a decade. In Asia, an extra 130 million people will be at risk of hunger by the middle of the century. By 2100, crop revenues in Africa will drop 90 percent. And scientists see Bangladesh as ground zero.

The country's 150 million inhabitants live in the delta of three waterways about the size of Iowa, and the majority of the country sits less than 20 feet above sea level. According to the IPCC, rising sea levels will wipe out more cultivated land in Bangladesh than anywhere in the world. By 2050, rice production is expected to drop 10 percent and wheat production by 30 percent.

By the end of the century, more than a quarter of the country will be inundated.

About 15 million people in Bangladesh alone could be displaced. That's the equivalent of every person in New York, Los Angeles and Chicago.

A migration that will change the face of the world

But while more climate migrants will come from Bangladesh than from any other country, scientists say that from Mozambique to Tuvalu, from Egypt to Vietnam, climate migration will change the face of the world.

"This will be the largest migration in history. This is not migration as we've known it before," said Edward Cameron, a former senior adviser to the government of the Maldives. "We're talking about people migrating from sensitive places into other very sensitive places."

In some ways, large-scale migration is nothing new. Humans, after all, have been on the move since early man left East Africa. But these shifts will not be the migrations of pioneers or adventurers seeking opportunities in new lands. Rather, social scientists say, they will be the movement of people who are rushed, unwanted and unprepared, into unfamiliar and perhaps hostile new environments. Most of those who will be uprooted already are living on less than $1 per day.

The first shifts will start within countries. Scientists see families flocking from rural and coastal areas to cities where livelihoods are less tied to fickle weather patterns. It's a pattern that is already happening against a background of rapid global urbanization, in which the desperate rate of growth far outpaces jobs and infrastructure.

Mohammad Ayub Ali, 40, is part of that mosaic. He left the central Bangladesh town of Sherpur because the failing crops couldn't earn him a living. A ruinous flood in September was the final straw.

Now Ali drives an eye-catching pink and orange rickshaw through the capital city Dhaka's teeming streets, where he earns the equivalent of $15 per month. He lives in a one-room metal shack with his mother, wife and two children.

"It's not that great over here, but it's better than over there," he said. Nearly 3.5 million people in Dhaka—about 40 percent of the population—live in slums, like Ali. The World Bank estimates that by midcentury, half of all Bengalis will live in urban centers.