Moving from one area of resource scarcity to another
The next step in the migration pattern is across national borders. Military experts predict a downward spiral of violence and conflict as people desperate for food, water and jobs cross into neighboring countries where resources may be only slightly less scarce.
Wealthy nations like the United States and the European Union, meanwhile, could also be asked to take in millions of the world's displaced people even as they negotiate international disputes.
"Those people who are most vulnerable right now, and having a problem just surviving, and having the normal development challenges of clean water, fighting disease, getting an education—those are the ones most affected," said Koko Warner, who heads the Environmental Migration, Social Vulnerability and Adaptation Section at the U.N. University.
In Bangladesh, the issues are magnified by the density of the population. Any climate-induced disaster "inevitably affects millions of people," researcher James Pender wrote in a recent sweeping report on Bangladesh. He estimated that by 2080, almost all the 51 million to 97 million people currently living in coastal zones may have to leave. The worst off won't even be able to do that.
"If those who are causing the greenhouse gas emissions are unable to control carbon emissions, the people in the vulnerable areas, many of the coastal areas, are going to be inundated," said Khawaja Minnatullah, a water specialist at the World Bank's Dhaka office.
"The vulnerable, the uneducated, the lowest of the communities will never be able to migrate to the U.S., to Canada, to Australia. There will be pressure on the not-so-vulnerable part of Bangladesh," he said.
In the village of Gabura in southwest Bangladesh, 20-year-old Amina lives with the fractured collarbone she suffered when a tidal flood smashed a wall of her home, crushing her. She and her husband have no money for a doctor, much less a move.
"Everyone that's living here, we're all poor people," she said, sitting in front of her partially repaired mud and thatch house. "We don't have anywhere to go."
Swelling overcrowded cities; scaring neighbors who have built a fence
But in Gabura and other parts of Bangladesh where the land can become the sea in the blink of an eye, climate migration has already begun.
Cities like Dhaka are bursting at the seams. Migration to bordering India appears to be occurring at a higher rate, as well, though government leaders are reluctant to acknowledge it. India, meanwhile, is wide awake to the possibility of migration from Bangladesh, and is building a fence much like the one along the U.S.-Mexico border to keep illegal immigrants out.
There is a human tendency to deny mind-numbing futures like this one, and Bangladeshi experts are positioned on both sides of this verbal fence. Some insist that climate migration is a reality that needs to be addressed sooner than later.