Mahe Noor left her village in southern Bangladesh after Cyclone Sidr flattened her family's home and small market in 2007. Jobless and homeless, she and her husband, Nizam Hawladar, moved to this crowded megalopolis, hoping that they might soon return home.
Two years later, they are still here. Ms. Noor, 25, and Mr. Hawladar, 35, work long hours at low-paying jobs - she at a garment factory and he at a roadside tea stall. They are unable to save money after paying for food and rent on their dark shanty in Korail, one of the largest slums in Dhaka. And in their village, more people are leaving because of river erosion and dwindling job opportunities.
"We're trapped," Ms. Noor said.
Natural calamities have plagued humanity for generations. But with the prospect of worsening climate conditions over the next few decades, experts on migration say tens of millions more people in the developing world could be on the move because of disasters.
Rather than seeking a new life elsewhere in a mass international "climate migration," as some analysts had once predicted, many of these migrants are now expected to move to nearby megacities in their own countries.
"Environmental refugees have lost everything," said Rabab Fatima, the South Asia representative of the International Organization for Migration. "They don't have the money to make a big move. They move to the next village, the next town and eventually to a city."
Such rapid and unplanned urbanization is expected to put even further strains on scarce water, energy and food resources, said Koko Warner, who works in environmental migration at the United Nations University Institute for Environment and Human Security in Bonn.
In Bangladesh, a largely flat, riverine nation where more than 140 million people live in one of the most densely populated countries in the world, past generations often moved to cities seasonally. They worked to send money home to their villages and usually returned there during planting season.
But in recent years, the moves are more likely to be permanent. More intense storms and floods, salinization damage to crops caused by the encroaching sea and especially worsening river erosion have left many people rootless, Ms. Fatima said.
Dhaka, the capital, is often the only real option in this region. It is the fastest-growing megacity in the world, according to the World Bank. At least 12 million people live in Dhaka, and there are more than 400,000 newcomers each year. The World Bank predicts that the population could grow dramatically by 2020.
Like the rest of Bangladesh, Dhaka is also extremely vulnerable to climate change: It is just a few meters above sea level and is regularly hit by cyclones and floods. The environmental group WWF recently rated it among the megacities most vulnerable to the effects of global warming, after Jakarta and Manila.
As many as half of the people in Dhaka live in shantytowns and slums, says Atiq Rahman, a climate change researcher and executive director of the Bangladesh Center for Advanced Studies. Of those, Mr. Rahman and Ms. Fatima estimate that three million people have been displaced by environmental degradation or disasters. The most destitute people live in clusters of improvised tents made of plastic sheets and discarded bamboo and often erected on private land near markets, railroad tracks and the city's rivers.
Most poor, working-class families end up in minicities like Korail, where Ms. Noor, the migrant from southern Bangladesh, lives with her husband and two daughters. Ms. Noor's third child, a son, lives with his grandmother in the family's village.
Korail sits on public land and is shared by at least 40,000 people crowded into cramped, cockroach-infested rental shanties made of mud, bamboo and corrugated tin.
Barefoot children play with broken marbles on narrow mud lanes filled with garbage and streams of raw sewage. A few enterprising residents have opened vegetable stands, tailor shops, carpentry mills and teahouses in tiny shacks.
Aid groups run primary schools in Korail and other slums, but many children work or stay home to mind younger siblings while their parents work.
Child trafficking and arson are serious problems, experts here say. Ms. Noor says she worries she will come home one day and find her young daughters kidnapped, or worse.
"Every day I hear about a fire or about someone's child missing," Ms. Noor said.
Ms. Noor's next-door neighbor, Aklima Akhter, 22, also lost her home and her family's small market in her southern Bangladesh village to flooding caused by Cyclone Sidr.
Another neighbor, Mukhles Rahman, 38, and his brother Mohammad Farid Uddin, 56, left their village of Chawlakathi in the division of Barisal eight years ago because of river erosion.
Their family once grew rice, jute, sugar cane, mustard seed and radishes on four hectares, or 10 acres. Over a couple of decades, the Sandhya River washed away the farmland and the family home.
"My father could cross the river just by jumping across," said Mr. Uddin, who finished high school and ran a small school in his village. "Even when I was a youngster in the 1960s, we could swim across. Now it's so big."
These days, the two brothers live in a tiny shanty with Mukhles Rahman's wife and young son.
"We are trying to find another place to go, because all the land back home is dissolving," said Mukhles Rahman, who works as a security guard at a garment factory. "But there aren't jobs in other cities or in villages."
For migrants displaced because of the environment, Bangladesh hopes to begin helping them find opportunities in cities other than Dhaka, said Saleemul Huq, a Bangladeshi scientist who is a senior fellow in the climate change group of the International Institute for Environment and Development in London.
Bangladesh and other countries hit hard by climate change are supposed to receive money from a $100 billion annual green climate fund approved by the negotiators at the Copenhagen summit meeting in December. Smaller amounts of aid have been pledged for the next few years. The larger amounts of environmental aid are not expected to begin flowing until 2020.
Later this year, Mr. Huq will open and lead the International Center for Climate Change and Development, an institute based just outside of Dhaka and aimed at helping vulnerable countries come up with practical ways to adapt to climate change.
"We are going to have low-lying areas in Bangladesh that are not going to be inhabitable anymore, so<