DHAKA, BANGLADESH — 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.