Rising sea levels will flood Bangladesh–and its impoverished population. It’s already happening, creating a new class of climate change refugees:

Masud, 19, lives in Korail, Dhaka’s largest slum. Its roughly 70,000 residents dwell in the shadow of the affluent Gulshan neighbourhood, with its mansions, restaurants and western-style shopping centres.

Masud, her husband Mohammed, and their year-old daughter Karima share a one-room shanty that can be crossed in two strides. It is made of corrugated steel sheets held up by wooden poles. A bed takes up a large portion of the room and a battered TV sits in one corner. In another corner are Masud’s dishes. The kitchen, a cramped space with a couple of shelves, is in the back, about a metre from a toilet shared by two dozen other families.

It is not just the smell. Every monsoon, when the slum is overrun with rainwater, cholera and malaria outbreaks are common. Masud has to cook while standing in puddles of muck; nearby, excrement gets stuck in the overflowing drains. She says she almost gags.

“I don’t want to live like this … who wants to?” says Masud, a pretty woman with big eyes and a sad smile. But after her husband’s family farm and home in Barisal, in southern Bangladesh, were inhaled by a powerful cyclone in 2008, they had no choice.

She is a climate change refugee.

Climate change is expected to trigger a migration like no other.

Experts expect about 250 million people worldwide to move by 2050. Of those, 20 million to 30 million climate change refugees are expected to be in Bangladesh, likely the largest number from one place.

As extreme weather, floods and drought force them to flee their homes, most will head to the capital.

Dhaka is the fastest-growing megacity in the world; its population is about 17 million, up from 12 million in 2005 and six million in 1990. By 2025, the UN says the city will be home to more than 20 million people.

Fast-growing urban areas like Dhaka will bear the brunt of climate change-related disasters, particularly because so many of them are located in coastal zones. Dhaka, on the banks of the Buriganga River in the low-lying Ganges Delta, is prone to flooding during monsoons.

As much as 40 per cent of Dhaka’s population — almost seven million — lives in tiny hovels in slums, beside railway tracks, along riverbanks and even on swampy lowlands in the shadow of glittering hotels.

The International Organization for Migration says about 70 per cent of these slum dwellers have come to Dhaka because of climate-change adversity.