Some of the world’s most climate-vulnerable nations will see a mass reshuffling of their populations as the extreme environmental and weather events prompt entire communities to migrate from their home to already crowded cities within their borders.

Extreme flooding in Bangladesh could send as many as 3.1 million internally displaced migrants to the densely populated capital of Dhaka by 2050, adding to the 12 million people who already live there, according to a new report from the C40 Cities coalition and the Mayors Migration Council.

And in Colombia, nearly 600,000 climate migrants — representing a fifth of all migrants moving within the country — are expected to settle in the capital Bogotá, where a water shortage already affects some 8 million people.

The report is among the first to provide city-level climate migration projections across multiple regions, with a focus on the so-called Global South where both climate impacts and urban challenges are felt more intensely. Without considerable reduction in global carbon emissions, 10 of the fastest-growing megacities across Africa, South America, South Asia and the Middle East could see a total influx of 8 million internal migrants by mid-century, according to projections. That’s on top of economic and political refugees, and migrants from other countries.

“People are moving to cities where they can find opportunities, housing and social connections, and they don’t need to go through the complications of moving abroad to find that,” said Claudia Huerta, senior manager of climate and migration campaigns at C40.

The population influx will put even more pressure on local services and accelerate unchecked urbanization. Receiving cities also face their own climate challenges, meaning incoming migrants — most of whom will settle in vulnerable neighborhoods — will likely give up one risky area for another one.

In Brazil, internal migrants driven out by events like flooding, water shortages and poor crop yields could find themselves facing dangerous levels of air pollution and devastating wildfires in São Paulo, or extreme heat and flooding in the favelas of Rio de Janeiro.

“The climate impacts they face are not eliminated,” researchers write. “They simply become urban in nature.”

The report also shows that if global temperature rise is limited to 1.5°C as set by the Paris Agreement, cities can substantially mitigate the impacts of climate migration. The internal climate migrant population in Bogotá, Rio de Janeiro and Karachi, in particular, could be three times lower than if global emissions don’t meet the Paris Agreement goal.

In the absence of notable progress yet in emissions reduction, some cities in the study are already preparing for the population influx. Freetown, Sierra Leone, and Accra, Ghana, are supporting informal waste collectors, many of whom are migrants, by providing financial and health resources. Amman, Jordan — a city that is no stranger to hosting millions of refugees from across the Middle East — is creating green space and education programs for young newcomers. And in Bangladesh, officials are hoping to alleviate overcrowding in Dhaka by diverting newcomers to migrant-friendly towns nearby.

But researchers say the burden should not fall on the receiving cities themselves, calling for national governments and the private sector to do their part in reducing climate risks.

“The biggest issue we need to address to reduce the impacts of climate migration on cities is to reduce emissions,” said Jazmin Burgess, director for inclusive climate action at C40. “And that responsibility doesn’t sit exclusively with cities but it demands action from everyone.”