Sprawling coastal cities in South and Southeast Asia are sinking faster than anywhere else in the world, making millions more vulnerable to sea-level rise, a new study says.

Rapid urbanization has led these cities to rely heavily on groundwater to support their growing populations, according to a study by Singapore’s Nanyang Technological University (NTU) published last week in the journal Nature Sustainability.

“This puts cities experiencing rapid local land subsidence at greater risk from coastal hazards than already exist from climate-induced sea-level rise,” the study said.

Vietnam’s most populous urban center and top business hub, Ho Chi Minh City, sank an average of 16.2 millimeters (0.6 inches) annually, beating the study’s survey of satellite data from 48 major coastal cities around the world.

The southern Bangladeshi port of Chittagong was second on the list, while the western Indian city of Ahmedabad, the Indonesian capital Jakarta and Myanmar’s commercial hub Yangon also sank more than 20 millimeters in peak years.

“Many of these rapidly sinking coastal cities are fast-growing megacities where … high demands on groundwater abstraction and loading from densely built-up building structures contribute to local land subsidence,” the study states.

Sinking cities aren’t per se the result of climate change, but the researchers said their work would provide better insight into how the phenomenon would “magnify the effects of climate-induced mean sea-level rise.”

According to the United Nations Intergovernmental Panel on Climate Change, by 2050 more than a billion people will be living in coastal cities threatened by sea level rise.

According to the IPCC, global sea levels could rise by up to 60 centimeters by the end of the century, even if greenhouse gas emissions were sharply reduced.