More than 16 million people in the contiguous US — roughly 5 percent of the population — live in a place with heightening flood risk and a shrinking population, according to new research. It makes the case that “climate abandonment areas” are becoming a more prevalent phenomenon in the US as people avoid places particularly vulnerable to climate-related disasters.

What’s a climate abandonment area? It’s a census block where flood risk has grown high enough to start pushing people to leave. Many of these areas lie along the Texas Gulf Coast, coastal Florida, and the mid-Atlantic.

But it’s by no means confined to these regions, which can get hit repeatedly by storms during the Atlantic hurricane season. Climate abandonment areas are spread throughout the US in places where heavy rainfall, tropical cyclones, and coastal and river flooding are becoming bigger problems.

“People understand which parts of their community to avoid and which parts of their community are more safe, and they’re acting on that,” says Jeremy Porter, demographer and head of climate implications research at the First Street Foundation that led the study. “People, if you look within housing markets, are being much more thoughtful about where to live.”

The phenomenon is more pronounced when you zoom in to see how people are moving from neighborhood to neighborhood. When people think about climate change affecting migration, they might picture someone moving far from home to another part of the country. But that’s just a small slice of overall migration trends. The majority of people move within the same city, county, or metro area, Porter points out.

First Street Foundation is a nonprofit research group that has developed tools to help residents assess risks from flood, fire, and extreme weather that individual properties face. Its latest research was published in the journal Nature Communications today. Researchers from several universities and the nonprofit Environmental Defense Fund also contributed to the study.

They used First Street’s flood risk data and the Census Bureau’s most recent population count for 2020 to build a mathematical model that can pinpoint the relationship between flooding and population change, controlling for other factors like job availability that might also affect someone’s decision to move.

That’s how they found “tipping points” for each Census block, a threshold at which flooding is so bad that people start to leave the area. The tipping point ranges from place to place, showing that people started to move away when roughly 5 to 15 percent of properties in an area were at risk of flooding.

More than 3.2 million people in the US left neighborhoods with high flood risk between 2000 and 2020, it finds. Climate abandonment areas are expected to shrink by another 16 percent over the next 30 years, losing an additional 2.5 million residents.

There are still people moving into areas where other people have left because of floods. The study also identifies “risky growth areas,” where despite rising flood risk, populations are still growing (albeit not as quickly as they might have without the risk). Nearly 30 percent of the population in the contiguous US, 97.2 million people, live in these risky growth areas.

“The population exposure over the next 30 years is a serious concern,” Evelyn Shu, senior research analyst at the First Street Foundation and lead author of the paper, said in a press release. “For decades we’ve chosen to build and develop in areas that we believed did not have significant risk, but due to the impacts of climate change, those areas are very rapidly beginning to look like areas we’ve avoided in the past.”