Affordable housing is scarce in the United States. There’s already a national shortage of more than 7 million affordable units. As climate change continues to cause sea level rise and increase the risk of floods, there may be even less: By 2050, as many as 24,500 affordable housing units could be at risk of flooding, and 10,000 of those units could see four or more floods a year.

That estimate is triple the number of affordable units at risk just 20 years ago, and a sign of how climate change is increasing the risk of sea level rise across the country. It comes from a report published in the journal Environmental Research Letters and managed by the nonprofit Climate Central, which has been researching the ways climate change will impact the public, whether through sea level rise or increasing temperatures.

Homes in any coastal area are at risk of flooding, but affordable housing is especially vulnerable, the researchers say, because it tends to be older and of lower quality than other housing. And people who live in affordable housing are more likely to be disabled and single parents, and might not have the savings to bounce back from an emergency like a flood.

“[General] housing is also at risk, but the important point is that people living in affordable housing units are low-income and have less resources to deal with impacts from flooding,” says Lara Cushing, an environmental health sciences professor at UCLA and co-author of the research. “That’s really want we wanted to highlight here: The impact will be worse among families in affordable housing than it would be to others in the general housing stock.”

Using models for elevation of high tide, sea level rise projections, and the probability of annual floods, the researchers were able to see where affordable housing is most at risk. They looked at both subsidized and unsubsidized affordable housing, and analyzed how many units were at risk, not just buildings, since affordable housing buildings can vary from single-family homes to apartment complexes. In a large apartment, multiple units can be affected if a flood leads to an electricity outage or damages the stairs.

New Jersey, Massachusetts, New York, and California are states with the most danger, though how much floods and rising seas will affect the overall housing availability in each area differs. New York, for example, is the city with the highest number of units exposed to flood risk by 2050 (more than 4,700), but those units are only 1.3% of the city’s affordable housing stock. In Atlantic City, though, about 3,100 units will be at risk by 2050—more than half of all available affordable housing there.

“Our study shows that if we want to preserve affordable housing in coastal areas, investments need to be made to protect against sea level rise,” Cushing says. Those investments could include flood mitigation, things like levies or improvements to buildings, and, she adds, investments in new affordable housing not in places likely to see floods in the future. We already know that climate change will worsen homelessness as fires, floods, and more disasters devastate housing. Though it’s hard to predict what exactly will happen to these affordable-housing residents if they do experience a flood, we know that sea levels are rising, and that brings a lot of risk. “More and more housing, and affordable housing, is going to be at risk of flooding in the future if we don’t do something about it.”