When a home is referred to as “underwater,” it generally means that a homeowner owes more on the mortgage than the home is worth. In the future, the term might take on a new, much more literal meaning.

With the United States withdrawing from the 2016 Paris climate agreement, Zillow is examining how housing in coastal cities could be effected by future rising sea levels that scientists agree climate change will bring. Sea levels are actually rising faster than the scientific community previously thought, but Zillow takes the long view: it looked at the National Oceanic and Atmosphere Administration’s predictions for sea level rise by 2100 and what it will mean for housing in America.

If sea levels rise as much as scientists predict by 2100, nearly 300 U.S. cities would lose at least half their homes, Zillow reports. As many as 36 cities could be completely lost.

Nationwide, almost 2 million homes – about 2 percent of all homes countrywide – are at risk of being underwater by 2100. That represents $882 billion in property value, Zillow said. And because more expensive coastal cities would be affected the most, many of the at-risk homes are costly: the median value of an at-risk home is $296,296, while the value of the average U.S. home is $187,000, Zillow reports.

Local effect

In Boston, 21,626 homes would be at-risk of being underwater, or about 18 percent of the city’s housing stock. That represents $24.5 billion in home values, according to Zillow.

Throughout Massachusetts, 62,069 homes would be at least partially underwater by 2100, or about 3 percent of the state’s housing stock and represents $51 billion in property value. That’s the fourth-most lost property value in the country, behind Pennsylvania, Florida and New York.

Of course, 2100 is nearly 80 years away, and much can happen by then. Zillow said its estimates could go either way, depending on technological advances and societal preferences:

It’s important to note that 2100 is a long way off, and it’s certainly possible that communities take steps to mitigate these risks. Then again, given the enduring popularity of living near the sea despite its many dangers and drawbacks, it may be that even more homes will be located closer to the water in a century’s time, and these estimates could turn out to be very conservative. Either way, left unchecked, it is clear the threats posed by climate change and rising sea levels have the potential to destroy housing values on an enormous scale.

Check out which homes in Boston will be at-risk by rising sea levels in 2100, courtesy of Zillow: