Illustration: Eniola Odetunde/Axios

Nearly 70% more properties in the U.S. are at substantial risk of flooding compared to government estimates, new peer-reviewed analysis shows.

Why it matters: Increased flooding, including from sea level rise and intensifying rains, is one of the clearest and most expensive impacts from rising global temperatures.

Driving the news: The data will soon feed a new feature on Realtor.com that displays flood risk rankings of nearly every U.S. home. It's part of an initiative by First Street Foundation, a nonprofit research and technology firm.

  • You can search now for your home now on FloodFactor.com, a website run by First Street, which is backed by private-foundation money.
  • Realtor.com plans to integrate the data soon on its website, though it doesn't have a specific launch date yet, a spokeswoman said Sunday.

What they’re saying:

“There are millions of Americans who have substantial flood risk and have no idea and now they’ll be able to access that ... Having that data available will change the perspective of flood risk in this country. It now makes it meaningful and personal.”
— Matthew Eby, founder and executive director, First Street Foundation

Where it stands:

  • The Federal Emergency Management Agency, which manages America’s flood risk for the government, classifies 8.7 million properties as facing substantial risk from flooding.
  • First Street’s data shows 14.6 million properties.
  • That’s six million households and property owners who the organization says have “underestimated or been unaware of their current risk.”
  • The discrepancy exists, the group says, because it uses more up-to-date climate data, analyzes precipitation as a standalone risk and includes areas FEMA has not.

How it works:

  • The areas with the largest newly revealed risk include large swaths of the Midwest and inland Western states that face risk from rain or river flooding. FEMA's traditional focus has been coastal areas. (Check out the chart on page nine of its report.)
  • Still, the risk is objectively larger for coastal states and grows more quickly in those regions out to 2050, as you can see in the graphic below.
  • That’s because rising sea levels and coastal storms grow more intense more quickly — fueled by a warmer atmosphere and sea surface temperatures — compared to inland precipitation events, according to Jeremy Porter, director of research and development at First Street.
Adapted from First Street Foundation; Chart: Naema Ahmed/Axios

The intrigue: Every property in the U.S. — some 142 million — will receive a “Flood Factor” score from 1 to 10, based on its cumulative flood risk over a 30-year mortgage, and will be included on an interactive tool, FloodFactor.com, hosted by First Street.

  • Some companies have proprietary models that are similar, but nothing exists like this that’s publicly available and that will be attached to a consumer-facing service like Realtor.com, First Street officials say.

Yes, but: Witold Krajewski, a hydrology professor at the University of Iowa who is part of a panel of outside experts reviewing the analysis, said users should keep in mind the amount of uncertainty inherent in any dataset as comprehensive as this. "As long as people approach it with caution, this can be very useful," he told Axios.

  • One specific shortcoming of the model for now is that it doesn’t account for current and potential future adaptation measures property owners may take, such as building ever-higher seawalls or more levees. First Street experts intend to incorporate that into the data over time.

What’s next: First Street plans to sell its data to for-profit firms, such as those that have already expressed interest from the insurance and real-estate sectors. This will help fund parallel models they plan to launch on other climate-driven weather phenomena, such as heat and drought.

Go deeper:

Editor’s note: The name Federal Emergency Management Agency was corrected.

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