The risk assessments affect a huge number of properties in Massachusetts: more than 150,000, and nationwide, nearly 13 percent of new listings on Zillow are now considered at high risk of flooding.

“This is the reality of climate change for housing, and that’s why this information is so important,” said Skylar Olsen, chief economist at Zillow.

That reality has been brutally enforced in recent weeks as Hurricanes Helene and Milton caused billions of dollars in damage to homes and properties across much of the Southeast.

Zillow relies on data from climate risk financial modeling firm First Street and uses a score from 1 to 10 for the risk of floods, fires, wind, poor air quality and heat. With sea levels on the Massachusetts coast 14 inches higher than they were a century ago, flooding appears to be the biggest threat to properties, both along the coast and inland near rivers.

This type of information, which includes historical flooding data and levels of projections about how climate change will increase sea levels and precipitation, has been available for several years but has been used primarily by institutional investors, said Jeremy Porter, head of climate impact research in of First Street. “Our goal was to make sure everyone could understand climate risk,” he said. Other sites, including Redfin and Realtor.com, have made First Street's climate risk data available in recent years, but the addition of Zillow — the most popular real estate website — marks a major shift in saturating the market with this information.

The flood forecasts are based on data from the Federal Emergency Management Agency and NASA's global climate models. The analysis shown on Zillow reflects a future that roughly follows the path we are currently on, in which greenhouse gas emissions continue and the planet moves from 4.3 to 5.4 degrees Fahrenheit (2.5 degrees Fahrenheit) by the end of the century .4 to 3 degrees Celsius) above pre-industrial times.

As the consequences of extreme weather events have become more severe in recent years, homebuyers are increasingly asking about climate risks, said Theresa Hatton, executive director of the Massachusetts Association of Realtors.

The new risk ratings on Zillow will result in buyers becoming “more cautious and aware of the risks associated with these properties,” Hatton said, and more likely to ensure they have the proper insurance when deciding to purchase a property decide which may be associated with an increased risk.

But not everyone in the real estate business is happy.

“It's getting people thinking about my offer that wouldn't normally be there,” said Dorothy McLaughlin, a Hingham-based real estate agent who represents the seller of the Revere four-bedroom home. Located several blocks from the shoreline and behind MBTA tracks, the property already requires flood insurance. But McLaughlin says it's unlikely the water would ever reach the house.

“How do you know what the climate will be like in 35 years?” she asked.

However, that is exactly what the data is trying to project. For this home, Zillow analysis estimates there is only a 4 percent chance of flooding this year. But with rising sea levels and stronger storms, that risk rises to 72 percent in the next 15 years and 97 percent in the next 30 years.

Over in Scituate, real estate agent Andrea Campbell says even the risk assessment doesn't give a complete picture. Zillow's results are based on models, not home inspections. “What they don’t take into account is the actual structure on the property,” she said. She noted that Zillow rated the listing she created a flood risk of 9 out of 10, but the building is built on 55 concrete posts. “Elevation is very important during floods.”

Emily Norton, executive director of the Charles River Watershed Association, said the Zillow listings are a good start, but noted that Massachusetts is still a long way from addressing flood risks. According to an analysis by the Natural Resources Defense Council, Massachusetts is one of only 14 states — and the only state in New England — that does not require flood risk disclosure when selling a property.

“There are disclosures about lead, there are disclosures about other things. “Why is there no disclosure about flooding?” Norton said. Therefore, it is possible for a buyer in Massachusetts to sign a mortgage on a home that regularly floods and is likely to flood more as climate change advances without even realizing it.

For now, the data available on Zillow “will be beneficial for future generations,” said Evan Blaustein, a Brookline-based broker who encourages his clients to think about ways to minimize the carbon emissions produced by their new home.

Zillow's new reviews are useful, he said could also make it difficult for current sellers whose properties have suddenly been deemed high risk. “That’s obviously a concern that a lot of homeowners will have.”

There's another reality potential buyers face in Massachusetts: a housing shortage.

“There is not enough housing to have a significant impact,” said Hatton of the Massachusetts Association of Realtors. “What it will do is make buyers more cautious and aware of the risks associated with these properties.”

This means that if you sell, you know in advance that some changes will need to be made or that additional insurance may be required. This can also be difficult. While experts say it's unlikely insurers will flee risky parts of Massachusetts in the same way they did in California or Florida, premiums are expected to reflect the increased risks of climate change. And some companies may refuse to provide flood insurance for properties deemed too risky.

For Rebecca Kleppe, whose two-bedroom apartment in Haverhill received a 10/10 rating for extreme flood risk, it's all a bit of a mystery. When she bought her condo in 2006, she chose one that was nearly a block from the Merrimack River, knowing that other condos in the development that were closer to the river had been flooded in recent years. She said she hasn't had any problems, although she has to take out additional insurance.

“That just doesn’t ring true to me,” she said.

But the good news is that she's no longer worried that Zillow scores will deter a potential buyer: “I've already accepted an offer.”

Sabrina Shankman can be reached at [email protected]. Follow her @shankman.