The number of people threatened by rising seas fueled by climate change in the U.S. could be three times greater than previously estimated, with more than six million Floridians at risk under a worst-case scenario, according to a study published Monday.

For the first time, a team of researchers looked at ongoing population growth in areas where the National Oceanic and Atmospheric Administration has created flood maps that more accurately reflect local conditions. What they found was startling: projections that failed to factor in population growth in dense states like Florida hugely underestimated the number of people at risk and the cost of protecting them.

Combined with the findings from a 2015 report, that means Florida can claim two titles: most property at risk and now, most people.

“In terms of sheer number of people living in harm’s way [South Florida] is way at the top basically,” said Stetson University ecologist Jason Evans, one of three co-authors of the paper published in the journal Nature Climate Change. “It just pops out.”

Using the most conservative estimate of sea rise — three feet by 2100 — the team found that 4.3 million people are projected to live in coastal areas across the country expected to flood. Floridians account for 1.2 million. Using a higher estimate of six feet by 2100 — a number scientists increasingly say could be more likely given the faster melting of polar ice — the number of people triples to 13.1 million, with nearly half living in Florida. South Floridians account for a quarter of the statewide estimate, the authors said.

The findings could help planners determine where, when and what kind of fixes they want to make in advance of rising waters.

“It’s a little bit like when you have diabetes. You want to monitor the situation,” said Ben Kirtman, an atmospheric scientist and director of the University of Miami’s Cooperative Institute for Marine & Atmospheric Studies.

While other studies have looked at the number of people living in coastal cities and towns expected to face increased flooding, the authors say they didn’t adequately account for population growth. Part of the problem is that populations can change quickly on a local scale, Evans said. For example, after Hurricane Andrew destroyed or damaged more than 125,000 homes in 1992, many residents fled neighborhoods ravaged by the storm.

To address such changes, the authors came up with a model that incorporates small-scale census blocks and tested their calculations against past years, factoring in figures for 319 coastal communities in 22 states. They projected growth through 2100 to align with sea rise projections widely used by scientists and the United Nation’s influential International Panel on Climate Change.

“It ends up being within the range of what’s acceptable,” Evans said.

They found that impacts — or daily flooding from sea rise — tended to be hyper localized compared to previous studies that generalized risks. Low areas, including the Florida Keys, they found not surprisingly, will likely see “catastrophic” flooding under the six-foot projection.

The findings mean the cost of sea rise in the state, from protecting its power grid to protecting water supplies, could be enormous. Last year, the Risky Business Project led by former New York City Mayor Michael Bloomberg and U.S. Treasury Secretary Henry Paulson placed the value of Florida property at risk from climate change first in the nation. By 2030, they estimated $69 billion at risk, with sea rise alone expected to threaten $15 billion in property.

But those estimates fail to take into account growth — both in people and the beachfront homes, hotels and businesses they build before 2100.

The new study, the authors say, should serve as a warning to planners to take a closer look at estimates they believe could be be “deceptively low” and complicated. Planners will need to balance the cost of staying or retreating. In Alaska, native villages are already starting to wrestle with the decision and calculated the cost of relocating at $1 million per resident. Some areas may find managing growth and adding flood control measures more reasonable. For example, Miami Beach has so far planned to spend between $400 and $500 million on pumps alone.

“Population projections are not a panacea for these problems, but they move us towards evaluating the potential [sea level rise] impacts on the future,” the authors wrote.

For Florida, governments may have to take a tougher position on growth, Evans said.

“You’ve got to give permits to build in a vulnerable area and local governments are going to have a responsibility to protect them,” he said. “Counties and cities need to look at their vulnerabilities and be thinking, hmm, in 30 years what kind of infrastructure am I going to be maintaining.”

Given the state’s track record, Evans said he’s worried.

“I’ve lived in Florida my whole life and there’s been a long history of short-sighted decisions,” he said. “So what we need to recognize is as we make those decisions, there are costs.”

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