JACKSONVILLE, Fla.—For most of his life, Steve Salem has led an existence closely linked with the rise and fall of the tides.

Salem is a 50-year boat captain who designed and built his 65-foot vessel by hand.

“Me and Noah, we’re related somewhere,” said Salem, 75, whose silver beard evokes Ernest Hemingway.

Salem is familiar with how the sun and moon influence the tides and feels an innate sense for their ebb and flow, although the tides here are beginning to test even his intuition.

He and his wife live in a rust-colored ranch-style house along a tributary of the St. Johns River, Florida’s longest. Before they moved in the house had flooded, in 2017, as Hurricane Irma swirled by. The house flooded again in 2022, when Hurricane Nicole defied his expectations. But Salem believes the house is sturdy and that he can manage the tides, as he always has.

“I’m a water dog to begin with. I’ve always been on the water,” said Salem, who prefers to go by Captain Steve. “I worry about things that I have to do something about. If I can’t do anything about it, then worrying about it is going to do what?”

Across the American South, tides are rising at accelerating rates that are among the most extreme on Earth, constituting a surge that has startled scientists such as Jeff Chanton, professor in the Department of Earth, Ocean and Atmospheric Science at Florida State University.

“It’s pretty shocking,” he said. “You would think it would increase gradually, it would be a gradual thing. But this is like a major shift.”

Worldwide sea levels have climbed since 1900 by some 1.5 millimeters a year, a pace that is unprecedented in at least 3,000 years and generally attributable to melting ice sheets and glaciers and also the expansion of the oceans as their temperatures warm. Since the middle of the 20th century the rate has gained speed, exceeding 3 millimeters a year since 1992.

In the South the pace has quickened further, jumping from about 1.7 millimeters a year at the turn of the 20th century to at least 8.4 millimeters by 2021, according to a 2023 study published in Nature Communications based on tidal gauge records from throughout the region. In Pensacola, a beachy community on the western side of the Florida Panhandle, the rate soared to roughly 11 millimeters a year by the end of 2021. 

“I think people just really have no idea what is coming, because we have no way of visualizing that through our own personal experiences, or that of the last 250 years,” said Randall Parkinson, a coastal geologist at Florida International University. “It’s not something where you go, ‘I know what that might look like because I’ve seen that.’ Because we haven’t.

“It’s the same everywhere, from North Carolina all the way down to the Florida Keys and all the way up into Alabama,” he said. “All of these areas are extremely vulnerable.”

The acceleration is poised to amplify impacts such as hurricane storm surges, nuisance flooding and land loss. In recent years the rising tides have coincided with record-breaking hurricane seasons, pushing storm surges higher and farther inland. In 2022 Hurricane Ian, which came ashore in southwest Florida, was the costliest hurricane in state history and third-costliest to date in the United States, after Katrina in 2005 and Harvey in 2017.

“It doesn’t even take a major storm event anymore. You just get these compounding effects,” said Rachel Cleetus, a policy director at the Union for Concerned Scientists, an advocacy group. “All of a sudden you have a much more impactful flooding event, and a lot of the infrastructure, frankly, like the stormwater infrastructure, it’s just not built for this.”

“It’s not something where you go, ‘I know what that might look like because I’ve seen that.’ Because we haven’t.”

Across most U.S. coastlines, the rate of sea level rise would suggest future trajectories that would be considered “intermediate” under a series of scenarios projected by the National Oceanic and Atmospheric Administration. The sea level rise scenarios, which range from “low” and “intermediate low” to “intermediate high” and “high,” are based on various uncertainties including the global resolve to address the greenhouse gas emissions warming the climate.

In the South the trajectories would fall within and in some places exceed NOAA’s “high” scenario, according to the Nature Communications study. Further acceleration in the region would go as far as to threaten national security and outpace adaptation measures, the report warned.

Scientists have offered several reasons for the surging sea levels in the South, including changes in land elevations and ocean circulation. A separate study pointed to variations in the Gulf Stream, a current coursing along the Gulf Coast, throughout the Caribbean and up the Atlantic Coast. Yet another study suggested that warming within the depths of the Gulf of Mexico is redistributing water from deeper areas to the shallows, contributing to the trend.

Florida is uniquely vulnerable to sea level rise, with some 1,350 miles of coastlines, flat topography and porous geology. The state’s 35 coastal counties are home to some 76 percent of the population. In Florida, the sea level rise similarly has accelerated, from a rate of some 3.1 millimeters a year since the mid 20th century to 5.9 millimeters a year since 1993 and 8.2 millimeters a year since 2003, according to a study based on 14 tidal gauges in the state.

The data suggest that by 2050 the sea level rise in Florida would correspond with NOAA’s “intermediate,” “intermediate-high” and “high” scenarios, according to the study.

“The rate of sea level rise that we see now is four or five times faster than it was over the last several thousand years,” said Parkinson, who was involved in the study. “That’s like an inch every two and a half years, is where we’re at right now. So it’s in a person’s lifetime. Now you can notice that sea levels are rising.”

The rising tides would test the state’s $1.8 billion Resilient Florida program that Gov. Ron DeSantis, a former Republican presidential candidate, initiated in 2021. The program, characterized by his administration as an historic investment to prepare communities for sea level rise, more intense storms and flooding, provides grant funding for vulnerability assessments and resilience projects.

DeSantis earlier this year signed legislation that made several updates to the program, including one that adjusted the requirements for vulnerability assessments. No longer will municipalities have to base the assessments on NOAA’s “intermediate-low” and “intermediate-high” sea level rise scenarios. Now municipalities may base the assessments on the federal agency’s “intermediate-low” and “intermediate” scenarios, a change that seems imprudent to some.  

“There is no logical reason whatsoever to be planning for anything but at least the ‘intermediate-high.’ And in my opinion the only planning should be the ‘high,’ because we’re there. If you use those other ones,” Parkinson said, “you’re going to have to do it all over again.”

“That would be malpractice to use low-end projections right now,” Cleetus said. “We need to be really managing this risk with an abundance of caution, and this is where the latest data is really concerning because it is showing that not only have we blown past that lowest end, we are getting all too close to some of the higher-end scenarios.”

Erin Deady, an environmental attorney specializing in land use and resilience, said the new requirement acknowledges the diversity of issues facing Florida’s municipalities.  

“That’s the minimum for what they want to see for state compliance with a vulnerability assessment,” Deady said. “If a community wants to run a NOAA ‘high’ or a NOAA ‘intermediate-high’ scenario, nothing prevents them from doing that.”

Multiple emails to the DeSantis administration for comment on the legislation were not returned.

The creep of sea water is poised to exert profound changes in Florida. By 2045, some $26 billion in residential real estate is projected to face chronic flooding, with Miami, the Florida Keys and the Tampa-St. Petersburg area especially at risk, according to the Union for Concerned Scientists. A separate study from the same group predicted the number of critical infrastructure assets in the state facing disruptive flooding will double by 2050. The infrastructure assets include public housing buildings and affordable housing units, as well as public health and safety buildings.

Many of the state’s natural landscapes also are at risk. In the Big Bend region, named for the way the peninsula meets the Panhandle, rising salt water has laid waste to freshwater forests, leaving the dead trees looking bony and skeletal as they loom above the newly created salt marshes. Some scientists describe these former thickets as “ghost forests.”

The state’s mangroves, peculiar trees that appear to hover above the coastal waters where they are found as their roots reach beneath the surface, serve as important buffers that protect the shorelines against the violence of storms and hurricanes. These mangroves are migrating inland.

In the Florida Keys, upland pine forests are fading. The recent loss of a stand of tree cacti constitutes the first local extinction of a species caused by sea level rise in the United States, according to a study published in the Journal of the Botanical Research Institute of Texas. The Key Largo tree cactus is scattered across a few islands of the Caribbean, including northern Cuba and parts of the Bahamas, although the cactus no longer is found in the Florida Keys.

Over time, the rapid sea level rise in the South may wane, although it will not retreat, said William Sweet, an oceanographer at NOAA.

“That’s what the concern is. We’re not going backward,” he said. “Sea levels are going to continue to rise, and flooding is only going to continue to get worse.”

Jacksonville is particularly vulnerable. Situated in the northeast corner of Florida, the city is spliced through by the St. Johns, a meandering river that experiences little change in elevation as it lazily courses 310 miles before emptying into the Atlantic Ocean.

“What makes Jacksonville so vulnerable, we’re so flat,” said St. Johns Riverkeeper Lisa Rinaman, part of the worldwide Waterkeeper Alliance, an advocacy group. “We’re low-lying and very susceptible. Every inch makes a huge difference and truly matters to our community.”

Here the sea level rise has accelerated from a rate of some 2.80 millimeters a year since 1928 to 4.69 millimeters a year since 1993 and 8.67 millimeters a year since 2003, according to the study Parkinson was involved in. Some 20,000 residential properties are at risk, and more than a third of them were built either before floodplain requirements existed or outside the floodplain, where elevation requirements do not apply, according to the city’s vulnerability assessment.

In 2017 Hurricane Irma left widespread damage across the state including in Jacksonville, where the storm coincided with a nor’easter. Together the weather systems joined to push sea water upstream into the St. Johns, causing flooding that surprised city leaders.

Since then the city has been preparing for another such disaster by, for instance, modeling a multitude of sea level rise scenarios that consider various factors including the tides, to identify vulnerabilities, said Allison Boss, resilience project coordinator for the city of Jacksonville.

“Whether it’s hurricane season or not we get flooding just from a summer storm,” Boss said. “The community everywhere is at risk.”

The flooding increasingly has become a way of life for residents here. Rinaman finds herself checking the weather first thing every morning to assess how Florida’s frequent rains may affect the roadways. Irma left her stranded at home for a couple of days.

Among the most vulnerable areas is the neighborhood where Captain Steve Salem lives along the Ribault River, a tributary of the St. Johns. It is the sort of low-lying, working-class neighborhood where many residents lack the resources to deal with the water, or move to higher ground. Many of the houses along the river flooded during Hurricane Irma and again during Hurricane Nicole.

For his part, Salem is preparing to retire and in recent years has put his boat up for sale and moved with his wife from Miami to Jacksonville, where he invested in a trio of houses. Two of the houses are situated side-by-side along the Ribault River. He resides in one of them.

He and his wife were able to save most of their belongings during Nicole by moving them to tables and countertops, although they lost their appliances and her car. Still, for them this is home, at least for now. On cool evenings they like to sit beneath the pecan tree in their backyard and watch the sun fade over the river, although Salem does think about the tides.

“That is something that’s been on my mind all my life,” he said.