A team of scientists led by a Tulane geologist is urging h to begin planning now for how to away from the coast, warning that sea levels may eventually rise high enough to overtake New Orleans.
The paper, published Monday in the journal Nature Sustainability with Tulane coastal geologist Torbjörn Törnqvist as lead author, gives a concrete picture of what southeast h could look like in the coming centuries as sea levels rise. The team identified, for the first time, an ancient shoreline about 30 miles north of New Orleans that marks where the Gulf of Mexico once reached.
That finding underpins an argument the paper makes, one its authors acknowledge won't be popular: that the state should begin planning now for a gradual, multi-generational relocation of coastal residents to higher ground. New Orleans, the paper argues, will eventually become “physically impossible” to defend with floodwalls and levees.
“They are saying out loud the things that no one wants to say,” A.R. Siders, an associate professor at the University of Delaware who studies how people are adapting to climate change, said of the study’s authors. “No one wants to say New Orleans won’t be there in 200 years.” Siders reviewed the paper but was not involved with the study.
The argument isn’t entirely new, and Törnqvist has been warning about New Orleans' long-term future for years. In a , he wrote that h's wetlands had crossed a tipping point and that the eventual shoreline would likely settle near the Baton Rouge Fault, a geological line where land to the south sinks faster than land to the north.
What the new paper adds is direct geological evidence of where that shoreline once was, and a sharper policy stance from an interdisciplinary team that combines geology with archaeology, demography and policy. It’s a perspective article, a format in which authors advance an argument supported by evidence rather than report a standalone research finding.
“What is new about this study is that it lays out in clear scientific terms with abundant data and supporting information that we are facing right now a multi-generational need to relocate our coastal communities,” said Sam Bentley, a professor of sedimentary geology at h State University who was not involved with the paper. “That has never been spelled out so clearly and with such urgency.”
Robert Young, a professor of geology at Western Carolina University who was not involved in the study, agreed with its findings but doubted they would move h's political class.
“It is going to be fairly straightforward for the political class to dismiss it because the timeframe is so long,” he said. “Politicians are not going to get re-elected by telling communities that they need to leave.”
Not everyone in the region accepts the idea that the city might have to relocate. Greater New Orleans Inc., the regional economic development organization, said the paper goes too far in advocating for wholesale relocation of the city.
“The study’s findings are academically compelling, but empirically unconvincing,” said Michael Hecht, the organization’s president and CEO. “Experience around the world, and here in h, demonstrates that relocation is not inevitable, nor always desirable.”
Finding an ancient shoreline
The discovery happened almost by accident.
About a decade ago, Tulane geologist Zhi-Xiong Shen — now at Coastal Carolina University and a co-author on the paper — was using lidar, a laser-based mapping technology, to study fault lines that contribute to southeast h's sinking. He noticed a low, curving feature in the elevation data, drilled into the ground, and found sand: the remains of an ancient beach.
That beach, now called the Ponchatoula Ridge, runs from roughly Mandeville in the east to Ponchatoula in the west. The sand had been shaped by wave action, indicating it is likely where the Gulf of Mexico lapped against the shoreline about 125,000 years ago, when the Earth was warmer than it was during most of human civilization. The shoreline from that period was known across much of the Gulf coast, but it was never located in h until now.
The discovery of the Ponchatoula Ridge is “a small detail, maybe, to some readers, but the implications of it are epic,” Bentley said. To him, it underscores that an area where more than a million people live will eventually be submerged.
Earth is now roughly as warm as it was during that last interglacial period, when temperatures peaked at as much as 2.7 degrees Fahrenheit above pre-industrial levels. Sea levels lag behind temperature, though, as ice sheets take centuries to melt, meaning the warming already locked in is expected to keep raising seas for centuries.
That means the city could become a kind of fortified island in the Gulf by the time sea levels rise by roughly ten feet above where they are now, the study finds. And if sea levels rise by over 20 feet — a level the researchers say is locked in over the long term, though the timing is uncertain — sea water could slosh over the French Quarter’s wrought-iron galleries.
Current projections from the National Oceanic and Atmospheric Administration show seas rising off h by
A photograph by Ted Jackson in 2007 is held at the site where the pictured U.S. Geological survey benchmark once stood at the surface of the water at the southwestern corner of Couba Island near St. Charles Parish, Friday, Sept. 8, 2023. The marker was installed on dry land in 1932. After Hurricane Ida, the marker was lost. An 8 foot metal pole was placed into the ground to indicate the depth. (Photo by Sophia Germer, , The Times-Picayune) ORG XMIT: BAT2309081456571746
STAFF PHOTO BY SOPHIA GERMER
Fewer diversions, less time
The state’s recent cancellation of two major Mississippi River sediment diversions, the Mid-Barataria and Mid-Breton projects, both opposed by shrimpers and oyster farmers, has shortened the timeline.
“New Orleans is in a terminal state,” said Jesse Keenan, an associate professor at the Tulane School of Architecture and a co-author of the study. “The question is how much time do we have, and how much time can we buy?”
The U.S. Army Corps of Engineers, which built and maintains the post-Katrina levee system, is designing new additions to extend protection through 2073. Beyond that, the Corps said in a written response, “additional evaluations” will be needed.
Without the diversions, the city has less time, Törnqvist said. Future projects could buy some or all of that time back, though they will not change the eventual outcome.
“The negative outcomes of continued sea level rise are just going to happen earlier,” Törnqvist said. “The time window to do this managed relocation is just going to be shortened by maybe a couple of decades. And that’s a lot of time.”
“There’s a kind of mortality here,” Keenan said. “We’re facing our own mortality with New Orleans, and that’s fine. There’s nothing wrong with that. It just means we have to think about palliative care and how we are going to treat those who are immobile and those that are getting left behind.”
The paper also argues that getting ahead of relocation could give New Orleans a “first mover advantage,” or a chance to develop expertise other coastal cities will eventually need.
Young, the Western Carolina geologist, wasn’t convinced that was really an advantage.
“The main problem I have with the paper is at the very end when it talks about the possibility of making New Orleans and places south a model for what could happen elsewhere,” he said. “I’m just really skeptical that that can happen anytime soon.”
Without a state-managed plan, the paper warns, residents with savings will leave first, taking the tax base with them and stranding lower-income residents in communities with eroding services and worsening climate hazards.
“Those people with abundant resources will move, and then who do you leave behind?” Keenan said. “A highly impoverished and immobile population.”
Articulating all of this and publishing it in a reputable scientific journal puts the researchers in a somewhat uncomfortable position, they said, having to tell people what they don’t want to hear. Törnqvist said he was “preparing for some hate mail.” Keenan said he would not have published the paper if he didn’t have tenure.
“Many of us have taken the approach that advocating for a particular stance can threaten our apparent objectivity in reaching a solution,” said Bentley. “But sometimes you have to do it. And I think this is an exceptional case."