The Louisiana Coast: Last Call — How We Got This Way: Rising Seas, Sinking Land
The clang of tidegaugesthroughout parts of southeast Louisiana aren’t from a science fiction movie, though they may make residents feel like they’re caught in one.
Those sounds tell the stories of rising tides along the Gulf Coast and melting glaciers in the Arctic. And they tell how scientists believe those two events, taking place thousands of miles apart, are the reasons why the Gulf of Mexico is on pace to submerge most of southeast Louisiana by the end of the century — if nothing is done.
Those are the sounds that explain why climate change is the biggest threat ever to the future of New Orleans and its surrounding landscape.
“If you were a doctor talking to a patient, the issue really comes down to: What do we do to manage — manage the patient, manage the coastal landscape — given the fact that it’s reaching a period of its lifespan that literally is nearing the end,” says NOAA researcher Tim Osborn. “[We are] going to see large conversions of coastal areas to open water, and that open water then is going to create a whole new set of challenges that we’re going to have to face in the future.”
Tide gauges are instruments used in NOAA stations along Louisiana’s coast measuring the daily high tide. When researchers plot those readings in a chart spanning decades, it shouts headlines right out of that science fiction disaster movie: Sea level is rising faster here than on any coastal landscape in the world — so fast that over the next six decades much of this region will be inundated by over four feet, and possibly much, much more.
“The landscape’s going to be continuing to sink, sea levels are rising, and the conversion to open water is going to be the highest we’re going to see anywhere in the world,” says Osborn.
By comparison the national climate assessment says the mid-range predictions for the rest of the nation are around two feet. Even Key West, built on an island near the tip of Florida, doesn’t face as much danger.
The reason for that disparity is well known: Because this part of the state is built on deltas of the Mississippi River that have been starved of sediment by levees and sliced by canals for oil, gas and shipping, it is sinking at the fastest rate of any coastal landscape on the planet — an inch every 30 months in some basins.
“The whole area has been sinking at an accelerated rate, and that’s speeding up what geologists call a transgression,” says Baton Rouge geologist Woody Gagliano. “The sea is invading the land, and it’s invading it largely because the land is sinking, but it’s also invading it because the land is not building itself up again.”
Coastal scientists here were confident for decades that they could keep up with subsidence, because the raw material to rebuild those sinking basins still flowed through them inside the Mississippi River. If the marshes were the sick patients, the medicine was right next door — and because the medicine would never run out, the patient could remain healthy indefinitely.
But in 2007 science delivered the state a sucker punch.
The Intergovernmental Panel on Climate Change confirmed global warming was real and that the world’s seas were rising at the fastest rate in more than 4000 years.
Suddenly, the challenge facing Louisiana’s coastal restoration effort got steeper, because the southeast corner of the state was sinking at the same time the Gulf was rising.
While many of the state’s best known politicians — including its governor and some of its congressional delegation — cast doubt on the science, some of the world’s foremost experts on that science actually live in the state, and know what it means to Louisiana.
That includes Virginia Burkett, chief scientist for climate change at the U.S. Geological Survey, and a co-author of the IPCC reports.
“The atmosphere is warming because the concentration of greenhouse gases is increasing, and there’s unequivocal scientific evidence that supports that conclusion, that humans are influencing the amount of CO2 and other greenhouse gases in the atmosphere to the point that has caused a warming of the atmosphere,” says Burkett.
And why does that cause the sea to rise?
“The sea rises because of two drivers,” Burkett says. “One is — we call it steric sea level rise — which is the change in the temperature of the mass of the ocean, which increases ocean volume,” because when you heat water it expands.
“The second driver is the increase in ocean volume that is associated with the decline in the ice sheets and glaciers, the land ice, that has declined rapidly,” which also contributes to a rise in the sea level, she says. “Those two drivers together we call eustatic sea level rise, which is the global increase in mean sea level that is caused ultimately by the warming atmosphere.”
Burkett says Louisiana has a much a higher rate of sea level rise than other places in the country because the rise is relative.
“The rate of sea level rise appears to be more rapid in Louisiana because the land itself is sinking,” she says.
But as the state’s political establishment fought against the first headlines about global warming, it was about to get much, much worse.
Since the initial IPCC report, water stored as glaciers and ice fields over land areas has been melting at a much more rapid pace than predicted, which steadily adds to the volume of the oceans. Recently, instruments have confirmed sea level rise has accelerated 60 percent faster than predicted.
And that worries experts like Burkett.
“If you look at the geologic record of how sea level has changed in the past it’s not a nice smooth curve, and it’s probably not going to be over the next hundred years, either,” she says.
Burkettsays over the last 7000 years the sea level has been relatively stable, and that is how coasts as we know them have evolved to today.
However, projections show the rate of sea level change climbing dramatically. “And, as it starts to climb, the rate of coastal change will respond accordingly, and so we won’t have the stable coastal system that people have built on for the past 7000 years,” Burkett says. “Based upon what the scientific literature published over the past five years indicates, the highest rate, physically tenable rate of change, is 2 meters; that’s 6.6 feet.”
Unlike the state’s politicians, the scientists and administrators planning Louisiana’s effort to protect its coast haven’t ignored the evolving science on global warming. In fact, it’s written into the state’s Master Plan for the coast, a 40-year, $50 billion project that planners say can adapt to highest rates of sea level rise currently predicted — if its built in time.
“What we did in our Master Plan is we looked at a range of sea level rise scenarios, we looked at a range of subsidence scenarios, at a range of other things, hurricane scenarios and other factors, that have a high degree of uncertainty in some cases, and we chose the projects or project portfolios that perform best or transcend all of those scenarios,” says Garret Graves, who heads Louisiana’s coastal protection and restoration efforts.
More on that story tomorrow.
Support for The Louisiana Coast: Last Call comes from the Greater New Orleans Foundation, an organization that addresses the challenges facing people who live and work in the coastal communities of Southeast Louisiana.The Louisiana Coast: Last Call is written and reported by Bob Marshall of The Lens, and produced by Fred Kasten.
Copyright 2021 WWNO - New Orleans Public Radio. To see more, visit WWNO - New Orleans Public Radio.