Sea levels have risen faster in the Gulf of Mexico and the Southeast coast than other parts of the U.S. in the last decade, according to two new studies.
The data shows a rapid rise in areas such as Pensacola, Cedar Key and Lake Pontchartrain in New Orleans, as well as accelerated rises on the east coast, from Miami up to Cape Hatteras.
The rate of acceleration exceeds the long-term trend, says a recent paper by Jianjun Yin, of the University of Arizona, and seems to have begun around 2010. The paper warns that the trend coincides with record-breaking North Atlantic hurricane seasons in recent years, resulting in elevated storm surge and more intense coastal flooding during storm events.
The second study, published in Nature by scientists from Tulane, Old Dominion and the University of Central Florida, looked at tide gauge records along the U.S. Southeast and Gulf coasts. Data revealed that while global mean sea-level rise has been about 1.5 millimeters per year since 1900, and more than 3 millimeters per year since 1992, in parts of the eastern Gulf, it’s been a rapid 10 mm per year since 2010. Those rates are “unprecedented in at least 120 years,” the report said. Data from the studies showed sea levels are 8 inches higher now than they were in 2006 at Lake Pontchartrain.
In other words, the Gulf and Southeastern U.S. in general are currently seeing much greater sea-level rise per year than the global average.
“There was this area on the Gulf coast where these extrapolation curves were tracking the highest sea-level-rise scenarios we have in the model runs,” said Thomas Wahl, of the University of Central Florida, and one of the co-authors of the Tulane study.
“That was surprising to many, indicating that we were already tracking a very high sea-level-rise scenario in the Gulf of Mexico.”
Wahl said that the acceleration is also happening on the Southeast coast from Florida up to Cape Hatteras in North Carolina. “You see this pretty strong acceleration,” he said.
The recent spike may be a temporary acceleration, say the reports, but scientists can’t say when it would slow down. There was a similar ramping up of sea-level rise in the 1940s that later returned to normal, meaning sea level continued to rise, but more slowly.
William Sweet, an oceanographer for the National Oceanic and Atmospheric Administration who read through the studies, said that these higher seas, both in the Gulf and on the east coast, create more of an opportunity for flooding, be it from a king tide or a hurricane.
The ramp up in sea-level rise is happening at different rates in different places. The west coast of the U.S., for example, is not experiencing nearly as much sea-level rise as the Gulf of Mexico and South Florida. For example, sea-level-rise trend in Oregon as relatively flat, while in Florida, both on the Gulf and Atlantic coast, the trend rises, and ramp up steeply after 2010.
What’s behind the spike?
Sweet said there’s a large volume of water right now in the Gulf and Southeastern United States. “There are two reasons: It’s human-induced, it’s the climate signal at play. But they also found that there are some natural reasons why this water might be accumulating.”
The Tulane study suggests that Rossby waves are partially to blame for the current spike. Rossby waves are massive 600-mile-wide undulations in the ocean that are so large and slow that they’re invisible to the human eye.
“There can be one- to two-decade oscillation to these trends,” said Sweet, and the NOAA website says they can take decades to cross a given ocean. They also move in a westerly direction, meaning they pile water up on the eastern seaboard of the U.S. and into the Gulf. “Right now we’re starting to get some high accumulations,” Sweet said.
Hot water expands, creates bulge in Gulf
A second reason for the burgeoning seas in the region is that water changes size — the volume of hot water is larger than cool water, Sweet said. As it takes up more space, it has to go somewhere. That somewhere is up, and then out into flood zones.
“There’s definitely indications of warming in the upper 500 to 1,000 meters of the ocean,” Sweet said. Warm water is mixing deep into the water column.
“There’s a lot of hot water in the Gulf of Mexico,” he said. “The summation of all the heating that’s going on, whether in the top, middle or bottom, it’s going to affect the height of the water column above it.”
Satellite altimeter maps of the ocean show changes in sea surface height. The Gulf has more rapid changes than most areas, almost like a bulge, as if the water is swelling at depth and pushing the surface up.
The good and bad news
The findings come as NOAA released a statement analyzing greenhouse gas emissions in 2022. “Levels of carbon dioxide (CO2), methane and nitrous oxide, the three greenhouse gasses emitted by human activity that are the most significant contributors to climate change, continued their historically high rates of growth in the atmosphere during 2022,” said the release.
“The observations collected by NOAA scientists in 2022 show that greenhouse gas emissions continue to rise at an alarming pace and will persist in the atmosphere for thousands of years,” said NOAA administrator Rick Spinrad.
The bad news, Wahl said, is that we are in a natural phase that exacerbates flooding.
“High-tide flooding has doubled in many places,” he said. “Miami is one of the hot spots for high-tide flooding. The semi-good news is that if there is a strong influence of natural variability, we can expect that in the not-too-distant future we will hopefully see again lower rates of sea-level rise. That does not mean we will see a stop of rise. It’ll continue to rise.” He said they don’t know when the slowdown would happen, but that it would eventually come.
Sweet said it’s a question of how fast rise will occur, not whether it will stop.
“What we’re not expecting is an overall deceleration in sea-level rise,” he said. “Whether or not we rise 15 inches or 12 inches in the next 30 years, we can debate that. But the facts on the ground are that communities are flooding. … We should not take our guard down. Sea-level rise of a foot or more in the next 30 years is a good bet. Whether it comes a decade early or a few years late, it’s coming,” Sweet said.
“I think there is more work to be done to connect the dots,” Wahl said. “But we have a much better understanding of what it means for flood risk in the future.”
