Hurricane Francine barreled into southern Louisiana on Wednesday as a Category 2 storm, packing 100 mph winds and sending a surge of water into coastal communities. Because so much of southern Louisiana lies at or below sea level, the surge can rush inland unimpeded. The last hurricane to hit the state was Ida in 2021, which was a catastrophic storm surge and cause $75 billion in damages and killed 55 people.
“Storm surge is really a nasty, nasty thing,” said Brian McNoldy, a hurricane researcher at the University of Miami. “These are hurricane winds that essentially push the sea onto land. It has nowhere else to go.”
The Gulf Coast’s storm surge problem will only get worse from here, scientists warn, due to colliding phenomena. Climate change is increasing hurricanes, as well as raising sea levels, and the coastline along Louisiana and Texas is sinking in some places, a process known as subsidence.
With every bit of elevation lost, sea level rise and storm surges get worse, but predictions have long neglected subsidence because researchers lack the data. This could mean that some parts of the Gulf Coast underestimate the potential damage. Louisiana’s coastal congregation has already more than lost 2,000 square miles of land between 1932 to 2016 to rise and fall of sea level. The state’s wetlands act as a natural buffer against storm surges, but the ecosystems can be approaching collapse.
Warmer waters in the Gulf of Mexico helped turn Francine into a fearsome cyclone. A hurricane is like an atmospheric engine. Its fuel is warm seawater, which evaporates and sends energy into the air. If the wind conditions are right, the storm will surge and march across the ocean. And if the water in its path is extra warm, the fuel is extra powerful, allowing a hurricane to intensify into a monster. “They can start growing very quickly under very warm sea surface temperatures,” said Daniel Gilford, who studies hurricanes at Climate Central, a nonprofit research organization. “Almost like when your foot hits the accelerator and that fuel flows into your engine to ignite.”
The Gulf Coast is naturally warm because it is heated over the summer. But according to a analysis by Climate Central, as Francine formed, it fed on ocean temperatures made at least 200 times more likely by climate change.
“What we’re seeing right now in the Atlantic Ocean and the Gulf of Mexico,” Gilford said, “is certainly an environment that is much more susceptible to stronger storms that spin faster and also bring much more moisture with them, which can lead to to increased rainfall.” In general, a warmer atmosphere holds more moisture, which means there is more water for a given storm to wring out of the sky.
As that water falls from above, the storm surge pushes water in from the side. The stronger the winds, the bigger the storm surge. This is happening on top of the base layer of additional sea level rise brought on by climate change. “So if sea levels are just higher on average than the built environment is prepared to handle, that could increase the amount of flooding associated with these storms,” Gilford said.
At the same time, communities are anticipating subsidence as parts of the Gulf Coast gradually lose elevation. Subsidence occurs when humans extract too much groundwater, oil or gas, causing the earth to crumple up like an empty water bottle. This also happens naturally when sediments settle over time. (Besides considering sea level rise, subsidence can destabilize roads, levees and other critical infrastructure.)
In a paper published last week in the Journal of Geophysical Research: Earth Surface, scientists used radar measurements from satellites to quantify subsidence across the Gulf Coast, from Corpus Christi to New Orleans, and found that parts are sinking at more than half an inch a year . This may not sound like much, but it happens year after year – just as sea levels steadily rise. As a result, the researchers concluded that the subsidence will greatly increase the risk of hurricane-induced flooding in the future.
However, the rate of subsidence is far from uniform: Some places along the Gulf Coast, such as Galveston County in Texas and New Orleans in Louisiana, are sinking rapidly while others remain stagnant. This makes subsidence a difficult problem to account for, as government agencies need precise data to assess the risk a given stretch of coastline faces. They can’t get a full picture of how much land they’ll lose as sea levels rise — and how bad storm surges will get — if they don’t account for the subsidence that’s happening at the same time.
“Once that land surface is lost,” says Ann Jingyi Chen, a geophysicist at the University of Texas at Austin and co-author of the paper, “and the buildings, the trees, the structures will be lost, which are actually some of the protective barriers, so that the storm surge can move further inland.”
Chen’s analysis found that cities that stopped overdrawing groundwater nearly halted their subsidence. And with more radar data, scientists can incorporate subsidence rates into models of storm surges, helping to find problem areas and take action to reduce the sink. Any bit of avoided subsidence will make storm surges like Francine’s that much less severe. “For planning purposes,” Chen said, “it’s good to know, so we don’t wait until it’s too late.”
