The Gulf Coast Is Sinking, Making Hurricanes Like Francine Even More Dangerous

“What we’re seeing in the Atlantic and the Gulf of Mexico right now,” Gilford said, “is certainly an environment that is much more susceptible to stronger storms that spin faster and also carry a lot more moisture with them, which can lead to increased rainfall.” In general, a warmer atmosphere holds more moisture, meaning there’s more water for a given storm to wring out of the sky. 

While that water is falling from above, the storm surge is pushing water in from the side. The stronger the winds, the bigger the storm surge. That’s happening on top of the base layer of additional sea level rise brought by climate change. “So if the sea levels, just on average, are higher than the built environment is prepared to handle, that can increase the amount of flooding that is associated with these storms,” Gilford said. 

At the same time, communities are reckoning with subsidence, as parts of the Gulf Coast are steadily losing elevation. Subsidence happens when people extract too much groundwater, oil, or gas, causing the earth to crumple like an empty water bottle. It also happens naturally when sediments settle over time. (Beyond the consideration of 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, finding that parts are sinking by more than half an inch a year. That may not sound like much, but that’s happening year after year — just as sea levels are steadily rising. Accordingly, the researchers concluded that the subsidence will significantly increase the risk of hurricane-induced floods in the future. 

The rate of subsidence is far from uniform, though: Some places along the Gulf Coast, like Galveston County in Texas and New Orleans in Louisiana, are rapidly sinking while others are staying put. That makes subsidence a difficult problem to reckon with, since state agencies need precise data to determine the risk that a given stretch of coastline faces. They can’t get a complete picture of how much land they’ll lose to sea level rise — and how bad storm surges will get — if they aren’t accounting for the subsidence happening at the same time.

“Once that land surface is lost,” said 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, that actually loses some of the protective barriers, so the storm surge can move further inland.”

Chen’s analysis found that cities that stopped over-extracting groundwater saw their subsidence pretty much stop. And with more radar data, scientists can incorporate subsidence rates into models of storm surges, helping find problem areas and take action to reduce the sinking. Any little 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 is too late.”

Matt Simon is Senior Staff Writer at Grist. This story was originally published by Grist. You can subscribe to its weekly newsletter here.