This story was originally published by Grist and is reproduced here as part of the Climate Desk collaboration.

It has been 20 years since New Orleans’ faulty levee system failed during Hurricane Katrina, causing a flood that claimed almost 1,400 lives and inflicted more than $150 billion in economic damage. The catastrophe was so bad that some doubted the city could continue to exist at all — the US House speaker at the time declared that rebuilding New Orleans “doesn’t make sense” and that much of it “could be bulldozed.”

Rather than just patch up the damage, which would have left one of the country’s most iconic cities exposed to every future storm, the federal government doubled down on flood protection, building a new $14.4 billion levee system that ranks as one of the most sophisticated anywhere in the world.

Over the course of a decade, the US Army Corps of Engineers rebuilt and expanded almost 200 miles of levees across three parishes. It outfitted every major channel and canal with a gate that could swing shut during surge events. On the east side of the city, where storm surge had overtopped its old levees, it built the Lake Borgne Surge Barrier, a 2-mile wall that could stop as much as 26 feet of surge. On the three canals where it had built shoddy flood walls, it built new ones and massive pump stations that can remove an Olympics-sized swimming pool of water from the city every 3.5 seconds. It also decommissioned the Mississippi River-Gulf Outlet, or “Mr. Go,” a large shipping channel that had destroyed protective marshland around New Orleans and funneled Katrina’s storm surge into the city.

But for all the success of the new levee system, the future of New Orleans remains uncertain.

The sea levels around the city are rising by about half an inch every year as climate change warms the oceans and melts glaciers. The city itself is sinking even faster than that, with some sections of the levee system settling by almost 2 inches each year — faster than the rate of change that the Corps projected when it built the system. This elevation change makes the new levee system less effective with each year, requiring constant repairs and expansions.

Even landmark structures like the Lake Borgne barrier may lose a few feet off their protection capacity by the middle of the century. That would put them within a hair’s breadth of being topped by storms such as Hurricane Michael, which delivered almost 20 feet of surge to Florida in 2018.

“Since 2005, several storms have made landfall on the Gulf Coast that far exceed the stated design capacity of the new ‘risk reduction system,’” said Andy Horowitz, a historian at the University of Connecticut and the author of a book on Hurricane Katrina. “It’s just chance, or luck, that one of them didn’t hit New Orleans. One day, inevitably, one will.”

The Corps maintains that the system is working as designed, but federal and state cuts could jeopardize the system’s resilience even further. The Trump administration has already eliminated funding for the Corps and the Federal Emergency Management Agency (FEMA) for

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