NO ONE really knows how fast global warming will make the oceans rise. But what is clear is that south-east Louisiana is sinking, rather rapidly by geological standards. Add in the prevalence of hurricanes in the neighbourhood, and it becomes a dangerous place to live.

Land subsidence in the Mississippi delta is largely caused by human interference. When the river was allowed to flood over its banks every year, some of the muck that spilled out remained, creating more and higher land nearby. (It is because of this that the highest ground in New Orleans, counter-intuitively, is along the riverfront, one of the only areas of town not to flood after Hurricane Katrina.)

Levees have changed that. Until Katrina, the Mississippi had not flooded in or around the city in almost 80 years. During that period, swampy lands in the region began to be drained for development. As the moisture was sucked out of it, the ground naturally sank, and without the once-annual dose of Mississippi mud, the sinking continued.

Scientists now think south-east Louisiana may sink between two and six feet (up to two metres) over the next century, and that perhaps half of that will be caused by subsidence. If they are right, New Orleans and its suburbs will become thin strips of low land walled off by levees and surrounded by open water on three sides, much more vulnerable than they were when Katrina struck.

This grim outlook, the subject of a recent series in the Times-Picayune, has reignited debate about how to protect the area from the sea, and how much to spend doing it. In the eyes of most south Louisianans, the federal government has been a bit stingy when it comes to reinforcing flood defences. Levee improvements are going on at the moment, but until they are finished New Orleans is vulnerable. Last year, though it did no major damage, Hurricane Gustav sent sea waves spilling over floodwalls in the heart of the city.

There have been some victories. The Army Corps of Engineers, the federal agency responsible for coastal defences, has successfully installed a couple of diversion projects, which push muddy river water into nearby marshland in an effort to mimic the land-building effect of the old springtime floods. They seem to work, but many more are needed, and soon.

What is clear is that an expensive, co-ordinated, all-hands-on-deck effort will be required to beat back the forces of nature in south-east Louisiana. That has yet to happen. Instead, plans have either been modest or have failed to get funding. One of the most successful diversions, near the river's mouth, is now in danger of being shut down because the shipping industry has complained that it is silting up a useful anchorage. The Corps of Engineers, meanwhile, has failed to produce plans for higher levees and coastal restoration that would provide so-called “Category 5” protection, as Congress has demanded.

Louisianans hope that the national infrastructure campaign Barack Obama wants to implement will include a healthy dose of levee- and wetland-building. In the meantime, the region continues to lose minute but measurable ground to the Gulf of Mexico every month.