Scientists say between 10,000 and 13,500 square kilometres of coastal land around New Orleans will go underwater due to rising sea levels and subsidence

A vast swath of the coastal lands around New Orleans will be underwater by the dawn of the next century because the rate of sediment deposit in the Mississippi delta can not keep up with rising sea levels, according to a study published today.

Between 10,000 and 13,500 square kilometres of coastal lands will drown due to rising sea levels and subsidence by 2100, a far greater loss than previous estimates.

For New Orleans, and other low-lying areas of Louisiana whose vulnerability was exposed by hurricane Katrina, the findings could bring some hard choices about how to defend the coast against the future sea level rises that will be produced by climate change.

They also revive the debate about the long-term sustainability of New Orleans and other low-lying areas.

Scientists say New Orleans and the barrier islands to the south will be severely affected by climate change by the end of this century, with sea level rise and growing intensity of hurricanes. Much of the land mass of the barrier island chain sheltering New Orleans was lost in the 2005 storm.

But the extent of the land that will be lost is far greater than earlier forecasts suggest, said Dr Michael Blum and Prof Harry Roberts, the authors of the study. “When you look at the numbers you come to the conclusion that the resources are just not there to restore all the coast, and that is one of the major points of this paper,” said Roberts, a professor emeritus of marine geology at Louisiana State University.

Blum, who was formerly at Louisiana State University, now works at Exxon. “I think every geologist that has worked on this problem realises the future does not look very bright unless we can come up with some innovative ways to get that sediment in the right spot,” said Roberts. “For managers and people who are squarely in the restoration business, this is going to force them to make some very hard decisions about which areas to save and which areas you can’t save.”

Efforts to keep pace with the accelerated rate of sea level rise due to global warming are compromised by the Mississippi’s declining ability to bear sediments downstream into the delta.

The authors used sediment data from the Mississippi flood plain to estimate the amount of sediment deposited on the river delta during the past 12,000 years. They then compared this with sediment deposition today.

In paper published in Nature Geoscience they calculate that due to dam and levee building on the Mississippi the sediment carried by the river has been reduced significantly. There are now about 8,000 dams on the Mississippi river system. Roberts said such constructions and the system of levees in Louisiana had cut in half the sediment carried down to the delta, inhibiting the river’s ability to compensate for the land lost to rising seas.

Sustaining the existing delta size would require 18 to 24bn tonnes of sediment, which the authors say is significantly more than can be drawn from the river in its current state. “We conclude that significant drowning is inevitable,” the authors wrote. “In the absence of sediment input, land surfaces that are now below 1m in elevation will be converted to open water or marsh.”