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
Suzanne Goldenberg guardian.co.uk 29 Jun 09;
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."
Mississippi River Delta to "Drown" by 2100?
Rebecca Carroll National Geographic News 29 Jun 09;
The Mississippi River Delta is drowning, according to new research that predicts the surrounding coastline will be inevitably reshaped in coming decades.
"There's just not enough sediment to sustain the delta plain," said study author Michael Blum of Louisiana State University in Baton Rouge.
Deltas are coastal landmasses created from a river's sediment deposits as the water flows out to sea.
The Mississippi River's delta plain, for example, includes the lacy "toe" of southern Louisiana.
All deltas are degrading to some extent, as their sediment settles and sinks. But a delta can sustain itself or even grow if its parent river regularly deposits enough new material.
Today sediments collected along the Mississippi cover about 23,360 square miles (60,500 square kilometers) ranging in thickness from less than 33 feet (10 meters) upstream near Memphis, Tennessee, to about 328 feet (100 meters) in the delta at the tip of southern Louisiana.
The drainage basin of the roughly 2,350-mile-long (3,782-kilometer-long) river, however, includes about 40,000 dams and levees built over the past century.
These structures control flooding and improve navigation, but they also trap sediment or funnel it completely through to the sea.
Previous studies suggested that dams and reservoirs built since 1950 have trapped as much as 70 percent of the river's natural amount of sediment. With less material feeding it, the delta plain has been experiencing erosion.
But even without the dams and levees, the amount of sediment flowing downriver would no longer be enough to sustain the delta because of rising seas, the study authors say.
Tough Choices
The researchers base their conclusions on estimated delta levels over the past 12,000 years, which show significant changes more than 7,000 years ago, when meltwater from the last ice age quickly filled the oceans.
The Mississippi Delta plain retreated inland at that point, and it was only after sea level rise had slowed considerably that the delta again grew seaward.
Current sea level rise, however, may be three times faster than it was the last time the delta was able to grow. (Related: "New York, Boston 'Directly in Path' of Sea Level Rise.")
With the added threat of rapid sea-level rise, sustaining the current extent of the delta plain would require 18 to 24 billion tons of sediment—way more than the entire Mississippi River currently carries, the researchers say.
The team therefore estimates that as much as 5,200 square miles (13,500 square kilometers) of delta land could disappear by 2100—an area only slightly smaller than Connecticut.
For now the study authors don't have a solution, and they add that plans to save the delta plain—such as redirecting and possibly adding sediment—will almost certainly involve sacrifices.
"They can [divert sediment to areas] downstream from, say, New Orleans, but that means that areas [of the delta plain] farther upstream will be submerged," Blum said.
"Tough choices have to be made, and they need to be made fast."
Findings appear online this week in the journal Nature Geoscience.
