Seas are rising too fast to save much of the Mississippi River Delta, scientists say
Author: isi网站管理员-刘成Source: Updated: 2018-04-12

This amplified color mosaic shows the Mississippi Delta in 2014. (NASA Earth Observatory image by Michael Taylor and Adam Voiland, using Landsat data from the U.S. Geological Survey)

The state of Louisiana is proceeding with ambitious plans to redirect the Mississippi River and rebuild some of its rapidly vanishing wetlands — but even this massive intervention may not be enough to save the most threatened lands from fast rising seas, scientists concluded in a study published Wednesday.

The study uses a methodology called “optical dating” to study how the river built an area called the Lafourche subdelta in coastal Louisiana, where the Mississippi dumped loads of sediment as much as 600 years ago, when it changed paths. The technology lets scientists identify the last time that long-buried sand was exposed to sunlight and, therefore, determine the rate at which the river naturally built up land by carrying sediment downstream.

“What we found was that, on average, it produced somewhere between 6 and 8 square kilometers of land per year, and the shoreline migrated seaward by somewhere between 100 and 150 meters per year,” said Torbjorn Tornqvist, a Tulane University geologist who was one of the study’s authors. “Those numbers in themselves I find pretty impressive.”

“But the problem is that if you put that in the context of the rates of wetland loss that we’ve seen over the last century, it doesn’t even come close,” he added.


The study reports that wetland loss at present is more like 45 square kilometers a year, or more than one acre an hour (an acre is close to the size of a football field).


Louisiana’s coastal wetlands are valuable as the home to human communities and also because they help protect New Orleans from hurricanes and sea-level rise. At the same time they’re a major habitat for birds, and they nourish fisheries on which humans rely.


The research was published in Science Advances and led by Tulane’s Elizabeth Chamberlain with colleagues at Tulane and other institutions in the United States, Britain, Austria and the Netherlands.


The scientists took their samples in areas of solid land where you would hardly expect there was once open water or a marsh — but, then, that’s the point. The Mississippi River is a great builder of land as it carries large volumes of sediment and silt downstream.


Zhixiong Shen, left, and Elizabeth Chamberlain collect a sample for optical dating along Bayou Lafourche in Louisiana, an abandoned course of the Mississippi River. (Anjali Fernandes)


Some of that sediment is now trapped behind dams along the length of the river, but much still reaches the delta. The problem is that factors that drive wetland loss are simply more powerful — the sinking of the land (subsidence), the intrusion of saltwater as seas rise, the dissolution of wetlands that have been cut into canals to support oil and gas pipes, and more.

And on top of that, sea-level rise is now occurring much faster than it did when the Bayou Lafourche land was built. The current rate is about 3.2 millimeters per year, and it is believed to be accelerating. But when the Lafourche subdelta grew, the sea-level rise rate was 0.6 millimeters per year.


“Lafourche formed during a relatively favorable time when the rate of sea-level rise was about as low as it can get in this region,” Tornqvist said.


The conclusion is that, well — the river just may not be able to keep pace. That’s even though the Louisiana Coastal Protection and Restoration Authority, using funds from the BP settlement, is moving forward with two large sediment “diversions” that within a few years could start channeling huge volumes of river water in new directions, in a bid to protect areas around New Orleans in particular. Many scientists have applauded the plan as a way of harnessing nature’s power to counter land loss.


But the rate at which the diversions may build land may not be enough for many areas, Tornqvist said.

“You focus your efforts on relatively select parts of the delta, and if these things are well designed, the very best you can expect is these rates you see from the Lafourche system,” he continued.


The fact that only part of the Mississippi Delta may be sustainable with rising seas is “not a new idea to coastal scientists and restoration planners in Louisiana,” said Andy Nyman, an expert on wetlands at Louisiana State University, who commented on the study but was not involved with it. “But it bears repeating with readers who might not realize this important fact.”


“The results are, on one hand, encouraging and helpful as the state of Louisiana moves forward with designing and implementing large diversions of sediment from the river that mimic the natural processes that built this landscape in the first place,” added Don Boesch, the president emeritus of the University of Maryland Center for Environmental Science.


“On the other hand, it is important to keep in mind that the delta studied was building when there was very little rise in ocean levels,” Boesch continued. “Already, global sea level is rising many times faster. To have a fighting chance in maintaining some semblance of a functioning delta, the world must quickly eliminate greenhouse gas emissions to limit global warming and slow sea-level rise.”

So the news is mixed — humans may be able to actively counter some of the wetland losses, but not all of it.


“We’re still convinced that these river diversions are basically the only real shot at getting something done in terms of coastal restoration,” Tornqvist said. “But you have to be really realistic about what you can expect, and it’s going to be have to be very focused on small and carefully selected parts of the delta. It means very difficult choices are going to have to be made.”




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