An abandoned boat sits in the water amid dead cypress trees in coastal waters and marsh Aug. 26 in Venice, La. Many oak trees and cypress trees throughout Louisiana's coastal marshes have died due to a combination of the saltwater intrusion and subsidence. © Drew Angerer/Getty Images
Rising seas are likely to overwhelm the wetlands that line the coast of Louisiana, destroying the protection they offer the city of New Orleans as well as the fisheries and tourism that contribute heavily to the state's economy, a new study says.
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The imperiled wetlands at the base of the Mississippi River have crossed a critical tipping point, according to the new research, which was published in Science Advances and is based on hundreds of measurements that revealed the fate of ancient marshes in this region.
Researchers found that the type of wetlands that exist in present-day coastal Louisiana have rarely persisted when rates of sea-level rise surpassed 3 millimeters per year over long periods of time. Current sea levels are rising at rates that already slightly exceed that, and those rates are increasing.
If sea-level rise exceeds 6 to 9 millimeters per year, the wetlands could be overcome by ocean water within 50 years, according to Torbjörn Törnqvist, a professor of geology at Tulane University in New Orleans who led the study.
As the world emerged from an ice age into the present Holocene era 8,500 years ago, seas rose at those high rates and the wetlands tended to vanish, the study found. Now, as climate change continues, sea levels may rise that quickly again.
"We are, if you believe this study, past the tipping point," Törnqvist said.
"We know that the rate of sea-level rise, even with the best action you can imagine, it's still going to ramp up further," he added. "Given the slowness of the ocean response, it's going to last for a very long time."
The research is based on decades of work extracting hundreds of "sediment cores," or thick cylinders of mud and peat, from across the Mississippi Delta. These cores provide a record of the history of the region stretching back thousands of years, and allow scientists to determine when it supported wetlands, and alternatively, when much of the region was simply open water.
Coastal Louisiana is already losing land at an alarming pace. Between 1932 and 2016, according to the U.S. Geological Survey, an area roughly the size of Delaware disappeared under the waves. A quarter of the land that existed in this area at the beginning of the last century is now gone.
The cause isn't just sea-level rise - the land in this region is also sinking as upstream dams have starved the lower Mississippi delta of sediment, and the wetlands have been further compromised by industry, especially oil and gas pipelines which cut through much of the region.
This current state means the wetlands are even more vulnerable to rising seas than they were in the ancient past, Törnqvist said.
"When you think about this early Holocene, when the rate of sea-level rise was very rapid, and we look at what marshes did back then, and we see they all drowned, you have to consider that that was a pristine landscape," Törnqvist said. "It was virtually untouched by humans. Compare that to what we have now. This region, it's severely wounded."
The study did not take into account the additional sinking that is occurring because of a variety of factors, including oil and gas drilling. Instead, it merely compared past sea-level rise rates with the current global sea level trend. Including these other factors, which are causing the marshes to sink, would only make the situation even more perilous, as Törnqvist notes.
At stake most of all is the future of New Orleans. The wetlands weaken storm surges during hurricanes, and so help to protect the city. New Orleans also has a massive and powerful flood defense around it, built by the U.S. Army Corps of Engineers following the devastation of Hurricane Katrina. But if wetlands recede, these powerful barriers could find themselves closer and closer to the Gulf of Mexico.
Donald Boesch, a professor of marine sciences at the University of Maryland Center for Environmental Science who has closely studied the Louisiana wetlands, called the new study "well-documented, analyzed and reasoned." But he pointed out that in the last decade, the marshes have been faring better than expected, especially compared with what the new research suggests is likely in the longer term. And it isn't clear why.
"Resolving this mismatch requires further research," he said.
During its worst stretches in recent decades, coastal Louisiana has lost wetlands at a speed equivalent to the area of a football field every 34 minutes. But in recent years, that rate has slowed to a football field every 100 minutes, according to the U.S. Geological Survey. The reasons for this change are not clear, but may include the fact there haven't been major hurricanes since Katrina in 2005 and Gustav in 2008, which can dramatically tear up the marshes, so the wetlands have had a relative reprieve, according to the agency.
Törnqvist, who completed the research with colleagues from Tulane University, Nanjing University in China, and the University of Texas at Rio Grande Valley, says that he believes that over time, erosion, the formation of ponds and other processes will inevitably undermine the wetlands.
The new research appears to challenge other papers such as a 2018 study in the journal Nature that have presented a more optimistic view. The Nature study suggested that global wetlands could actually gain area as sea level rises, and that losses can be avoided by creating buffers and limiting development in wetland areas.
Matthew Kirwan, a professor at the Virginia Institute of Marine Sciences who was one of the authors of that study, said research into what happened to Louisiana's marshes long ago is "important," but added that "we know almost nothing about what an 8,500-year-old marsh looked like and how and why it drowned."
"We clearly need to know more about why marshes that survived decades to centuries of rapid sea-level rise would eventually drown," Kirwan said. "That part is a mystery to me. Marshes in other places in the United States record a history of successful adaptation to sea-level rise for thousands of years."
In the Chesapeake Bay, as seas have risen, some marsh has been swallowed but a roughly equal new amount has formed closer inland, Kirwan said. He suggested something similar could happen in Louisiana.
"There is about as much land predicted to be inundated by 1m of sea-level rise along the Gulf of Mexico as there is current marshland," Kirwan wrote in an email. "So even if all the existing marshland in coastal Louisiana drowned, there is an incredible opportunity for new marshes to develop in other places."
The state of Louisiana is not letting its wetlands just disappear. It is planning vast "sediment diversions," in which large flows from the Mississippi River would be redirected to use its remaining sediment to build additional wetlands in strategic locations. This can fight off worst-case scenarios and buy time, Törnqvist said.
"If we do that in a targeted way, just downstream of the largest population center, which happens to be where I live, maybe that portion of the delta can be sustained for a bit longer than if we do nothing," he said, referring to New Orleans. "I think a couple of decades is incredibly valuable because it could be the difference between a somewhat managed retreat versus complete chaos."
How humans respond to climate change will also have an impact. Sea level rise can be mitigated by cuts in carbon emissions, scientists say. At the current rate of just over 3 millimeters of sea level rise per year, Törnqvist said, his research finds that marshes do tend to last for centuries, even if they ultimately seem to succumb. The real issue is avoiding rates closer to 6 to 9 millimeters per year, or even higher.
"Only if you ramp it up further to that higher rate, then you start talking about half a century," Törnqvist said. "Of course, that's the direction we're headed right now, but that is something we still have some control over. And if we manage to reduce emissions in a serious way, we should be able to avoid the worst possible scenarios."