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Spokane, Washington  Est. May 19, 1883

Sea level rise could destroy three-quarters of Louisiana’s wetlands

By Brady Dennis and Chris Mooney Washington Post

Rapidly rising seas are wreaking havoc on Louisiana’s coastal wetlands, and could devastate three-quarters of the state’s natural buffer against hurricanes in the coming decades, scientists found in a study published Thursday.

The new research documents how a sudden burst of sea level rise over the past 13 years – the type of surge once not expected until later this century – has left the overwhelming majority of the state’s coastal wetland sites in a state of current or expected “drowning,” where the seas are rising faster than wetlands can grow.

“We (can) treat the past decade or so like a really large-scale natural experiment, trying to evaluate how the natural system responds to such a high rate of sea level rise,” said Guandong Li, the lead author of a group of scientists at Tulane University who published the study Thursday in the journal Nature Communications.

The researchers arrived at their findings based on measurements of sea and wetland heights at 253 sites throughout coastal Louisiana. “Over the past decade, about 90 percent of the monitoring sites are unable to keep pace with the water level nearby,” Li said.

The news is dire for a state that has already lost over 2,000 square miles of wetland area since 1932, bringing the ocean ever closer to New Orleans and other population centers and leaving them more vulnerable to storms. Louisiana has launched major efforts to restore its coastal wetlands, replete with billion-dollar expenditures and massive engineering projects – but the state could also use a little help from the Earth itself.

The new research suggests the opposite is happening.

“If this rate of sea level rise continues for another 10 or 20 years, then we would probably lose the vast majority of our wetlands in that time period,” said Torbjorn Tornqvist, a Tulane wetlands expert and the second of the study’s three authors, along with sea level expert Sonke Dangendorf.

Brady Couvillion, a wetlands expert with the U.S. Geological Survey, has also found a recent speedup in the rate of Louisiana wetland losses, specifically when it comes to saltwater and brackish wetlands.

Because of the crucial role wetlands play in the state’s economy and storm protection, losing large swaths of marsh “is a problem of utmost scientific and societal importance,” the authors of Thursday’s paper write. In addition to offering a buffer from hurricanes and other storms, wetlands draw in tourists, provide habitats for birds and fish, naturally filter pollutants from the water and store carbon.

Along the coast of the Gulf of Mexico, where the range between high and low tide is small compared with the East Coast, wetland grasses grow at heights just a bit above the average daily water level. Such wetlands naturally spend some time underwater during high tide, and that’s when they’re typically nourished by sediment floating in the water.

But if the sea starts to rise too quickly, that same wetland will spend more and more time underwater – not necessarily just at high tide, but during lower parts of the daily tidal cycle as well. And at some point, that submersion becomes too much.

“The plants literally will drown when there’s too much water for too long in the day,” said Pam Mason, a senior research scientist at the Virginia Institute of Marine Science who was not involved in Thursday’s study.

That’s just what the study says is happening, and it’s likely to continue to happen along Louisiana’s coastline.

Seas in the region have risen at an unusually rapid rate of over 10 millimeters – or about 0.4 inches – per year since 2010, the research found. At the same time, many of the marsh sites in Louisiana are also experiencing major land sinking, or subsidence. This added effect can in some cases double the total speed at which the wetlands are sinking, relative to the height of the ocean.

Most just can’t keep up.

Out of the 253 wetland monitoring sites, 87 percent “are unable to keep up with rising water levels,” the authors wrote.

They distinguish in the study among three different types of wetland “drowning.”

Entirely drowned sites – which constitute 6 percent of the total – are those that are now underwater even at low tide. Sites “in the process of drowning” are spending between 10 and 90 percent of the time underwater; 69 percent of sites were found to be in this condition at low tide. Finally, sites “projected” to drown – 12 percent of the total at low tidal conditions – are currently spending less than 10 percent of the time underwater, but are in a location where the water is rising faster than the surface of the wetland, meaning that this percentage is set to grow.

The Tulane researchers think the current bout of rapid sea level rise is partly caused by a natural oceanic cycle that may subside. If so, the wetlands may catch a break.

Nonetheless, sea level rise is expected to continue to accelerate around the world throughout this century. Scientists also expect sea levels in the Gulf of Mexico to continue to rise, although not necessarily at so rapid a pace. Based on the marshes’ current response, the new study also tries to predict their fate in a scenario reasonably aligned with where the world appears to be heading in its greenhouse gas emissions. It finds that Louisiana could lose 75 percent of all of its coastal wetlands by 2070.

“I would never predict that wetlands are going to disappear entirely. We will probably keep something, but it’s just going to be very, very small,” Tornqvist said.

One question is whether in the future, even as wetlands are lost in much of coastal Louisiana, they will form in new places instead, offsetting some of the retreat.

“In the last five years or so, there’s been an emerging realization that existing marshes are drowning, but that we are getting new marshes in places where they have never existed before,” said Matthew Kirwan, a coastal wetlands expert at the Virginia Institute of Marine Science who was not involved in the study. One example, he said, is in the Chesapeake Bay. “You have to look at the balance between gains and losses, not just the losses.”

In a separate 2022 study, researchers used high-resolution satellite imagery to analyze the global extent of wetlands over the past two decades. They found a nuanced story – that while more than 5,000 square miles of tidal wetlands have been lost around the planet, those losses “have been substantially offset” by the creation of new wetlands.

The Tulane team, however, does not think the ability of wetlands to migrate is likely to help Louisiana much. Tornqvist said that they can migrate in the right conditions, but even that will likely be smaller than what was lost.

While wetlands can migrate, Mason said, doing so is hardly simple. Many shorelines are already developed. Roads, bridges, sea walls, farmland and buildings stand in the way of shifting wetlands.

“It’s only going to get harder, not easier” to create room for wetlands to move, Mason said. “And I don’t see it getting cheaper. … There’s going to be more and more competition for that land.”

Adam Langley, a wetlands researcher and biology professor at Villanova University who was not involved in Thursday’s study, said the new paper’s findings are broadly consistent with what scientists around the world are documenting. While the trends vary from place to place, existing wetlands are vanishing more rapidly than they are being replaced.

“The Earth is mostly ocean, and it’s becoming more ocean,” Langley said. “That’s the bottom line.”