Since 1932, coastal wetlands in Louisiana have declined by about 25 percent. At its fastest, the decline was around one football field lost every 34 minutes; at its slowest, every 100 minutes. The Pelican State is losing ground faster than any other state in the contiguous United States. And those losses reach far beyond its borders: coastal Louisiana plays a crucial role in fisheries, shipping, and oil and gas production.
In recent years, the wetlands have been faring better than in previous decades, possibly because there hasn’t been a Katrina-level storm in that time. But a study published last week in Science Advances suggests that this is a temporary reprieve. With sea levels rising as rapidly as they are, the wetlands, including the famed Mississippi Delta, are likely to be gone in a matter of decades—or, at most, centuries.
Coastal Louisiana is currently home to 15,000 square kilometers of marshland, a critical ecosystem held together by a complex, interlocking set of processes. The tide washes in; plants grow and die; sediment is brought in by rivers and builds up. If the system changes—for instance, if sea levels rise rapidly—the marsh changes, too.
As with many ecosystems, that change might happen gradually and then all at once, write geologist Torbjörn Törnqvist and his colleagues. Ecosystems like wetlands have some resilience as the environment becomes less ideal, adapting and surviving at the outer extremities of their livable range. But once a tipping point is reached, the ecosystem can quickly change state—open water may suddenly appear where there was marsh before.
Recent research has found that marshes may be resilient to rising sea levels. But these models use relatively short timescales—years or decades—in the historical data they draw on to make their predictions.
Looking further back, as Törnqvist and colleagues found, yields a completely different answer. They drew on hundreds of sediment cores taken from the area—samples of mud extracted from the Earth and studied to determine what kinds of materials were being deposited as the centuries rolled by. Their samples allowed them to analyze the survival of the region’s marshes as far back as 8,500 years ago.
They looked for indicators that the area they were sampling had signifiers of having once been open water. For instance, finding preserved samples of a species of clam that lives in brackish water was a sign that the marsh had become an open-water lagoon during that period. Then they compared the history of marshiness in the area to data on sea-level rise.
The researchers found that when the sea rose at less than 2mm per year, the sites survived as marshes. When sea levels rose at more than 3mm per year, most of the marshes gradually drowned over a period of centuries. And when sea-level rise increased past 6mm per year, the marshes drowned rapidly and completely, taking less than 50 years to do so.
Delaying the inevitable
Obviously, there are important differences between the marshes of thousands of years ago and the marshes of today. But many of those differences don’t work in our favor. For instance, thanks to damming and other human interference, the Mississippi River is washing less sediment to its delta than it would be in a world with no problematic human interference. So the historical record doesn’t necessarily superimpose perfectly onto the present—but those differences make the future look more, not less, dire.
Still, this doesn’t mean that people should shrug and give up. There’s still damage that can be mitigated, with different climate change scenarios associated with more and less drastic rates of sea-level rise; the lower end of the range is still a death knell for the marshes, but it will take longer, giving us a cushion to get climate change into check. Louisiana is also responding to the loss of marshlands with a program to divert sediments to strategic wetland locations, which could help to mitigate some of the worst human impacts and buy crucial decades.
With the conflicting conclusions from this and other studies, it is important for researchers to gather more data—both from Louisiana’s marshes as well as other regions where sediments can be studied to understand how sea-level rise affected wetlands.
But based on this evidence, write Törnqvist and colleagues, drowning of the marsh is inevitable. “This is a major threat,” they write, “not only to one of the ecologically richest environments of the US but also for the 1.2 million inhabitants and associated economic assets that are surrounded by [Mississippi Delta] marshland.”