“I think it’s a point worth making that we are finding here that what people recently have considered worst case scenarios are actually conditions that we already see right now,” said Torbjörn Törnqvist, a geologist at Tulane University and a co-author on the new paper.
Scientists have long known that Louisiana is sinking. Subsidence is believed to be a natural process, which has likely been occurring in the region for thousands of years. But scientists believe the process has been enhanced by a variety of human activities in the Mississippi Delta over the past century, including oil and gas extraction, as well as the building of levees and other actions affecting the flow of the Mississippi River, which carries mud and sediment down toward the Gulf and helped build up the delta in the first place.
“When we started building levees along the river, we made it much harder for the sediment to disperse across the delta and beyond the delta,” Törnqvist said.
The combination of subsidence and sea-level rise along the Gulf shore has made coastal Louisiana increasingly vulnerable to erosion in recent years. Last year, residents of the region’s rapidly sinking Isle de Jean Charles received a $48 million grant from the federal government to be used for relocation, giving them the grim title of the nation’s first “climate refugees.” And in the future, as sea levels continue to rise, the problem is only expected to worsen along the Louisiana shore.
As a result, many researchers have attempted to quantify not only how quickly the seas are rising, but also how quickly the land is sinking. They’ve used a variety of methods to do so — one common technique involves tide gauges, which help measure changes in sea levels. But previous methods have not always accurately captured all the changes taking place along the shore, Törnqvist said.
The new map relies on data, published earlier this year in another paper Törnqvist also co-authored, that was assembled using a novel approach that the authors say is more accurate than other methods. The new approach involves a combination of GPS data and special rods, driven deep into the sediment below the surface of the water, which help measure the land’s elevation.
“This is definitely a very accurate way of doing it,” said Alex Kolker, an associate professor at the Louisiana Universities Marine Consortium, who was not involved with the new paper. The new method “looks only at the land motions, whereas tide gauges can be influenced by both land movements and water movements.”
Using the new data, Törnqvist and his colleagues created a map of subsidence in coastal Louisiana. They found that subsidence rates throughout the region averaged about nine millimeters per year, although the rate varies significantly from one location to the next. According to the authors, previous studies have generally suggested average subsidence rates of between one and six millimeters over the past few decades.
However, Kolker suggests that the nine-millimeter finding is perhaps not so surprising after all. Other studies have suggested high rates of subsidence in certain areas along the coast at various times, he pointed out, and said he believes a coastwide average of nine millimeters may not be “that far off from what many people expected.”
In his opinion, he said, the paper’s greater value is the spatial variation it reveals, with different subsidence rates at different locations. This suggests that different parts of the shoreline may need more attention than others.
“Knowing that things are slightly different from one place to the other is more important than knowing the average, particularly as the state moves forward with its master plan for coastal restoration,” he said. “In Louisiana, the amount that the area is sinking directly impacts the amount of mud that you need to fill the area and to restore a marsh.”
But although the map does reveal a high degree of variability from one location to the next, Törnqvist cautioned that there’s also a great deal of uncertainty about the subsidence rates in any specific place.
“This map should not be used for reading values for very specific locations,” he noted. “It’s more intended to show the big picture.”
And for now, the big picture suggests that coastal Louisiana is still sinking, and perhaps more quickly than some experts believed. Coupled with the continued invasion of the rising seas, the area remains one of the most vulnerable parts of the country. And while policymakers have already expressed deep concern about the region’s future — the state legislature recently approved a $50 billion plan for the protection and restoration of the coastline — experts note that there’s still need for more research on exactly how quickly the shoreline is slipping away.
Joshua Kent, an expert in geoinformatics at Louisiana State University, told The Washington Post by email that the new paper reiterates the need for increased monitoring along the coast. “Accurate and consistent monitoring will help reduce the uncertainties and improve strategies for sustaining this important landscape,” he said.
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