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Confusion over sinking coasts in Gulf

A new federal report states that land in Louisiana and probably throughout the Gulf Coast has been sinking at a relative rate of more than 1.5 meters (5 feet) per century for at least the last 100 years — a rate significantly higher than previous reports have shown. If that rate is correct and continues, it means much of the coastline, including New Orleans, could be underwater by the end of this century.

Tulane University geologist Torbjörn Törnqvist sets up a GPS instrument in front of an artificial levee along the Mississippi River in Louisiana. Based on detailed reconstructions of sea-level rise, he and his students have found subsidence rates of about 10 millimeters per century — two orders of magnitude less than the rate of 1.5 meters (15 feet) per century published in a recent federal report. Courtesy of Juan González.

Scientists agree that subsidence — the slow sinking of land — is occurring along coastal areas of Louisiana, Mississippi, Texas and Alabama. Louisiana, for example, loses an acre of wetlands every 35 minutes to inundation by the sea and has lost 65 to 100 square kilometers per year for the past several decades, according to the Louisiana Coastal Wetlands Conservation and Restoration Task Force. More than 1 million residents and businesses in New Orleans are below sea level and are at serious risk from subsidence, says Jeff Williams with the U.S. Geological Survey (USGS) in Woods Hole, Mass.

Many restoration efforts have been undertaken to save the coastline, including the Coast 2050 plan and the Louisiana Coastal Area plan, a $14 billion project to restore the structure and function of the coastal wetlands. But as coastal managers struggle to figure out how to mitigate problems, controversy keeps arising over just how rapidly the land is sinking relative to sea level, and why.

“Most scientists would agree that the subsidence is caused by a combination of geologic and anthropologic factors,” Williams says. These factors include natural fault movement, compaction of sediments, and withdrawal of oil and gas, common in the oil-producing Gulf. But, he says, scientists don’t know what the individual contribution of the factors is to the overall subsidence rate.

Roy Dokka of Louisiana State University in Baton Rouge, co-author of the new subsidence report, says that the sinking is probably almost all natural and thus likely to continue over the next 100 to 1,000 years. After all, he says, “the bottom of the Mississippi delta is 100 meters down, and began to form 8,000 years ago, showing a natural compaction” of sediments at a rate of 12.5 millimeters per year.

Dokka and co-author Kurt Shinkle of the National Geodetic Survey (NGS), which published the report, looked at benchmark leveling measurements from NGS archives from 1920 to 1995, and then compared those data to tide-gauge measurements and other independent datasets. The benchmarks, often brass disks attached to rods stuck in the ground or on the sides of buildings, are points of comparison for elevation measurements over time.

“Subsidence is much more widespread and much faster than previously thought,” Dokka says. While these rates are specific to the past 100 years and “may not reflect the current rate of subsidence,” according to the NGS report, Dokka says that “you can’t extrapolate these rates without looking at the future,” and he “absolutely” thinks the rates are natural and will continue.

Many scientists, however, disagree. Rates this high are only possible locally — and due to human influences — and are not likely to apply today, says Bob Morton of USGS in St. Petersburg, Fla. Additionally, he says, “we have good evidence that the highest rates of subsidence, near oil and gas withdrawal sites, are in decline, and that wetland loss rates — which are closely tied to subsidence — have slowed dramatically, leading us to infer that overall subsidence rates may have also slowed dramatically.”

Indeed, other studies show a significantly slower rate of subsidence throughout the region. Geologic data from old beach deposits in western Louisiana, for example, have revealed a rate of about 1 millimeter per year, or 1 meter per 1,000 years (almost 100 times slower than the NGS report), says Kristy Milliken, a graduate student at Rice University in Texas. “And our data are in line with other geological data collected along the coastal area,” she says.

If the area had been subsiding at the rate of 1.5 meters per 100 years that the NGS report suggests, this area would be about 57 meters below ground, instead of 3.8 meters, says John Anderson, a geologist at Rice and Milliken’s advisor. “It is difficult to reconcile those subsidence rates,” he says, unless “humans have had far more of an impact than we thought. That would be really interesting if it were true.”

This report, and the conclusions drawn from it, Williams says, “need to be based on the best interpretations of the data available and I’m not confident that they are.” Part of the problem, he says, is that the NGS raw elevation data were not published in the report and are not publicly available for peer review.

Still, Dokka stands by the report and its conclusions. “This has the potential to become the biggest environmental disaster North America has ever seen in less than a century — as most of Louisiana and much of the coast is below 3 feet above sea level, we could lose as much land as the size of Massachusetts over 100 years,” Dokka says.

The problem, Morton says, is that with the different datasets and interpretations, “there is no absolute truth.” So what are city and community planners to do? “That’s an excellent question,” Williams says.

Megan Sever

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