For coastal communities, the sea level rise propelled by melting ice and warming oceans is bad enough. But people living on the soft, compressible sediments of river deltas have another factor to contend with: sinking land. Scientists have traditionally inferred the sinking from tide gauge readings or measured it directly at GPS stations. But a team of scientists now says these methods significantly underestimate subsidence at many deltas and low-lying coastlines worldwide.
In recent years, scientists at Tulane University in New Orleans, Louisiana, have shown that in the Mississippi River delta, fluffy, young sediments within a few meters of the surface are compacting rapidly. They estimate the effect more than doubles the region's rate of sea-level rise to a total of 13 millimeters a year. Tide gauges and GPS stations miss that subsidence because they are anchored to deeper layers, which are less susceptible to compaction.
The same mechanism is likely at play in many low-lying coastal areas worldwide, which host some 10% of the global population, the team argues in a paper published this week in Ocean Science. "Tide gauges are not measuring what we need," says Torbjörn Törnqvist, a geologist at Tulane and co-author on the study. "We need to really rethink how we monitor these areas."
Satellites are the main tools for monitoring the absolute changes in ocean height, which reflect the biggest drivers of sea level rise: melting ice and the expansion of warming water. But for people and ecosystems, the relative impact of rising or falling land is just as important. Some regions are still rebounding thousands of years after ancient ice sheets melted, lifting a colossal weight off Earth's elastic mantle. Many more are subsiding. "It's something we've been overlooking too long in sea level projections," says Aimée Slangen, a climate scientist at the Royal Netherlands Institute for Sea Research in Yerseke and a lead author of the sea level chapter of the next United Nations climate report.
Louisiana, for example, is sinking fast. Although compaction is the primary culprit, the extraction of groundwater, oil, and gas also play a role. Sediment washed down the Mississippi River once compensated for the subsidence, but levees and other engineered structures now shunt it out into the Gulf of Mexico. To monitor the sediment loss, the state over the past few decades has deployed a network of some 400 simple wetland-monitoring instruments, called surface elevation tables.
The table, a metal arm that juts out parallel to the swamp's surface, is anchored to a pole driven deep below. Twice a year, a series of pins are lowered from the table until they just touch the marsh surface—giving a regular measure of how fast the surface is sinking relative to deeper layers. Five years ago, when Törnqvist's group began to use this network to divine the source of Louisiana's subsidence, researchers realized the problem is not just sediment loss. Shallow soils, deposited in earlier centuries when the river ran free, are simply compressing. "Tide gauges were not capturing that," says Molly Keogh, the Tulane graduate student who led the new work.
The new paper lays out why. The region's 131 tide gauges measure the tide in comparison with benchmarks anchored in deep sediments, often tens of meters down—"as close as we get to bedrock in Louisiana," Keogh says. The region's 10 GPS stations with known benchmarks were also anchored, on average, 14 meters deep in the mud. To both devices, the zone of the most compaction—a source of half the sea level rise—was invisible. The scenario could be true in other delta regions that also rely on tide gauges, casting doubt on estimates of regional sea level rise, says Mark Schuerch, a physical geographer at the University of Lincoln in the United Kingdom. "It's quite innovative and quite exciting—or scary, really."
Figuring out the anchor depths of tide gauges elsewhere in the world will be a herculean task, warns Philip Woodworth, former director of the Permanent Service for Mean Sea Level in Liverpool, U.K., who reviewed the paper. Tide gauge records do not typically include the depth of their benchmarks; that knowledge, if it exists, is buried in country bureaucracies.
Moreover, the rate of shallow compaction probably varies greatly from wetland to wetland. In some marshes, plants compensate for compaction by capturing new sediment with their roots. And in some regions, such as Bangladesh, compaction occurs more uniformly across shallow and deep layers, says Céline Grall, a marine geologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York. "These assumptions are not true there."
Deploying elevation tables in deltas around the world could resolve those uncertainties, creating a global database, Törnqvist says. The tool is simple, cheap, and effective, and has already been used in more than 30 countries. For an area the size of coastal Louisiana, only 40 would be needed to keep track of subsidence—and determine how fast seas are truly rising. The millions of people living in the world's deltas need to know the answer, Grall says. "That's a legacy we should work on."