Sea levels began rising precipitously in the late 19th century and have since tripled the rate of climb seen at any time in at least two millennia, a detailed analysis of North Carolina marsh sediments shows.
“This clearly shows the recent trend is not part of a natural cycle,” says Ken Miller of Rutgers University in Piscataway, New Jersey, who was not associated with the analysis.
Andrew Kemp of the University of Pennsylvania and his colleagues spent five years plumbing salt marsh sediments that had remained largely undisturbed for millennia. Kemp, now at Yale, and his team drilled cores at two sites, unearthing the microscopic remains of single-celled shelled organisms known as foraminifera.
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Foraminifera vary in their salt tolerance. So as sea level changed over millennia, so did the mix of species living at any given site, explains University of Pennsylvania coauthor Benjamin Horton. Knowing the modern-day distribution of foraminifera at various water depths along the modern-day coast, the researchers could infer past sea levels at the two core sites from the abundance of different species in successive sediment layers. Radioisotope dating showed that the sediments recorded 2,100 years of sea level history, the researchers report online June 20 in the Proceedings of the National Academy of Sciences.
“We know what sea level has done, in a broad sense, going back 20,000 years,” Miller says. But detailed records of what’s happened over the past 2,000 years have been spotty, he says.
The cores show that sea level at the North Carolina sites was largely unchanging from 100 B.C. until A.D. 950. Then sea level underwent a four-century rise averaging 0.6 millimeters per year. Sea level didn’t rise again until after 1865. Since then, it’s been climbing an average of 2.1 millimeters annually. And at least for the last 80 years, Horton says, “the fit with North Carolina tide gauge data is one to one: It’s perfect.”
The results validate the use of general equations relating past temperatures and sea level changes to predict sea level rise as the climate continues to warm, says Aslak Grinsted of the University of Copenhagen’s Centre for Ice and Climate.
“What’s great about this new record is that it’s really high resolution and continuous,” Grinsted says, “and quite consistent with records all around the world.”