Detective story behind Rutgers report on rising sea levels

Tide gauges show that average sea levels have been steadily rising since the late 1800s, a worrisome trend that scientists blame on emissions of heat-trapping greenhouse gases.

But what about the centuries before then, when those gauges were mostly nonexistent?

Part of the answer, a team of researchers reported this week, lies in the salt marshes of South Jersey.

In a study published in Proceedings of the National Academy of Sciences, authors calculated sea levels over thousands of years by analyzing cores of sediment from the salt marshes and 23 other geological sites around the world.

The results were sobering.

The research team found it "extremely likely" that average sea levels rose faster in the 20th century than in any previous century since at least 800 B.C., at a rate of about 1.4 millimeters per year.

That may not sound like much, but it adds up - 5.5 inches during the 20th century, due to thermal expansion of seawater and the addition of liquid water from melting glaciers.

There is more to come, the authors added.

Even with strong cutbacks in burning fossil fuels such as gas and coal, sea levels could still rise anywhere from nine inches to two feet during this century, the scientists projected. With few or no reductions, average sea levels could rise by anywhere from 1.7 feet to more than four feet, they said.

"It clearly illustrates that we're living in an unusual time," said Rutgers University professor Benjamin P. Horton, one of the authors. "The paper also shows we're the cause of this."

Previous studies have calculated past sea-level rises for various locales, but this is the first to weave together a global picture with such rich sources of data spanning the globe, Horton said.

Lead author Robert E. Kopp, also of Rutgers, helped to develop the statistical approach for synthesizing the various geological records.

But first, someone had to get them - an often tedious process that took well more than a decade.

The scientists drilled for sediment cores along the Atlantic coast of North America, including at the Edwin B. Forsythe National Wildlife Refuge, north of Atlantic City. They analyzed other sediment data from such diverse sources as corals in the South Pacific and mangroves in Seychelles, in the Indian Ocean.

The sea level associated with each sample of sediment was calculated by counting the numbers of various one-celled organisms that lay within. Certain species of these creatures, called foraminifera, thrive at different levels of salinity, enabling scientists to discern the depth of the waters when the sediment was deposited.

The age of the sediments was then calculated by carbon-dating the remains of plants found at the same depths.

As a reality check, the scientists then looked to see if measurements from the modern tidal gauges matched data taken from sediments of similarly recent vintage. They did.

One source of data had a human origin: fish tanks made by the ancient Romans. The stone tanks were built to keep fish alive until it was time to prepare them. They were set just high enough in the water that fish could not swim out, but were low enough that the tanks received regular infusions of fresh tidal nutrients.

By measuring the current depth of the waters surrounding these tanks, scientists determined how much the seas had risen in the last 2,000 years, said Horton, a professor in the Rutgers department of marine and coastal science.

A key impact of rising seas is flooding. In a separate report posted this week on the educational site climatecentral.org, Kopp and others calculated that human-induced sea-level rise was responsible for more than 80 percent of the increase in "nuisance flood days" between two 30-year periods: 1955 to 1984 and 1985 to 2014.

Sea levels rise by varying amounts in different parts of the world due to factors other than climate change. For example, seas are rising faster in coastal New Jersey because the underlying land is subsiding.

So the scientists used statistics to extract a global average sea level over the centuries, said Kopp, an associate professor in the department of earth and planetary sciences.

"What we did was synthesize those local records to come up with a truly global curve," Kopp said.

One limitation: The scientists did not have high-resolution data from seas bordering Asia and most of South America. They hope that others in the field will develop new ways to measure the depths of those oceans in eons past.

But for now, the research provides the widest-yet picture of the rising seas, Horton said.

"It's really like a geological detective story," he said.

tavril@phillynews.com

215-854-2430@TomAvril1