North Carolina shore

In the summer of 2002, a few years after receiving a PhD in geography in England, Ben Horton flew across the Atlantic to the U.S. and joined a climate research team from East Carolina University. They scrambled around the North Carolina’s Outer Banks, a 200-mile-long string of narrow islands and peninsulas vulnerable to hurricanes and tropical storms barreling up from the Caribbean.

The team was documenting sea level on the North Carolina coast in the distant past, knowledge that will help governments, insurers, and others in the private sector plan for the future.

Horton examined long, skinny cylinders of muck he and his colleagues drilled in salt marshes.

“Sometimes I felt as though I was the first human to be walking through some of marshes we worked at,” he wrote recently in an e-mail recalling that trip. The beauty of the region convinced him to immigrate to the U.S., where he’s now a professor at Rutgers University’s Institute of Marine and Coastal Studies.

Re-enforcing previous studies on sea-level rise

Horton’s research on the North Carolina coast has joined a vast array of other studies that work to prove how global sea level is rising – and is doing so at an ever-faster pace: A new paper that uses Horton’s salt marsh research concludes that sea level is rising more quickly than ever in the last three millennia. The research underscores that the world’s coastal cities likely will face punishing floods, requiring huge investments in protection, and in some cases forcing large numbers of people to abandon some areas for higher ground.

We’ve all heard that global warming is melting mountain glaciers and ice sheets. It’s also expanding ocean water. But figuring out how fast sea level is rising and how today’s rate compares with past rates is difficult. NASA first mounted radar range finders on satellites in 1992, measuring sea level to within a fraction of an inch. Agency researchers found that the ocean’s surface is rising by about a tenth of an inch annually. But how does that compare with rises in the 20th century? Or in the centuries before greenhouse gasses began warming the planet?

Head in the sand
Accelerating sea-level rise obviates head-in-sand approach.

Tide gauges – simple floats that rise and fall across oceans worldwide – have been monitored at ports around the world. A few tide-gauge diaries actually date back to before 1800. By measuring natural processes that provide insights into sea surface elevation, scientists have found ways to determine sea level even before industrialization.

Techniques for measuring past sea levels

Before moving to the U.S., Horton had perfected a method of studying past sea level. Marine organisms tolerate a limited range of physical and chemical conditions. Horton had learned that shells of foraminifera, or forams, a protozoan, remain undecayed in sediment long after the organism had died. There are thousands of foram species, mostly smaller than the period at the end of this sentence. Many have distinctive spikes, whorls, or folds, and can be sorted.

Tides uncover marshes twice daily. But the duration spent submerged varies within a wetland. A marsh’s mucky floor slopes gradually from dry land to open water. Areas nearest the shore are uncovered longest. Horton realized that forams vary in predilection for drier or wetter zones.

Sorting thousands of shells, analyzing progressively deeper mud lead to new sea-level rise insights. Click To Tweet

In his PhD dissertation, Horton pioneered a technique for calculating the elevation of sea level above a muddy sampling site using ratios of the abundance of several of the foram species. Cross sections of marsh sediment, deposited in earlier times, entomb chronicles of past sea level. He examined progressively deeper mud.

For more than a decade after his first visit to North Carolina, Horton and, beginning a few years later, his colleague Andrew Kemp sliced marsh mud cylinders they had collected, creating a chronological series with carbon 14 and other dating methods. Kemp sorted through more than 45,000 shells and compiled a database of the relative abundance of foram species from about 50 BC to the present.

Looking back 2,100 years

In 2011, with several other colleagues, the researchers published a continuous record of sea level over the past 2,100 years. They showed that for the past 1,000 years, sea level along the North Carolina coast had closely tracked global temperature. It rose between 950 and 1400 AD, including what climatologists call the Medieval Warm Period. Sea level stabilized during a cooler time called the Little Ice Age. Between 1865 and 1892, sea level off North Carolina abruptly began to climb again, several times more quickly than during any time they had studied.

Maureen Raymo, a research professor of geology at the Lamont-Doherty Earth Observatory in Palisades, N.Y., praised the research for a making “a convincing link between man-made global warming and sea level rise.”

Horton’s North Carolina results didn’t accurately show sea-level variance on a global scale. Local sea level is useful, but it’s a “cloudy lens,” says Jerry Mitrovica, a geophysicist at Harvard University. Sea level varies by many feet from location to location. Winds and currents pile up water near shores at one point and withdraw it somewhere else. Large surface features such as polar ice sheets and mountain ranges gravitationally attract water, raising sea levels for hundreds of miles around.

Coastlines themselves continually move up and down relative to the sea, at rates that vary from place to place, and not always steadily. For instance, during the last ice age massive glaciers depressed parts of North America by hundreds of feet. To compensate, land south of the ice sheets, such as present-day North Carolina, bounced up, like the bulging edges of a waterbed after someone lies down. Land in New England and farther north is still rebounding from the ice that retreated 12,000 years ago. And North Carolina on the whole is subsiding about half an inch every decade.

Horton and his colleagues could account for some of these factors but to chart the trajectory of global sea level with a less clouded lens, he needed sea level measurements at many locations. He collected samples at other sites along the U.S. East Coast and recruited researchers gathering similar records around the world. It was then that Robert Kopp, a sea level specialist in Rutgers’ Earth & Planetary Science department, joined the team.

Mitrovica and other scientists helped to analyze the data. They published the results of their research in February in the Proceedings of the National Academy of Sciences. Their paper confirms that global sea level began rising rapidly in the late 19th century.

Projection of 1.5 to four feet sea-level rise by 2100

“It is extremely likely,” they write in their paper, that in the 20th century sea level is rising faster than at any time since 800 AD. Their research paper reinforces earlier studies suggesting that unless global warming is slowed, sea level will increase by between 1.5 and four feet by 2100, with devastating consequences in many low-lying areas. If the ice sheets at the poles liquefy ever more rapidly into surrounding waters, as some scientists predict, sea level could climb still higher.

This summer Horton plans to vacation on the Outer Banks. By coincidence, at about the same time, the state is slated to issue new coastal protection rules that he hopes his findings will influence.

In 2012, the Republican-controlled North Carolina legislature nearly enacted a law requiring its Coastal Resources Commission to base policies on the counter-scientific assumption that the rate of sea-level rise would not accelerate. A coalition of realtors and developers, fearing that science-based assumptions about future sea level would reduce the value of coastal property, had supported that legislative effort. But at the last minute, the legislature allowed the commission to consider higher rates of sea level, if justified – after July 1, 2016.

Horton’s and his colleagues’ research appears to have produced that justification.

Will the coastal commission now accept that sea level is found to be rising at an accelerating rate? “I would hope so,” Horton says. “If you put your heads in the sands of the Outer Banks you’re going to destroy them.”

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