Scientists recreate the past to predict rising sea level

By Hannah Lazo 

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Scientists have found that melting sea ice is contributing to sea level increases.  Photo courtesy of NASA.

WASHINGTON, D.C.–What can we learn from an ancient organism preserved in the coastal salt marshes of North Carolina? Plenty, according to Benjamin Horton, a researcher at Rutgers University who’s looking at the distant past to make sense of the current increases in global sea levels.

Horton, along with Jerry Mitrovica of Harvard University and Eric Rignot of University of California-Irvine, presented research on the relationship between rising sea level and melting ice caps on Friday, Feb. 12, 2016, at the annual conference of the American Academy for the Advancement of Science. All three scientists agreed that global sea level is rising faster than it has at any point during the last two millennia.

Horton and his coauthors took a historical look at sea levels in their paper “Climate related sea-level changes over the past two millennia” published in the Proceedings of the National Academy of Sciences. They found that sea level was stable from at least BC 100 until AD 950. Then the sea level increased for four centuries at a rate of 2.36 inches per century. This was followed by a period of stable, or slightly falling sea level that lasted until the late 19th century. Since then sea level has risen at an average rate of about 8.5 inches per century.

“When you look at the past sea level changes,” Horton said, “It gives context to the current changes we are experiencing.”

Those changes were a topic of discussion at the 2015 Paris Climate Conference where world leaders agreed to take measures to prevent the temperature from rising more than 1.5 degrees Celsius. Rignot said that even an increase of 1.5 degrees would melt ice and said millions could be impacted as sea levels continue to rise.

It’s no easy business to reconstruct ancient sea levels, but it is not impossible. Horton’s team collected dozens of core samples from salt marshes in Sand Point and Tump Point, North Carolina. The team then spent 2 years looking for the presence of various strains of foraminifera, single-celled organisms with porous shells, in more than 46,000 microscope slides in order to put together a record of where the fossils are deposited. The locations of these deposits are inextricably tied to the elevation of the coastal salt marshes where forams lived during the last 2,100 years.

“Since forams can only survive under specific conditions, it makes them a good indicator of where these salt marshes were,” said Horton.

Horton said the data reveals four phases of persistent sea-level change after correcting for glacial isostatic adjustment (GIA). GIA is the ongoing movement of the land once strained by ice-age glaciers. Once the ice melts, Earth’s crust slowly rises back up, the way a mattress does after someone lying on it gets up.

Mitrovica said the icecaps are so massive that they exert a powerful gravitational pull on the sea close by. The icecaps actually pull the ocean towards them. As they melt and lose mass, their gravitational pull is diminished and the water recedes, but not uniformly.

“A lot of people like to think if Greenland melts it would be like filling up a bathtub, but that’s just not true,” Mitrovica said. “All that displaced water has to go somewhere.” 

The unequal distribution of the water means that some locations will be impacted more severely than others as the melting continues, said Mitrovica.

Both Mitrovica and Horton said scientists look to the past so they can ask how unusual the last 20 years have been. Horton said the data show that we are living in very, very unusual times.

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