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Bruce E. Johansen: A day at the beach in the Pliocene

Published 12:30 pm, Monday, April 8, 2013
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Scientists are looking for shells on beaches along the East Coast, from near Washington, D.C. to Orlando, Fla. These are not ordinary shells on today's beaches, but the surf line of the Pliocene, about 3 million years ago.

The shells are fossilized, and they washed up the last time the carbon-dioxide level in the atmosphere was 400 parts per million, the level we will be breathing in a few years at present rates of increase.

The project, funded by the National Science Foundation, is called "Pliomax."

The shoreline during the Pliocene between Washington, D.C. and Orlando roughly followed today's "fall line," where most rivers become too narrow and rocky to navigate in most commercial ships, between 60 and 100 miles inland. The question on the scientists' minds that has become especially salient following Hurricane Sandy, which pushed the New Jersey coast back about 35 feet, is this: How high will the ocean will get after the temperature rises the 4 to 5 degrees F. anticipated by the end of this century by climate modelers?

Today's carbon-dioxide level does not instantly melt the glaciers that raise sea level. First, thermal inertia requires about a century and a half, give or take, to warm seawater.

After that, another century or two will be necessary to melt the ice at these temperatures at Pliocene levels, if the carbon-dioxide level does not rise above the 400 p.p.m. of that period.

By that time, today's greenhouse gas levels (which may be higher in 300 years, speeding this process) will have melted much of Greenland's ice, as well as the West Antarctic Ice Sheet.

Portions of the larger East Antarctic Ice Sheet will be eroding. Mountain glaciers and the Arctic Ocean's ice cap will exist only in history books and dusty copies of the National Geographic.

NASA satellites have detected a widening rift in the Antarctica's Pine Island glacier. This glacier acts as a plug, like a cork in a bottle, between the ocean and the West Antarctic Ice Sheet.

Should this ice sheet melt into the sea, the world ocean would rise 16 to 18 feet. This "cork" is now breaking up. Greenland's ice is also melting at an accelerating rate.

Not if, but when

Given today's CO2 levels, major sea-level rise is not a question of if, but when. Three hundred years may seem like a long time to you and me, but it is a blink of the eye in geologic time, much less than the time period required by the natural causes that raised the ocean to the levels that washed up shells near present-day Washington, D.C.

Unless the levels of greenhouse gases (mainly CO2 and methane) stop rising, and then decline to no higher than 350 p.m., this script has already been written. Every pump of the gas pedal today is making it more likely.

We do not know the exact timetable, because land itself also slowly rises and falls. We do know the seas will rise, and that storms will cause short-term inundation above the average.

We also know that most of the land that comprises the Eastern Seaboard is subsiding -- slowly sinking.

The height at which some of the fossilized shells have been found indicate that the level of sea-level rise is not a question with a single, linear, answer.

In South Africa, some shells have been found 64 feet above modern shorelines. The most commonly cited figure for Pliocene sea level is about 80 feet above today.

"I wish I could take people that question the significance of sea level rise out in the field with me," Maureen E. Raymo, who works at Lamont-Doherty Earth Observatory, a unit of Columbia University just outside New York City, told the New York Times. "Because you just walk them up 30 or 40 feet in elevation above today's sea level and show them a fossil beach, with shells the size of a fist eroding out, and they can look at it with their own eyes and say, `Wow, you didn't just make that up.' "

Faster than nature

"Absolutely, unequivocally, nature has changed before," Richard B. Alley, a leading climate scientist at Pennsylvania State University told the Times. "But it looks like we're going to do something bigger and faster than nature ever has."

"I can merely tell you that every time in recent earth history where we've had these kinds of temperatures for any protracted period of time, two polar ice sheets have catastrophically collapsed," said Jerry X. Mitrovica, an earth physicist at Harvard University.

"At every point, as our knowledge increases," Dr. Raymo told the Times, "we've always discovered that the climate system is more sensitive than we thought it could be, not less."

Bruce E. Johansen is a professor in the Communication and Native American Studies department at the University of Nebraska at Omaha.