Scientist tries to predict rise in ocean levels

Scientist tries to predict rise in ocean levels
In 2010, Maureen Raymo traveled to Western Australia's Roe Plain to survey the elevation of shoreline features and sediments as old as 3 million years. Credit: Michael O'Leary

Columbia climatologist Maureen Raymo is trying to predict the planet's future by looking to its past.

About 3 million years ago, prior to the last Ice Age, carbon dioxide (CO2) levels were roughly the same level they are now – about 400 parts per million. But they arrived there far more gradually.

Raymo, a marine geologist and paleoclimatologist at Columbia's Lamont Doherty Earth Observatory, is studying how much those levels caused the oceans to rise. From that, scientists can figure out how much of Earth's land mass may be inundated as the climate warms and polar ice caps melt.

"The big question is what impact global warming will have on the East Antarctic ice sheet," says Raymo, who earned her Ph.D. in geology from Columbia in 1989 before returning in 2011 to head Lamont's Core Repository, an archive of sediment, rock and coral, as well as digital data related to the material, used for climate research.

Raymo is one of a sizable group of Columbia researchers examining 's impact on melting and rising sea levels.

"If we took the amount of ice on East Antarctica and melted it, sea level globally would rise more than 50 meters (150 feet). You'd drown most of the major coastal cities of the world." (Morningside Heights is about 37 meters above sea level.)

Scientist tries to predict rise in ocean levels
Ancient rocks can reveal where the seas once stood. Credit: Dan Grossman

CO2 is linked to melting because it absorbs heat energy from Earth's surface and prevents it from escaping into space; the amount of CO2 in the atmosphere has spiked dramatically in recent decades due to deforestation and fossil fuel consumption.

Raymo is not the first person to look to the period known as the Pliocene for answers about what lies ahead. For many years, most experts believed that sea levels rose about 25 meters the last time CO2 levels reached current levels. But Raymo was never convinced.

Geologists came up with their estimates by examining the elevation above present sea level of 3 million-year-old fossils and used them to infer ancient sea levels. Australia's West Coast, for instance, is mantled by fossilized coral reefs, which rise progressively higher as one moves inland. Since reefs must be covered with water for coral to grow, scientists assumed that if you could determine the age of the fossilized reefs, their height relative to the current sea level would be an accurate proxy for the sea level at the time that coral stopped growing.

But Raymo still had questions. If sea levels rose, wouldn't the weight of the water alone push the down? And if the land has rebounded as polar caps expanded, wouldn't that cause ancient sea levels to look higher than they were?

Raymo took her suspicions to a specialist who studied such phenomena: Jerry Mitrovica, an expert in mantle geophysics at Harvard University. That led to a series of papers that forced Raymo's entire field to reevaluate traditional assumptions.

After highlighting the shifting land heights caused by water weight and explaining how researchers would have correct for it, the two researchers began work on another variable climatologists had failed to consider – dynamic topography.

Deep within the Earth, slow convective motions of our planet's viscous interior can, over time, change the buoyancy of different parts of Earth's mantle – the layer between Earth's crust and core. Hotter, more buoyant areas, often hundreds to thousands of miles across, can push up the surface of the planet from below causing areas of high elevation spanning similar distances.

In 2011, Raymo and Mitrovica won a $4.2 million grant from the National Science Foundation for a study to correct for such factors.

Raymo and her collaborators have been scouring the globe for what are known as paleo-shorelines, exposed rock in structures such as cliffs and ridges that allow them to measure the relative elevations of shorelines that formed during a small window of time before the last Ice Age began.

It's too early to determine how much sea levels rose during the climatological period 3 million years ago that is very similar to ours. But the researchers have gathered enough data to confirm Raymo's initial suspicion – that 25 meters, the typical number cited for expected rise once the planet adjusts to the current level of CO2 in the atmosphere, is far from certain. Raymo believes that the number could be as low as 12 meters or as high as 35 meters. And as a global database of Pliocene shorelines grows, the uncertainty in these numbers will inevitably shrink.

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