Past Antarctic Warming Raised Global Sea Levels

Newswise — In two papers in the March 19 issue of Nature, climate scientists at the University of Massachusetts Amherst and elsewhere report new evidence that a slight rise in past ocean temperatures has affected the stability of Antarctic ice, which traps one of the largest supplies of fresh water on Earth. As the ice melted, sea levels rose.

Climate modeler Rob DeConto, a UMass Amherst geoscientist and a co-author on both studies, says together they present a new reality for climate scientists to digest: "An ice sheet model and a new geological record based on a sediment core converge on the same finding, that the Western Antarctic Ice Sheet (WAIS), the smaller of the two ice sheets there, is highly variable and capable of rapid retreat to a point near complete collapse, within a few thousand years."

A total of 56 researchers took part in the multi-disciplinary, international studies described in the two companion articles, one describing findings based on new sediment core, the other based on modeling. The work refines previous findings about the relationship between atmospheric carbon dioxide levels, atmospheric and oceanic temperatures, sea-level rise and natural cycles in Earth's orbit around the Sun.

One project, by DeConto and David Pollard of Pennsylvania State University, combined ice sheet and ice shelf models to simulate Antarctic ice sheet variations over the past five million years. They tracked the border between land and floating ice, known as the grounding line. Results show the WAIS changed relatively rapidly in geologic terms (only a few thousands years) between full ice cover, intermediate and collapsed states. Further, each time the WAIS collapsed, some margins of the East Antarctic Ice Sheet melted as well. The combined effect was a global sea-level rise of almost 21 feet above present levels.

The Western ice sheet covers the polar continent on the Pacific side of the Transantarctic Mountains and sits mostly on rock that is below sea level. DeConto says his and Pollard's new modeling "suggests that the ice sheet is most sensitive to ocean temperature versus other factors like surface air temperatures, precipitation, or sea level."

If it were to completely collapse in the future, resulting higher sea levels would "clearly have devastating effects, although a complete collapse event (at least in our model) takes a few thousand years to occur," DeConto notes. Researchers do worry that the WAIS is unstable at present and even a small temperature rise might tip it toward crumbling and breakup.

But despite the new information from ANDRILL and the new model, researchers' ability to project future collapse is still limited because of uncertainties about future greenhouse gas levels, ensuing warming of the ocean around Antarctica and the link between ocean temperatures and melting under the ice shelves.

The other project, led by Tim Naish of the University of Wellington, New Zealand and Ross Powell at Northern Illinois University, used a 1,280-meter sedimentary rock core from the sea floor under Antarctica's Ross Ice Shelf. It was collected during the first project of the ANDRILL (ANtarctic geological DRILLing) research program, a multinational collaboration between Germany, Italy, New Zealand and the United States. The sediment core data allowed the scientists to look back in time millions of years to study past Antarctic climates, ice sheets and ocean conditions.

Naish and colleagues investigated the relationship between Earth's orbit and WAIS collapse. Consistent with DeConto and Pollard's modeling, they found evidence that the WAIS collapsed more frequently during the early Pliocene (about 5 million to 3 million years ago) in a pattern consistent with higher carbon dioxide levels at that time, which were in fact only a bit higher than those seen today. Their data also suggest the 40,000-year cycles in the tilt of Earth's rotational axis played a role.

The National Science Foundation, which manages the U.S. Antarctic Program, provided about $20 million to support ANDRILL as a focus during this International Polar Year.