Projections of Greenland Ice Melt May be Underestimated

2 September 2008

Based on their analysis of the disappearance of the Laurentide ice sheet, a team of researchers led by University of Wisconsin-Madison geologist Anders Carlson reports that sea level rise from greenhouse-induced warming of the Greenland ice sheet could be double or triple current estimates over the next century. A paper on their work was published online 31 August in the journal Nature Geoscience.

We’re not talking about something catastrophic, but we could see a much bigger response in terms of sea level from the Greenland ice sheet over the next 100 years than what is currently predicted.

—Anders Carlson, UW-Madison professor of geology and geophysics

Carlson worked with an international team of researchers, including Allegra LeGrande from the NASA Center for Climate Systems at Columbia University, and colleagues at the Woods Hole Oceanographic Institution, the California Institute of Technology, University of British Columbia and University of New Hampshire.

Scientists have yet to agree on how much melting of the Greenland ice sheet—a terrestrial ice mass encompassing 1.7 million square kilometers—will contribute to changes in sea level.

Carlson and his group were able to draw on the lessons of the disappearance of the Laurentide ice sheet, the last great ice mass to cover much of the northern hemisphere. The Laurentide ice sheet, which encompassed large parts of what are now Canada and the United States, began to melt about 10,000 years ago in response to increased solar radiation in the northern hemisphere due to a cyclic change in the orientation of the Earth’s axis. It experienced two rapid pulses of melting—one 9,000 years ago and another 7,600 years ago.

The researchers estimate that around the time of the first melting phase, the retreating ice sheet led to about approximately 7 meters of sea level rise at about 1.3 cm a year. The second phase accounts for around 5 meters of sea level rise at about 1.0 cm a year. These rates are comparable to evidence for global sea level rise for this interval derived from coral records.

Those pulses of melting, according to the new study, occurred when summer air temperatures were similar to what are predicted for Greenland by the end of this century, a finding the suggests estimates of global sea level rise due to a warming world climate may be seriously underestimated.

The most recent estimates of sea level rise due to melting of the Greenland ice sheet by the Intergovernmental Panel on Climate Change (IPCC) suggest a maximum sea level rise during the next 100 years of about 1 to 4 inches. That estimate, Carlson and his colleagues note, is based on limited data, mostly from the last decade, and contrasts sharply with results from computer models of future climate, casting doubt on current estimates of change in sea level due to melting ice sheets.

According to the new study, rising sea levels up to a third of an inch per year or 1 to 2 feet over the course of a century are possible.

Even slight rises in global sea level are problematic as a significant percentage of the world’s human population—hundreds of millions of people—lives in areas that can be affected by rising seas.

For planning purposes, we should see the IPCC projections as conservative. We think this is a very low estimate of what the Greenland ice sheet will contribute to sea level.

—Anders Carlson

The authors of the new Nature Geoscience report were able to document the retreat of the Laurentide ice sheet and its contributions to changes in sea level by measuring how long rocks once covered by ice have been exposed to cosmic radiation, estimates of ice retreat based on radiocarbon dates from organic material as well as changes in ocean salinity.

In an accompanying News and Views letter in Nature Geoscience, Mark Siddall, a researcher at Columbia’s Lamont-Doherty Earth Observatory, writes “Carlson and colleagues… show that the decay of the Laurentide ice sheet in the early Holocene was extremely fast during the periods they consider … Their work suggests that, in principle, future melt rates on the order of one metre per century are certainly not out of the question.”

In addition to Carlson and LeGrande, co-authors of the study, which was funded primarily by the National Science Foundation, are Gavin A. Schmidt of Columbia University, Delia W. Oppo of the Woods Hole Oceanographic Institution, Rosemarie E. Came of the California Institute of Technology, Faron S. Anslow of the University of British Columbia, Joseph M. Licciardi of the University of New Hampshire and Elizabeth A. Obbink of UW-Madison.

Resources

• Anders E. Carlson, Allegra N. LeGrande, Delia W. Oppo, Rosemarie E. Came, Gavin A. Schmidt, Faron S. Anslow, Joseph M. Licciardi and  Elizabeth A. Obbink (2008) Raphid early Holocene deglaciation of the Laurentide ice sheet, Nature Geoscience, doi: 10.1038/ngeo285