"The inspiration came from the IPCC report stating that we don't know how fast ice sheets will retreat and raise sea level in the future," says Anders Carlson, a paleoclimatologist at the University of Wisconsin–Madison and lead author of that study. "Well, we do have past records of ice sheet retreat under climates warmer than present so we decided to determine what those rates of retreat were."
At the same time, Greenland's glaciers were also smaller, though they persisted because of snowfall generated from the ocean. "More snow can partially offset the greater melting, helping the Greenland Ice Sheet to survive this interval," Carlson says. "But note it did retreat and this is what we are predicting for the future."
Based on this historical record and the fact that the Laurentide melted away under summertime temperatures similar to those expected in Greenland by the end of this century, Carlson and his colleagues forecast glacial melting that contributes somewhere between 2.8 inches (seven centimeters) and 5.1 inches (13 centimeters) of sea level rise per year, or as much as a 4.3-foot (1.3-meter) increase by 2100. Current rates are just 0.1 inch (3 millimeter) per year—and Greenland is contributing roughly 0.02 inches (0.4 millimeters) of that rise annually.
Pfeffer notes that the Laurentide and other ice sheets that disappeared in the past had an easier path to the sea than the glaciers in Greenland or Antarctica. "The analogies between those past climates and today aren't strong enough to say anything specific about the rate of sea level rise in the next century," he says.
The bottom line: sea levels will rise much more than predicted by the IPCC, based on both present understanding of current glacial melt as well as evidence from the geologic record. "The IPCC noted that their estimates should be seen as minimum estimates," Carlson notes, "and they are right."