Finally they measured oxygen atoms in plankton fossils in Labrador Bay, which is adjacent to the site of the historic ice sheet.
The atoms—oxygen isotopes—indicate the contribution of fresh water from the melting glacier, Carlson said, and therefore independently confirm the land-fossil measurements.
The authors said there were two major Laurentide melting pulses—9,000 years ago and 7,500 years ago—that added a total of almost 40 feet (12 meters) of depth to the world's oceans.
They say the entire Laurentide ice sheet was gone by about 6,500 years ago.
Upping the Ante
Covering about 650,000 square miles (1.7 million square kilometers), the Greenland ice sheet is Earth's second largest, after the Antarctic ice sheet. It's about the size that the Laurentide ice sheet was 8,000 years ago, after its first major melting pulse.
The Laurentide ice sheet was exposed to direct solar heat because Earth's tilt had it angling closer to the sun, Carlson said. The concentration of heat caused by greenhouse gases is having a similar effect on today's Greenland ice sheet.
The new findings also suggest the Greenland ice sheet will melt faster than the United Nations Intergovernmental Panel on Climate Change (IPCC) predicted last year.
The IPCC estimate was based on melt rates measured between 1992 and 2003 in Greenland and Antarctica and on a current rate of sea level rise of about .12 inches (3 millimeters) a year.
The IPCC scientists predicted the Greenland ice sheet would cause the the seas to rise up to four inches (ten centimeters) within a hundred years.
"It's basically showing the IPCC was right when they were predicting sea level rise," Carlson said of the new results. "But we're showing that they probably were conservative."
Cities Under Water?
Mark Siddall, a geologist at the University of Bristol in the United Kingdom, was not a co-author on the new study but wrote an editorial that accompanies it. He also helped write the IPCC's 2007 report.
"At the very least," he said, "this work suggests that the sea level rise in the next century will be at the upper end of the IPCC estimate and maybe some tens of centimeters more."
Any rise at all could threaten U.S. cities that are built, at least in part, at sea level. These cities include New York, Boston, Los Angeles, Philadelphia, New Orleans (read more in National Geographic magazine), and San Francisco, among others.
(Related story: "Greenland Melt May Swamp LA, Other Cities, Study Says" [April 8, 2004])
Large swaths of entire countries, including Bangladesh and the Maldives, would also be vulnerable to flooding.
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