Seas Will Rise Much Faster Than Thought, Study Says

Anne Minard
for National Geographic News

September 3, 2008

Melting Greenland ice could cause oceans to rise by more than a foot (30 centimeters) over the next hundred years.

The resulting sea level rise, spurred by global warming, may also happen three times faster than previously predicted.

When all other sources of melting ice are also factored in—such as the Antarctic ice sheet and smaller glaciers—the sea level has been predicted to increase by several more feet by 2100, according to previous studies.

(See a global warming interactive.)

The new estimates are based on disappearance rates of the ancient Laurentide ice sheet that long ago covered North America and melted between 9,000 and 6,000 years ago.

"We have never seen an ice sheet retreat significantly or even disappear before, yet this may happen for the Greenland ice sheet in the coming centuries to millennia," lead study author Anders Carlson, of the University of Wisconsin, said in a press release.

Carlson said his team's research on the Laurentide ice sheet "gives us a window into how fast these large blocks of ice can melt and raise sea level."

The study appears this week in the journal Nature Geoscience.

Massive Disappearance

At its peak, the Laurentide ice sheet was more than 5,000,000 square miles (13,000,000 square kilometers) across, with a thickness of up to 10,000 feet (3,000 meters) in some places, according to previous estimates.

To determine the demise of the massive sheet, Carlson and his team estimated the ages of boulders left in its wake based on how long they had been exposed to cosmic rays.

The geologists also obtained radiocarbon dates of trees and other organic materials that couldn't have existed until after the ice was gone.

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.