Greenland's meltdown: Could it raise sea level faster than expected?
In recent years, scientists have seen ominous signs of melting on Greenland's ice cap. According to a 2006 satellite study, the ice cap is releasing up to 240 cubic kilometers of water into the ocean every year. Greenland is a primary cause for the rising worries about how a rising sea will affect coastlines around the world.
The consensus predictions call for a total sea-level rise of 40 to 50 centimeters over the next century, but a new study of past melting in a vast ice sheet that covered most of Canada suggests the sea could rise by 50 to 80 centimeters, drowning a much larger coastal region.
New Orleans, which just dodged a bullet called Hurricane Gustav, would be in even more danger if the sea rises faster than expected.
A complete meltdown of Greenland could raise the sea by a catastrophic seven meters, wreaking havoc on islands and coastlines from New York City to Shanghai. One meter of sea-level rise could displace 145 million people, mainly in Asia (see "A tale...", below).
Anders Carlson, an assistant professor of geology and geophysics at the University of Wisconsin-Madison, was the primary author of a new study of the giant Laurentide ice sheet. Carlson's study found that during the global warming that ended the last ice age, a torrent of meltwater from the Laurentide raised sea level by up to 1.3 centimeters a year, more than four times as fast as today's total annual sea rise of 3 millimeters.
Ice retreats, but ocean rises
The sobering estimate was based on a new view of how fast the Laurentide ice retreated.To find when particular locations at the edges of the ice were re-exposed to the sun after long burial under the ice, researchers have used:
•carbon-dating of organic matter left by plants and animals that grew as the glacier departed; and
•studies of how long rocks around the edges had been exposed to cosmic rays, which are blocked by thick ice.
Carlson and colleagues used the rate of retreat to calculate the volume of the meltwater and then the effect on sea level. Using this approach, the researchers found that the melting Laurentide caused sea level to rise 0.7 centimeters per year about 7,600 years ago, and 1.3 centimeters (half an inch) per year about 9,000 years ago.
The climate during these meltdowns differed from today's in two key respects. Earth was closer to the sun, and thus received more radiation, but more heat could escape because the atmosphere contained less greenhouse gas. When the researchers put the two climates into a computer model, these differences cancelled each other out, Carlson adds. "In terms of energy transferred into the ice sheet, the net effects were about the same. The climate's effects on the ice sheets 9,000 years in the past, versus 100 years in the future, are similar."
The Intergovernmental Panel on Climate Change (IPCC) has published a consensus prediction that the melting of the Greenland Ice Sheet alone will raise sea level one to two millimeters annually by 2100, but Carlson says this is an underestimate. "Our data suggest that there could easily be up to 1 centimeter per year from the Greenland ice sheet alone."
Although scientists are uncertain about the larger question of how Antarctica's ice will affect sea level (ice that melts around the edges may be replaced by greater snow accumulation inland), Greenland alone may add 20 or 30 centimeters to the 40 to 50 centimeter total rise expected by the IPCC by year 2100.
A rising sea was one of the biggest bugaboos during the early global warming studies, in the 1980s, but the fretting faded until the early 2000s, when mammoth slabs of ice began leaving Antarctica , and rapid melting was observed in Greenland.
If the ocean rises half a meter in 90 years, people will have plenty of adjusting to do in places like Bangladesh and low-lying Pacific islands -- but the real rise may be even higher than that, Carlson says. "Sea level rise could be much underestimated. The geological evidence suggests that [Greenland] could cause a much greater sea level rise than currently predicted."
- David J. Tenenbaum
• Rapid early Holocene deglaciation of the Laurentide ice sheet, Anders Carlson et al, Nature Geoscience, 31 August 2008.
• A tale of two ice sheets, Mark Siddall et al, Nature Geoscience, 31 August 2008.
† Background photo of Greenland Ice Sheet fromNASA. Credit: Joughin/UW Polar Science Center