Meltdown: Satellites Show Accelerated Polar Ice Threat
By Robert Roy Britt
Senior Science Writer
posted: 07:01 am ET
17 January 2001
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Vast sheets of ice on the warming fringes of Antarctica may be on the verge of collapse and could eventually release rivers of ice that would cause sea levels worldwide to rise more rapidly than expected, according to new study of satellite images released Tuesday.
Scientists have long known that polar ice has been melting for the past 14,000 years or so, and that seas are on the rise. So far, the major contributor to rising seas is what's called thermal expansion: As oceans warm up, (as current data confirms), the water expands and sea levels actually rise.
The Environmental Protection Agency has estimated that sea levels have been rising nearly a foot (0.3 meter) per century and will rise at least that much, possibly more, over the next 100 years. The EPA and the United Nations have both said that in a worst-case scenario -- depending on how much global air temperatures increase -- seas could jump 3 feet (1 meter) by 2100.
And each vertical inch can translate into several horizontal feet of flat lowlands.
Many scientists expect that global warming will likely continue to warm the oceans for decades or centuries to come. And increasingly, experts warn that small islands, farmland and coastal cities occupied by hundreds of millions of people could be swamped. (Others dispute the extent and causes of global warming).
Either way, melting polar ice has so far not contributed significantly to the rising seas. Prevailing views hold that it could be between 100 and 1,000 years before enough polar ice melts to significantly affect sea levels.
But the latest evidence shows that floating sheets of ice, fed by major glaciers that inch inexorably to the sea, are at greater risk of breaking off during short periods of warm weather.
Satellite images of the Larsen Ice Shelf show a phenomenal retreat between December 1993 (left) and March 2000.
The researchers studied the crumbling Larsen Ice Shelf, a relatively small peninsula of ice that dropped an area of ice as big as Rhode Island into the sea between 1995 and 1998. This ice was already floating, and so had little effect on sea levels.
But in studying the Larsen shelf, researchers found that melting surface water, which can build up in just a few warm summers, seeps into cracks and forces a quicker-than-expected breakup.
"This result implies that other ice shelves are closer to the breaking point than we previously thought," says Ted Scambos of the National Snow and Ice Data Center at the University of Colorado.
"The Antarctic has been up to now a relatively minor player in sea-level rise," Scambos said in an interview. "But it is the 600-pound gorilla in a way."
Holding back the flow
Huge sheets of ice, larger than the Larsen shelf, act as dams that hold back virtual rivers of ice which extend far inland. If one of these lets go, a veritable gush of ice that had been resting on solid ground could flow rapidly into the sea.
Scambos said the new study shows that the Ross Ice Shelf, many times larger than the Larson shelf, could be at a critical juncture. If temperatures continue to rise, it could begin to break up in the next 20 to 50 years.
"It's clear that if you get rid of the ice shelf, then those things will flow much faster, explains Christina Hulbe, of NASAs Goddard Space Flight Center and a colleague of Scambos. "But it's not clear how much faster."
The study of these processes is in its infancy. And both researchers caution that much work needs to be done to determine any potential threat and how quickly it could come. It would still likely be 100 years, perhaps several hundred, before the process fed significant water into the sea.
Next page: No matter what, a potential global disaster
Looming global disaster?
But already, experts in many fields have warned that rising seas caused by global warming will gravely threaten beach areas around the world. A 1999 study by Britain's Meteorological Office concluded that rising global temperatures could force a 6-foot (1.8-meter) rise in sea levels during the next few centuries.
And while scientists debate the existence and extent of global warming, the sea is quietly gaining ground.
The Mississippi River Delta gave up about 16 square miles (41 square kilometers) of land to the Gulf of Mexico every year between 1930 and 1990. But the rate has accelerated in recent years to about 25 square miles (65 square kilometers) a year, according to a study released a year ago by University of New Orleans coastal geologist Shea Penland.
"We're living on the verge of a coastal collapse," Penland warned last January.
Some say New Orleans may be among the first major casualties. The city, which sits 8 feet (2.5 meters) below sea level at the mouth of the Mississippi, is protected by a system of manufactured levies and natural barrier islands. But many of the tiny islands are expected to disappear by 2050. Meanwhile, New Orleans is sinking, in a process called subsidence, at a rate of 3 feet (1 meter) per century.
Responding to the threat, the director of the U.S. Geological Survey, Chip Groat, issued a blunt warning in 2000: "With the projected rate of subsidence, wetland loss and sea-level rise, New Orleans will likely be on the verge of extinction by this time next century."
Ice experts say the greatest immediate danger doesn't even come from polar ice. Glaciers around the world are melting at ever-faster rates.
A 1999 study supported by the National Science Foundation found that the Columbia Glacier, near Anchorage, Alaska, was melting rapidly and had increased its forward speed from about 82 feet (25 meters) per day to around 115 feet (35 meters) per day. Researchers expect that within a couple of years the glacier might either begin breaking off icebergs at a rapid pace, or it could melt so quickly that it stops reaching the sea.
And less ice -- at the poles or anywhere -- means less sunlight is reflected and global warming is therefore accelerated. Water and earth, being darker than ice, absorb more sunlight and warm the whole planet.
Next page: A chunk of ice the size of France, at risk
Polar ice melt as a catalyst
Antarctic ice, if it breaks up more rapidly than expected, could add dramatically to the doom predicted by Groat and others.
Ice shelves are the enormous floating tongues of even larger land-dwelling glaciers. They grow and shrink, advance and retreat over decades. And now and then, large icebergs fall from their edges and crash into the sea. When air temperatures rise, more of the ice melts, setting up a catastrophic chain reaction.
Melted surface ice can form deep pools of water. The water forces its way into cracks in the ice shelf. It's there that the problem escalates.
"The weight of the water essentially forces the cracks open, so a relatively small amount of climate warming can destroy a large, centuries-old ice shelf," says Scambos, of the Snow and Ice Data Center.
Temperature data on Antarctica is scarce, but the researchers know that some areas have warmed several degrees in the past 50 years. NASA's Hulbe, who also works at the University of Maryland, used a computer model to recreate how this increased temperature caused the breakup of the Larsen ice shelf. She found that a water-filled crevasse just 15 to 50 feet (5 to 15 meters) deep could crack an ice shelf more than twice as deep as a football field is long.
"The importance of melt water implies that ice shelf stability may not be limited by the mean annual temperature, as has long been thought, but by the mean summer temperature," Hulbe said.
A chunk of ice the size of France
Possibly the greatest danger lies with the Ross Ice Shelf -- the largest mass of floating ice in Antarctica. The Ross shelf varies from 600 to 3,000 feet (182 to 915 meters) thick, covering an area about the size of France. It catches the outflow of several major glaciers and ice streams that drain ice from the continents interior.
A massive collapse of the Ross Ice Shelf could, in one fell swoop, cause a measurable rise in sea levels, the researchers say.
"We need to monitor the summertime temperatures to see what the future holds for these ice shelves," says Hulbe.
The new research is reported in the latest issue of the Journal of Glaciology.