WILKINS ICE SHELF IS “HANGING BY A THREAD”

Antarctic ice shelf set to collapse due to warming

By Alister Doyle, Environment Correspondent

Tue Jan 20, 2009 8:42am EST

WILKINS ICE SHELF, Antarctica (Reuters) – A huge Antarctic ice shelf is on the brink of collapse with just a sliver of ice holding it in place, the latest victim of global warming that is altering maps of the frozen continent.

Between 1981 and 2007, most of Antarctica warmed. Portions of West Antarctica experienced an especially rapid rise in temperature. (NASA image by Robert Simmon, based on data from Joey Comiso, GSFC.)

“We’ve come to the Wilkins Ice Shelf to see its final death throes,” David Vaughan, a glaciologist at the British Antarctic Survey (BAS), told Reuters after the first — and probably last — plane landed near the narrowest part of the ice.

The flat-topped shelf has an area of thousands of square kilometers, jutting 20 meters (65 ft) out of the sea off the Antarctic Peninsula.

But it is held together only by an ever-thinning 40-km (25-mile) strip of ice that has eroded to an hour-glass shape just 500 meters wide at its narrowest.

In 1950, the strip was almost 100 km wide.

“It really could go at any minute,” Vaughan said on slushy snow in bright sunshine beside a red Twin Otter plane that landed on skis. He added that the ice bridge could linger weeks or months.

The Wilkins once covered 16,000 sq km (6,000 sq miles). It has lost a third of its area but is still about the size of Jamaica or the U.S. state of Connecticut. Once the strip breaks up, the sea is likely to sweep away much of the remaining ice.

Icebergs the shape and size of shopping malls already dot the sea around the shelf as it disintegrates. Seals bask in the southern hemisphere summer sunshine on icebergs by expanses of open water.

High-resolution satellite data show the Wilkins Ice Shelf collapse in detail, including blocks of ice that have tipped over and blocks of ice that have remained upright. (Formosat image © 2008 Dr. Cheng-Chien Liu, National Cheng-Kung University and Dr. An-Ming Wu, National Space Organization, Taiwan.)

A year ago, BAS said the Wilkins was “hanging by a thread” after an aerial survey. “Miraculously we’ve come back a summer later and it’s still here. If it was hanging by a thread last year, it’s hanging by a filament this year,” Vaughan said.

Nine other shelves have receded or collapsed around the Antarctic peninsula in the past 50 years, often abruptly like the Larsen A in 1995 or the Larsen B in 2002. The trend is widely blamed on climate change caused by heat-trapping gases from burning fossil fuels.

WARMING TO BLAME

“This ice shelf and the nine other shelves that we have seen with a similar trajectory are a consequence of warming,” Vaughan said.

In total, about 25,000 sq km of ice shelves have been lost, changing maps of Antarctica. Ocean sediments indicate that some shelves had been in place for at least 10,000 years.

Early in the day on February 28, 2008, the Wilkins Ice Shelf remained intact. By March 17, 2008, the ice shelf had retreated significantly from its original extent. As the Antarctic summer drew to a close, sea ice began to freeze around the ice fragments. (NASA images by Jesse Allen and Robert Simmon, based on MODIS data.)

Vaughan stuck a GPS monitoring station on a long metal pole into the Wilkins ice on behalf of Dutch scientists. It will track ice movements via satellite.

The shelf is named after Australian George Hubert Wilkins, an early Antarctic aviator who is set to join an exclusive club of people who have a part of the globe named after them that later vanishes.

Loss of ice shelves does not raise sea levels significantly because the ice is floating and already mostly submerged by the ocean. But the big worry is that their loss will allow ice sheets on land to move faster, adding extra water to the seas.

Large blocks of ice, recently broken off the Wilkins Ice Shelf, float on the ocean in this aerial photograph. In between the large blocks of ice float much smaller pieces of ice with a rough surface. (Photograph © 2008 British Antarctic Survey.)

Wilkins has almost no pent-up glaciers behind it. But ice shelves further south hold back vast volumes of ice. “When those are removed the glaciers will flow faster,” Vaughan said.

Temperatures on the Antarctic Peninsula have warmed by about 3 Celsius (5.4 Fahrenheit) since 1950, the fastest rise in the southern hemisphere. There is little sign of warming elsewhere in Antarctica.

BAS scientists and two Reuters reporters stayed about an hour on the shelf at a point about 2 km wide.

“It’s very unlikely that our presence here is enough to initiate any cracks,” Vaughan said. “But it is likely to happen fairly soon, weeks to months, and I don’t want to be here when it does.”

The U.N. Climate Panel, of which Vaughan is a senior member, projected in 2007 that world sea levels were likely to rise by between 18 and 59 cm (7 and 23 inches) this century.

But it did not factor in any possible acceleration of ice loss from Antarctica. Even a small change in the rate could affect sea levels, and Antarctica’s ice sheets contain enough water in total to raise world sea levels by 57 meters.

About 190 nations have agreed to work out a new U.N. treaty by the end of 2009 to slow global warming, reining in emissions from burning fossil fuels in power plants, cars and factories.

Following are facts about ice shelves.

• Ice shelves are extensions of land-based ice sheets that float on the sea. They can be several hundred meters thick and are found mainly in bays around Antarctica, with some in the Arctic. Antarctica’s biggest, the Ross Ice Shelf, is the size of France.

• Scientists worry that that the collapse of ice shelves could prompt glaciers inland to start sliding faster toward the sea, raising sea levels. Antarctica holds enough fresh water to raise sea levels by 57 meters (187 ft), so even a limited melt would have big consequences.

• Since 1950, ten ice shelves on the Peninsula, which snakes up toward South America, have contracted or collapsed.

• The British Antarctic Survey reckons that 25,000 sq km (10,000 sq mile) of ice shelves have been lost in total — an area the size of Macedonia, Rwanda or the U.S. state of Vermont.

• Ice shelves that have broken up since 1950 are the Larsen A, Larsen B and Larsen C, Prince Gustav, Muller, Jones, Wordie, George VI north, George VI south and the Wilkins.

• Among the most dramatic collapses was that of the Larsen A within a few weeks in 1995, when satellite images abruptly showed the bay dotted with icebergs. The Larsen B also abruptly collapsed in 2002.

(Editing by Andrew Roche) – copyright the author or the news agency.

Freshening of deep Antarctic water could prove catasterophic

Scientists have detected changes in salinity of the antarctic water that could drmatically change the ocean currents and the world’s climate.

They found that salty, dense water that sinks near the edge of Antarctica to the bottom of the ocean about 5 km (3 miles) down is becoming fresher and more buoyant.

The Antarctic “bottom water” is responsible for the great ocean conveyor belt, a system of currents that move throughout the Southern, Pacific, Indian and Atlantic Oceans distributing warm water around the globe.

The edge of the remaining part of an ice shelf in the Weddell Sea in Antarctica photo taken on March 4, 2008. REUTERS/Mariano Caravaca/Handout

“The main reason we’re paying attention to this is because it is one of the switches in the climate system and we need to know if we are about to flip that switch or not,” said Rintoul of Australia’s research arm the CSIRO.

“If that freshening trend continues for long enough, eventually the water near Antarctica would be too light, too buoyant to sink and that limb of the global-scale circulation would shut down,” he said earlier today.

the great ocean conveyor belt delivers warm water into the north Atlantic, making Europe warmer than it would otherwise be. The slowing down or stopping of these currents could result in catastrophic changes in the world’s climate.

“We don’t see any evidence yet that the amount of bottom water that’s sinking has declined. But by becoming fresher and less dense it’s moving in the direction of an ultimate shutdown.” Report

The Thermohaline Circulation (THC)

The thermohaline circulation (THC) is the global density-driven circulation of the oceans. Derivation is from thermo- for heat and -haline for salt, which together determine the density of sea water. Wind-driven surface currents (such as the , ) head polewards from the equatorial Atlantic Ocean, cooling all the while and eventually sinking at high latitudes (forming North Atlantic Deep Water). This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters (with a transit time of around 1600 years) upwell in the North Pacific (Primeau, 2005). Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth’s ocean a global system. On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.

The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyer, the global conveyor belt, or, most commonly, the meridional overturning circulation (often abbreviated as MOC). (Source: wikipedia)