Researchers worry over Antarctica's shifting ice

PUNTA ARENAS, Chile -- Scientists looking southward from the tip of South America, over steel-gray waters toward icy Antarctica, see only questions on the horizon about the fate of the planet.

Now that one mammoth Antarctic ice shelf has collapsed into the ocean, when might another, bigger one crumble and slip into a warming sea? In 1,000 years? In 100 years? Sooner? Never?

"People don't have the answer to the question yet -- what the probability is of that collapse, if any," said scientist Gino Casassa. "But there's some indication of instability."

Casassa and fellow Chilean researchers had just flown back from the icy continent to this expedition staging point, and brought with them some potentially unsettling news.

On a two-month round-trip trek by snow tractor to the South Pole, they pointed their sophisticated radar at the ground and found that the West Antarctic Ice Sheet may be thicker than thought, many hundreds of feet thicker in parts.

Glaciologists like Casassa worry most about that western ice sheet, half a continent of frozen water believed enough, if gradually melted, to raise ocean levels worldwide by about 15 feet.

That would be a slow-motion catastrophe for global coastlines -- not instantly deadly like a tsunami, but more universal and permanent. And "the deeper the West Antarctic Ice Sheet, the greater the potential impact to sea level," Casassa pointed out, though cautioning that their data awaits full analysis.

Such pressing questions about the white continent and climate change -- and the impact each will have on the other -- are consuming more and more scientific resources these days, as hundreds of researchers migrate south in the southern summer to probe, measure and observe in an on-the-ice search for answers.

Advanced technology, like the radar lent by the University of Kansas, allows scientists to penetrate darker corners of polar science. ICESat, a NASA satellite launched two years ago, is giving them an unprecedentedly precise look at the state of the ice in both the Arctic and Antarctic. The same U.S. space agency, meanwhile, is boosting the power of its supercomputers to speed through millions of calculations to foresee temperature, evaporation, precipitation and other changes far into the future, using complex climate models.

The challenges remain huge, however, on the globe's most forbidding landscape. And technology sometimes fails. One of the ice-surveying satellite's laser eyes went dead, for one thing, reducing its useful data by more than half. For another, the vast continent has too few permanent monitoring stations to give scientists more than a sketchy grasp of its climate behavior.

"Even now, we're not so sure what's going on in all Antarctica. There aren't sufficient data," said Argentina's Pedro Skvarca, a veteran Antarctic glaciologist.

The hunt for data took on fresh urgency after Antarctica's "Larsen B," an ice shelf bigger than Rhode Island, collapsed into the Southern Ocean over the space of just 35 days in 2002.

The 1,300 square miles of ice had fringed the Antarctic Peninsula, a rocky arm of land that reaches north to within 750 miles of this southernmost Chilean city. In that peninsular region, average surface temperatures have risen by 4.5 degrees Fahrenheit over 50 years.

Temperatures globally rose about 1 degree Fahrenheit in the past century. It hasn't been established whether the Antarctic Peninsula warming stems directly from global changes, or from more localized conditions.

Because an ice "shelf" already floats on the sea, displacing its weight in water, Larsen B's disintegration -- and that of the smaller, nearby Larsen A in 1995 -- didn't raise ocean levels. But what has happened since did.

Skvarca and American researchers, collating aerial reconnaissance with ICESat images, reported last September that land-based glaciers backed up behind Larsen B have accelerated their flow since its break-up -- moving ice into the sea up to eight times faster than before.

Now scientists are warily watching the Larsen C ice shelf, farther south and 20 times larger.

Like the B sector before it collapsed, "Larsen C also appears to be thinning," said Robert Thomas, a researcher at NASA's Goddard Space Flight Center facility in Virginia. "It's quite possible that the thinning is the precursor to the breakup."

Scientists expect Larsen C to disintegrate sometime in this century, Skvarca said. "We should bear in mind what is happening to the Larsen ice shelves, because if it also happens to a big shelf, we are going to be in trouble," he said.

"Big" refers to the floating giants -- the Ross and Ronne ice shelves. Each is around 200,000 square miles, bigger than California. They lie deeper south in a more frigid zone, are thicker in depth and hold back immense streams of ice coming from the heights of the West Antarctic Ice Sheet, the 1,100-mile-wide, 9,000-foot-high dome that sits atop continental rock.

No one has spotted signs of instability yet in the two giant shelves. But scientists also worry about what Thomas calls the "back door" -- a stretch of Antarctic coast whose glaciers feed off the western ice sheet directly into the Amundsen Sea, or into small ice shelves on that sea.

"There's some indication of instability in the Amundsen Sea," Casassa said.

Casassa, Thomas and colleagues have watched the "back door" since the early 1990s, most recently via satellite tracking. They reported in the journal Science last September that a half-dozen glaciers there are now thinning and accelerating. The big Pine Island glacier is carrying ice faster toward the sea as far as 186 miles inland, deep within the western ice sheet.

"The instability and changes are migrating inland," Casassa said. "This could affect the whole of the West Antarctic Ice Sheet, because the ice sheet 'feels' what is happening at the coast."

Glaciologists are quick to point out they've found no basis for fearing an imminent, massive collapse of ice into the southern seas. The Intergovernmental Panel on Climate Change, a U.N.-organized network of scientists, concluded in a 2001 assessment that the impact of climate changes "will be realized slowly" in Antarctica.

Although scientists assume major change might take a millennium, much remains unknown about the links among ice, ocean and skies.

It isn't known, for example, how excess amounts of cold, fresh water from glaciers, pouring into the salty sea, will affect the ocean current that circles Antarctica from west to east -- a main driver of all the world's ocean currents, and hence of world climate.

Antarctica's own climate, meanwhile, may be verging on change.

Factors including the seasonal Antarctic "ozone hole" of recent decades are believed to have held temperatures in the interior stable while the world warmed. But NASA climatologists recently reported that a computer model study indicates things may rapidly reverse toward warming in Antarctica, in part because an international ban on ozone-destroying substances is expected to restore the ozone layer.

Drew Shindell of NASA said "there's a lot of room for improvement in the modeling" -- in upper-atmosphere dynamics, for example, including the chemistry of ozone.

"We're trying to get that kind of modeling. We have new computers, 10 times bigger," he said.