Experts Worry About a Disturbing Trend at the North Pole

The distinct possibility that the North Pole could be free of sea ice -- for the first time in recorded history -- may become a cold reality this summer.

The Arctic's thick, resilient multiyear sea ice (frozen sea surface), which usually accumulates and lasts through the annual melting season, has started to give way to thinner, vulnerable first-year ice.

Satellite data gathered by the University of Colorado's National Snow and Ice Data Center showed that young sea ice, which is no more than about 60 inches deep and much more susceptible to melting away, now makes up only 72 percent of the Arctic ice sheet. Using that estimate, scientists at the center see a 50 percent chance that ice at the highest point in the Arctic will melt by the summer's end.

Andy Mahoney, a center researcher, has pinpointed this year in particular as having the "greatest chance" of being ice-free.

Such a scenario, however, will depend on the weather during the next couple of months. "It will probably come down to how cloudy it is this summer," Mahoney says. "If there's clear skies and if atmospheric patterns resemble last year's, you're going to see a lot more melt."

Increased rates of Arctic melt have altered the region in unprecedented ways. Arctic sea ice dwindled to a record low in September, clearing a route through the fabled Northwest Passage that runs from Greenland to Alaska. Opening of the path has provided ships a shorter, more direct route between Asia and Europe.

"It's got a shock level for people because there's always ice at the North Pole, but there are also real implications," Mahoney said. "If the North Pole melted out, the shipping industry would be paying very close attention to that."

Wieslaw Maslowski, who conducts Arctic ice research from his base at the Naval Postgraduate School in Monterey, Calif., told ABC News last summer that there was a chance that the Arctic's entire ice sheet could vanish for the first time in just four or five years.

Such a statement was considered a daring projection at the time, given that climate prediction models estimated a few years before that it would take at least another 40 or 50 years before such a scenario is likely to occur.

But now, Maslowski says that "whether the Arctic sea ice disappears for the first time this summer or four or eight summers from now may be beside the point."

"The point," he noted, "is that we may well be passing through the sea-ice tipping point now. We'll just have to see what July and August weather have in store for the ice this summer."

The disappearance of Arctic sea ice may mean an even hotter planet, since the region's ice pack helps cool the earth by bouncing the sun's rays back into outer space. This reflective property, known as albedo, also prevents the rays from reaching the ocean, where heat is absorbed.

Less sea ice means more dark open water to absorb the heat, which melts the sea ice even further.

"Losing the ice sheet means losing an important way of cooling down," Mahoney said. "As a result, global warming would accelerate as the ice retreats."

BILL BLAKEMORE and TUAN C. NGUYEN