ON BOARD COAST GUARD FLIGHT ABOVE BEAUFORT SEA, Oct 2 (Reuters) - Out in the Arctic Ocean, about 200 miles (322 km ) north of the nearest human settlement, the future of the world’s climate is written in the patterns of ice patches on the water’s surface.

Old, “multiyear” ice -- the glue that holds the polar ice cap together and forms the Arctic’s defense against encroaching warming -- is slowly disintegrating, a process that is plain to see from the air.

Thick ice floes used to be kilometers (miles) wide just over a decade ago, said Jim Overland, a sea-ice expert with the U.S. National Oceanic and Atmospheric Administration, who has been surveying the site since the 1990s.

Now the narrow floes -- with bright-white tops and a blue underwater glow -- are just meters (yards) wide, observed Overland as he studied the patterns from the window of a U.S. Coast Guard C-130 aircraft.

The dense, high-quality ice is not coming back, Overland said.

“That’s a one-way street,” he said “We have the same amount of multiyear ice this year as last year, even though we have a little more ice overall.”

Overland said while there was broad awareness of the harmful effect of sea-ice loss on polar bears and other Arctic animals, its impact on weather elsewhere in the northern hemisphere and the rest of the world was potentially more critical.

A warmed Arctic Ocean emits heat into the atmosphere that drastically alters weather patterns, he said.

“That’s the big question: Who cares about the Arctic? Well, it’s going to change the whole heat engine of the planet,” he said.

Scientists have voiced concern for years about the alarming decline in the size of the Arctic ice cap, which functions as a giant air conditioner for the planet’s climate system as it reflects sunlight back into space.

As a greater portion of the ice melts, larger expanses of darker sea water are exposed, absorbing more sunlight and adding to the global warming effect attributed to rising levels of heat-trapping greenhouse gases emitted into the atmosphere by human activity.

SMALL CHANGE, BIG EFFECT

Arctic ice cover this year was 23 percent greater than the record-low levels of 2007, according to the latest data from the National Snow and Ice Data Center at the University of Colorado, which has been keeping records for 30 years. But it was the third-lowest coverage on record, after 2007 and 2008.

The one-year ice that accounts for the increase over 2007 and 2008 -- pancake-flat pieces with finger-like surface ridges etched by movements of the water -- is no substitute for the thick multiyear ice, Overland said.

“It’s thinner. It’s more broken up. And it moves faster,” he said. “And all of that contributes to melting earlier in the season.”

For the Arctic, incremental temperature changes have multiplied effects.

An increase of just a few degrees -- or even fractions of degrees -- can mean the difference between freeze and thaw. Thaw leads to more thaw, as dark-colored sea surfaces absorb solar radiation that would bounce off white snow and ice.

The additional warming caused by the melt itself, along with the greater absorption of solar heat into the now-uncovered northern waters, amplifies the warming in the polar region.

That feedback phenomenon is why the Arctic is warming at about three times the global rate and its ecosystems are changing so much, according to the International Panel on Climate Change.

Total summer ice cover in the Arctic Ocean is only about half the level it was in 1950, according to the IPCC. This year’s summer minimum was 20 percent below the 30-year average, according to the National Snow and Ice Data Center.

In the Beaufort Sea, winter is already encroaching just days after the autumnal equinox. Evidence is in the thin film of new ice appearing between existing multiyear and single-year chunks. But the seasonal build-up will be slower than in the past, Overland predicted.

There was “no indication of freezing whatsoever” in the open water next to the grouped ice, he said as the C-130 flew south to land at Barrow, Alaska. “In contrast to previous years, there’s absolutely no freezing outside that cluster.”