ABOARD THE USCGC HEALY -- Great chunks of sea ice knock against the bow of the U.S. Coast Guard Cutter Healy, jolting the ship like a series of moderate earthquakes.

The constant thumping soon fades to background noise as the 420-foot vessel plows through ice floes before its next stop in the Arctic Ocean beyond Alaska's northwestern coast. With the icecutter anchored again, the shaking stops, and scientists on board collect another round of specimens for the most ambitious study of global climate change of its kind.

Their 40-day mission ended Aug. 26, wrapping up initial research in the Western Arctic Shelf-Basin Interactions program. The 10-year, $17 million effort -- called SBI for short -- is jointly funded by the National Science Foundation and Office of Naval Research.

Participants are looking at possible indicators of climate change in the Chukchi and Beaufort seas, hoping to determine whether the apparent warming of the world is a modern phenomenon or part of an ancient cycle.

"Only two things can change life as we know it -- nuclear weapons and rapid climate change," said Lou Codispoti, a University of Maryland chemical oceanographer and one of 38 scientists on board.

Ice volume down

A significant shift in climate would show up first in polar areas, researchers say.

Some scientists estimate Arctic ice volume has decreased between 20 percent and 40 percent in recent decades. That would support what Arctic hunters have observed for the last decade: thinner shore ice, quicker seasonal thaws, more mosquitoes, more coastal caribou sightings.

More changes could further shrink shore ice, causing coastal erosion and displacing some walrus populations and other wildlife that depend on the ice.

The five-year research phase was launched on an earlier spring trip on the Healy following three years of planning, said chief SBI scientist Jackie Grebmeier, 47, a University of Tennessee biological oceanographer.

The current voyage followed a route similar to the first one, said Grebmeier, who split on-ship leadership duties with her husband, Lee Cooper, 46, a University of Tennessee research professor. Both treks started in Nome, traveling through the Bering, Chukchi and Beaufort seas to Barrow, then north to farther reaches of the Arctic Ocean. Smaller trips are planned next year.

"The ice has really pulled back since the first trip," Grebmeier said during a work break during the voyage. "In the spring the ice was in all the way to Nome. This time we didn't hit ice until Barrow."

'Snapshots in time'

On this particular day in late July, however, global warming seemed a distant concept on the decks of the Healy, anchored some 50 miles north of Point Barrow, Alaska's northernmost tip.

Bundled in parkas against a frigid wind, researchers delved into a dozen independent projects to study salinity, temperature, minerals, light transmission, plant life, bacteria and larger animals. They scooped and cored sediments from the bottom of the ocean. They collected water samples from different depths, using a cylindrical instrument ringed with 12 large tanks. The entire apparatus is about the size of a 55-gallon drum.

Meanwhile, scientists on board another Coast Guard cutter -- the Polar Star -- were planting other instruments elsewhere in the Arctic. Over the coming year, these will measure marine conditions like salinity, temperature and currents.

"Think of the work on the Healy as snapshots in time. The Polar Star is the long-term picture," said science foundation spokesman Peter West. "You've got two sets of data measuring much of the same thing."

The elements didn't always cooperate, as the scientists and Coast Guard crews discovered when they lowered the cylinder over the stern for another round of water samples. Strong currents drove massive floes toward the cable holding the equipment. Codispoti pushed the closest one away with a boat hook as larger chunks drifted toward the line.

"We should get some more people out here," he called over his shoulder. "Big-time problems out here today."

A small crowd gathered, some armed with boat hooks. But no amount of pushing could divert the larger floes. The cable began to creak against the side of the ship.

Despite the momentary hitch, most of the stops went as planned on board the Coast Guard's largest vessel.

Ultimately, researchers hope to share findings with various circumpolar nations studying climate change to develop a pan-Arctic model in the third and final phase of the project.

"Our research is like a puzzle," Grebmeier said. "Each part is like a piece, but until you put the pieces together you can't see the big picture."