JAKOBSHAVN GLACIER, Greenland — Gripping a bottle of Jack Daniel's between his knees, Jay Zwally savored the warmth inside the tiny plane as it flew low across Greenland's biggest and fastest-moving outlet glacier.
Mile upon mile of the steep fjord was choked with icy rubble from the glacier's disintegrated leading edge. More than six miles of the Jakobshavn had simply crumbled into open water.
"My God!" Zwally shouted over the hornet whine of the engines.
From satellite sensors and seasons in the field, Zwally, 67, knew the ice sheet below in a way that few could match. Even after a lifetime of study, the raffish NASA glaciologist with a silver dolphin in one pierced ear was dismayed by how quickly the breakup had occurred.
Wedged between boxes of scientific instruments, tent bags, duffels and survival gear, Zwally had no room to turn inside the cramped passenger compartment of the twin-engine Otter. He passed the whiskey bottle over his shoulder to geophysicist Jose Rial from the University of North Carolina, squeezed on a jump seat between a surveyor and a sleeping climatologist.
Homeward bound -- windburned, bone-chilled and greasy after weeks on this immense ice cap tilted like a beret flopped across the top of the world -- they all had been in a celebratory mood.
Somber now, Zwally and Rial shared a drink in silence as the shadow of the plane slipped across azure meltwater lakes, rust-red tundra and silver tongues of ice.
The Greenland ice sheet -- two miles thick and broad enough to blanket an area the size of Mexico -- shapes the world's weather, matched in influence by only Antarctica in the Southern Hemisphere.
It glows like milky mother-of-pearl. The sheen of ice blends with drifts of cloud as if snowbanks are taking flight.
In its heartland, snow that fell a quarter of a million years ago is still preserved. Temperatures dip as low as 86 degrees below zero. Ground winds can top 200 mph. Along the ice edge, meltwater rivers thread into fraying brown ropes of glacial outwash, where migrating herds of caribou and musk ox graze.
The ice is so massive that its weight presses the bedrock of Greenland below sea level, so all-concealing that not until recently did scientists discover that Greenland actually might be three islands.
Should all of the ice sheet ever thaw, the meltwater could raise sea level 21 feet and swamp the world's coastal cities, home to a billion people. It would cause higher tides, generate more powerful storm surges and, by altering ocean currents, drastically disrupt the global climate.
Climate experts have started to worry that the ice cap is disappearing in ways that computer models had not predicted.
By all accounts, the glaciers of Greenland are melting twice as fast as they were five years ago, even as the ice sheets of Antarctica -- the world's largest reservoir of fresh water -- also are shrinking, researchers at NASA's Jet Propulsion Laboratory and the University of Kansas reported in February.
Zwally and other researchers have focused their attention on a delicate ribbon -- the equilibrium line, which marks the fulcrum of frost and thaw in Greenland's seasonal balance.
The zone runs around the rim of the ice cap like a drawstring. Summer melting, on average, offsets the annual accumulation of snow.
Across the ice cap, however, the area of seasonal melting was broader last year than in 27 years of record-keeping, University of Colorado climate scientists reported. In early May, temperatures on the ice cap some days were almost 20 degrees above normal, hovering just below freezing.
From cores of ancient Greenland ice extracted by the National Science Foundation, researchers have identified at least 20 sudden climate changes in the last 110,000 years, in which average temperatures fluctuated as much as 15 degrees in a single decade.
The increasingly erratic behavior of the Greenland ice has scientists wondering whether the climate, after thousands of years of relative stability, may again start oscillating.
The Theory at Work
Huddled inside a red cook tent atop 3,900 feet of ice, Zwally shoveled snow into a pot simmering on a two-ring propane camp stove.
He had to melt enough to boil lobster tails for dinner. Zwally, who works at NASA's Goddard Space Flight Center in Greenbelt, Md., had purchased them at Costco and lugged them to Greenland.
The beating wings of a 30-mph wind slapped against the tent fabric. Every 15 minutes, a gust sucked open the door and frosted the room.
The tent -- buried in drifts and entered by a ladder through a hatch in the roof -- was part of Swiss Camp, located 155 miles north of the Arctic Circle.
For those assessing the effect of global warming, there may be no more perfect place than this warren of red tents on the Northern Hemisphere's largest ice cap. Here, the theoretical effects seen in computerized climate models take tangible form.