The austral summer of 1995 opened some eyes. Just as Larsen A underwent its now notorious collapse, the Prince Gustav ice shelf, 60 kilometers to the north, also vanished. “The disintegration came as a total surprise,” says Scambos, who, with scientists at the British Antarctic Survey, has been monitoring the continent's ice shelves continually via satellite for many years. The effects of these breakups have reverberated throughout the region. In aerial photographs taken before Prince Gustav disappeared, Sjögren Glacier was a smooth-surfaced plume that sloped gradually from the mainland far out into the fjord, inching toward the ice shelf and sea. But 15 years later Sjögren is a sorry sight, wrinkled with crevasses and sagging in the middle. After the Prince Gustav ice shelf disappeared, Sjögren accelerated toward the ocean at several times its former speed. Crevasses 20 meters wide opened across its surface as the 600-meter-thick ice below stretched under the seaward deformation. Enormous icebergs splintered, uncontrolled, off Sjögren's front edge; that edge now sits 15 kilometers farther back into the fjord than it used to.
“Every single glacier that flowed into an ice shelf, when the shelf was removed, suddenly accelerated,” Scambos says. “Not just a little bit but by a factor of two, three, five, up to eight times as fast.”
Seven summers later, in 2002, the Larsen B ice shelf, just south of Larsen A and 55 times larger than Manhattan, disintegrated into hundreds of shards the size of skyscrapers. “We could see whales in places where the ice was 300 meters thick a few days earlier,” says Pedro Skvarca, a glaciologist with the Argentine Antarctic Institute in Buenos Aires who flew over the site in a plane shortly afterward. “We were quite astonished.”
Once again, the demise of floating ice removed the backstop that stabilized glaciers behind it. As a result of such breakups, more than 150 cubic kilometers of glacial ice has slid off land into the ocean. So great a load has been removed that the earth's crust is literally springing up from below. After Larsen B's collapse, a sensitive GPS instrument bolted into the bedrock on Anvers Island, 150 kilometers west, showed that the rate of tectonic uplift had nearly tripled, from 0.3 to 0.8 centimeter a year.
Healthy ice shelves tend to shed, or “calve,” large, tabular icebergs, sometimes larger than the state of Rhode Island. But Larsen B broke up in a very different way. A series of seven sharp images from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite instrument, taken over 35 days, showed Larsen B splintering into hundreds of bergs on the order of 130 meters wide, 160 meters deep and a kilometer or more long. The bergs, shaped like the long, narrow geometric blocks that descend in the game Tetris, rolled off the edge of the ice shelf and into the ocean to reveal their cross sections of blue glacial ice. Researchers had never seen this pattern of calving before. The ice shelves were dying from some heretofore unrecognized mechanism.
Scambos and Skvarca first attempted to understand that mechanism of collapse in March 2006. On a dim, cold day an Argentine naval helicopter landed on a broad, tabular berg with a precarious, sideways bounce; the pilot, thrown off by the berg's uniform milky white color, did not realize that his spinning rotors had dipped dangerously low. Scambos, Skvarca and four other scientists climbed out of the helicopter. This iceberg, named UK211, had survived for three years since calving off the Larsen C ice shelf 385 kilometers south, but now it was drifting into warm climates north of the peninsula. Scambos and the others hoped to use it as an experimental analogue for ice shelf breakup.
The team installed an instrument station, dubbed AMIGOS (Automated Met-Ice Geophysics Observation Systems), that would monitor the berg's deteriorating health. A GPS unit tracked the berg's position, a meteorological station measured wind and temperature, and a camera documented snowmelt on the surface. The camera could be aimed at a marked pole driven into the berg to show how quickly the snow level dropped as the result of melting. The camera could also be aimed at a line of poles that the researchers planted 2.2 kilometers out toward the berg's edge. If that line started to curve, it would indicate that the berg was softening and bending.