One summer soon the Scar Inlet ice shelf will cross a critical threshold. Repeated cycles of melting and refreezing will harden its surface until it can hold large melt ponds. Those ponds will drain into exposed crevasses. As water accumulates in crevasses, its weight will drive the cracks deeper—“like a wedge,” Scambos says—until they reach the bottom of the ice, breaking off a long, skinny Tetris berg. The rupture of one crevasse will produce shock waves that will set off others closer to land's edge. The entire ice shelf might disintegrate within only a few days—maybe just hours.
That is how Scambos thinks Scar Inlet will die. The AMIGOS will let him test the theory. Their cameras will show melt ponds forming, crevasses opening and ponds draining into them. Shots of the pole lines will show the ice shelf straining and buckling. The ridgetop camera will record the pattern of iceberg calving. The AMIGOS on Flask and Leppard will show how the glaciers speed up as the ice shelf holding them back collapses. By having upstream and downstream stations on each glacier, Scambos will see the dynamic nature of glacial response—the manner in which the bottom of the glacier accelerates before its higher reaches do, thus causing it to stretch, thin and welt up with crevasses the way Sjögren Glacier did. The Scar Inlet ice shelf, Scambos says, “is teetering on the edge.”
Rock, Data, Scissors
Glaciers on the Antarctic Peninsula that have lost their ice shelves are indeed thinning at a rapid rate of five to 10 meters a year. The data come from laser altimetry measurements that were taken by the now defunct ICESat and, more recently, by aircraft. The crucial question is how this rate compares with the gradual thinning that has happened since the close of the last ice age 12,000 years ago—and in particular, whether the recent ice shelf breakups are truly unprecedented. Greg Balco, a geologist at the Berkeley Geochronology Center who was on the Palmer, wanted to answer this question.
On a cold, overcast morning a helicopter ferried Balco and me from the Palmer to Sjögren Glacier, 30 kilometers west. Sjögren's fjord held ice 600 meters thick as recently as 1995, right before the Prince Gustav ice shelf broke up, but now it holds seawater instead.
The helicopter dropped us on a bare, rounded mountain beside the fjord. The peak's gray-and-white-layered bedrock was worn into smooth curves and was raked with scrape marks—scars that a younger, thicker Sjögren Glacier left as it rode over this terrain thousands of years ago. “This is beautifully polished,” Balco said of the bedrock. “It looks like it deglaciated last week.” Scattered all around were stones that did not match the bedrock—a brown volcanic boulder here, granite over there. Sjögren had carried them in from far away and dropped them in their present locations as its ice melted.
Balco used these oddball rocks to figure out how quickly Sjögren Glacier has thinned over thousands of years. He picked his way uphill, collecting rocks at different elevations. Back home, he analyzed them to see how long they had been exposed to sunlight by measuring tiny amounts of a rare isotope called beryllium 10, which forms when cosmic rays strike stone. By measuring how long rocks at different heights on the mountain have seen sunlight, Balco could calculate how quickly the glacier thinned and reexposed the mountain.
A year after the expedition Balco had analyzed rocks collected from around two glaciers—Sjögren and Drygalski. His results suggested that the glaciers have undergone major retreats at least once in the past 4,000 years—indicating that both the Prince Gustav and Larsen A ice shelves had collapsed at least once in that time.