Balco never reached Larsen B because of the ship's problems with sea ice, but Eugene Domack, the marine geologist who led the 2010 expedition, has already estimated the age of the Larsen B ice shelf. Domack, an environmental studies professor at Hamilton College, managed to reach the Larsen B area during earlier cruises. His team bored several three-meter-long columns of sediment from parts of the seafloor that were covered by Larsen B until its collapse. Cores taken from under the open ocean are often stained green from microscopic plants called diatoms that settle to the seafloor after dying, but this core contained none. Layer on layer of fine, sandy mud created by grinding glaciers revealed that Larsen B had shaded this area for at least 11,000 years. Layers in the core were dated by analyzing the carbon 14 content of shells left by microscopic organisms called foraminafera.
Eleven thousand years is as far down as Domack's core reached. He says, however, Larsen B may have persisted as far back as 100,000 years, the beginning of the last ice age. Taken together, Balco's and Domack's results suggest that the northernmost ice shelves on the Antarctic Peninsula have come and gone in the recent past. But as the chain of ice shelf collapse pushes farther south from the peninsula's tip toward the mainland, to Larsen B and Scar Inlet, it is now entering the ominous realm of historical anomaly.
Implosion, then Acceleration
Eighteen months after the Palmer returned to port in Punta Arenas, Chile, Scambos reviewed the data streaming in, via satellite, to his office in Boulder. The Scar Inlet ice shelf still has not collapsed—but instruments on the ground had already revealed other insights that were totally unexpected. Researchers had assumed, for example, that even if the peninsula's ice shelves experienced brutal summers, the winters would still nourish them with new snow. Yet when Scambos and his team had returned in November 2010 to repair the station, they found the Scar Inlet ice shelf too crisscrossed with exposed crevasses for their plane to land. As the Twin Otter skimmed overhead, the boot and skid marks that they had left nine months earlier were still visible: a winter that should have nourished Scar Inlet with new snow left it, instead, one step closer to collapse.
Another surprise had occurred that same year between July 14 and 15, during the dark depths of the polar winter. The AMIGOS on Scar Inlet reported a heat wave. The temperature suddenly shot up by 43 degrees Celsius, topping out at a toasty 10 degrees C—shirtsleeve weather in Boulder. The heat was driven by westerly “foehn” winds, which formed as air sliding down the mountains of the peninsula compressed and warmed. At the same time, thermistors buried several meters into the ice at the AMIGOS site recorded a pulse of warmth—suggesting that water from snowmelt was percolating down.
No one knows how often these foehn winds happen—but, Scambos says, “we could be missing some important facts.” The average speed of winds blowing off Antarctica's coastlines has increased by 10 to 15 percent over the past 30 years. Wind now scours 50 billion to 150 billion metric tons of snow from Antarctica's surface each year, blowing it into the ocean, where it melts. As winds strengthen, scouring will likely increase, potentially worsening the prognosis for ice shelves in a way no one anticipated.
What is more, three precision GPS units that Domack had bolted into bedrock outcrops around the perimeters of Larsen B and Scar Inlet show that this region is now rising 1.8 centimeters a year. The disappearance of heavy glaciers is allowing the earth's crust below to rebound—“remarkably fast,” Domack says, and far greater than the 0.8 centimeter estimated from a GPS station 150 kilometers away. The rate of tectonic uplift will increase again when the Scar Inlet ice shelf implodes and the glaciers behind it surge into the ocean. Measure that uplift, Domack says, and you can estimate the amount of ice spilling out. Do that at Scar Inlet, and you can better predict how much ice will disappear as other ice shelves succumb farther south.