That more ice shelves will collapse is a foregone conclusion. An average summer temperature of zero degrees C seems to represent the highest temperature at which an ice shelf can exist. And the invisible line where summer averages zero degrees C is creeping south along the Antarctic Peninsula tip toward the mainland, along with higher mean annual temperatures. Every ice shelf that the line crosses has collapsed within a decade or so. Next up, south of Larsen B and Scar Inlet, is the Larsen C ice shelf, which covers 49,000 square kilometers—twice as large as the state of Maryland, or about 820 Manhattans. Larsen C has more glacial ice flowing into it than all the other ice shelves that have collapsed combined. It already sees summer melt ponds on its northern end.
Even more worrying are the ice shelves hanging off the mainland, which support much larger glaciers, such as Pine Island, Thwaites and Totten. They are melting from their undersides because of warmer ocean currents, rather than from the top down. The result is the same: Pine Island Glacier has thinned by only 15 percent since 1994, yet the massive glacier behind it has accelerated by 70 percent.
The full effects of ice shelf breakup on glacier demise will not be known for some time. A study published in 2011 by Scambos, Truffer and Pettit found that one glacier continues to accelerate even 15 years after losing its ice shelf: Röhss Glacier (which used to flow into the Prince Gustav ice shelf) has now reached nine times its former speed.
This acceleration in glacier flow may explain a recent observation by Eric Rignot and Isabella Velicogna of the NASA Jet Propulsion Laboratory. They found that the rate of ice loss from Antarctica is actually increasing by roughly 25 cubic kilometers a year. Those 2007 IPCC estimates of 18 to 59 centimeters of sea-level rise by 2100 do not account for any of these ice shelf effects. The estimates “actually send the wrong message,” Rignot says. “They're probably off by a factor of two to three.” By 2100, he says, “you could easily see a meter of sea-level rise.” An analysis published in 2009 by Martin Vermeer of the Helsinki University of Technology places the estimate between 75 and 190 centimeters.
Such hints beg further monitoring of the Larsen region—an area that punishes those who try to pry apart its secrets. Prior to the 2010 Palmer expedition Domack had sailed to the area on five earlier research cruises, three of which never reached their geographic target because of brutal sea ice. “It can be really frustrating,” he admits. But important questions are bound to keep him and Scambos coming back.