National Geographic : 2017 Jul
46 national geographic • july 2017 deep into the ice from below, the unsupported ice sags, causing the entire shelf to bend and warp. Crevasses erupt along the lines of stress, on both the top and the bottom of the ice. The pops and bangs the researchers heard and the daily opening of new cracks bore witness to the ice’s gradual failure as it thinned and broke down beneath them. As the Pine Island Ice Shelf has weakened and the glacier behind it has accelerated, the ice has stretched and thinned for 150 miles inland from the coast. The destabilizing effects spread farther into West Antarctica every year. “A little nudge can get you to several decades of retreating be- havior that’s hard to reverse,” Truffer says. In fact, research by Rignot and others over the past few years indicates that the collapse of sev- eral major glaciers flowing into the Amundsen Sea is now unstoppable. Between 2002 and 2009 alone, the ice shelf in front of the Smith Glacier thinned by 1,500 feet in some places, the one in front of the Pope Glacier by up to 800 feet. The grounding lines of the Amundsen glaciers have retreated so far—tens of miles in some cases— that they now rest on seafloor that slopes down toward the center of the ice sheet. Each incre- ment of retreat exposes a greater ice surface to warm ocean water. It’s a runaway process—and scientists are urgently trying to figure out how fast it will run. The ice shelves, Fricker says, “are the canary in the coal mine.” Because they’re already floating, they don’t raise sea level themselves when they melt—but they signal that a rise is imminent, as the glaciers behind them accelerate. Fricker and her team have found that from 1994 to 2012, the amount of ice disappearing from all Antarctic ice shelves, not just the ones in the Amundsen Sea, increased 12-fold, from six cubic miles to 74 cubic miles per year. “I think it’s time for us scientists to stop being so cautious” about communicating the risks, she says. The retreat and hemorrhage of these gla- ciers “will accelerate over time,” agrees Rignot. “Maybe you don’t care much about that for the next 30 to 40 years, but from 2050 to 2100 things could get really bad, and at that point listening to They tend to fan out and steer warm water to- ward the edges of the shelves. The ice there is crucial: It rubs against the stationary banks and slows the flow of the shelf and the glacier behind it. But that edge ice is also thinner than the rest. This “is something that bears watching,” Scam- bos said in early 2016. Ian Howat, of the Byrd Polar and Climate Research Center in Columbus, Ohio, is another glaciologist who’s watching Pine Island closely. Last November he reported two ominous new rifts spreading across the ice shelf that threat- en to prune it to its shortest length in recorded history. As Howat looked back through month- ly satellite photos, he realized that the rifts had been triggered by a singular event that had happened, unnoticed, three years before. The strip of torn-up ice anchoring the ice shelf to its northern bank had suddenly fallen apart, suggesting it had been undermined by melting from below. It blew out “just in a matter of days,” Howat says, “like a zipper, unzipping the side of the glacier.” It’s unclear when the entire ice shelf might dis- integrate. The “warm” water flowing underneath it from offshore is only 4 to 6 degrees Fahrenheit above freezing. But roughly 3,000 cubic miles of it arrives every year, which means the ice shelf is receiving an amount of heat that exceeds the output of a hundred nuclear power plants, oper- ating 24/7. When Truffer and his team camped on the shelf in December 2012, they could sense how it had already weakened. As the meltwater cuts Research indicates that the collapse of major glaciers that flow into the Amundsen Sea is now unstoppable.