A large ice crevasse begins forming on the Pine Island Glacier. For perspective, the crevasse in the foreground is approximately hundreds of feet deep. (Credit: Jeremy Harbeck/NASA) In the late 1940s, with World War II finally over, the USSPine Island was redeployed from the fight in the Pacific on yet another perilous mission: Operation Highjump. Pilots would launch from the ship, surveying Antarctica from seaplanes and helicopters, and performing some of the earliest photo mapping of the continent. Three airmen died during the mission. And a Texas-sized chunk of ice was named in honor of their ship: The Pine Island Glacier. Today that glacier — and its neighbors — are known as some of the fastest retreating glaciers on the planet. Pine Island is a major player in current and future sea level rise; however, records of its retreat are only as old as the satellite record, which starts a couple decades ago. So scientists weren’t sure when it started shrinking. But a new study published today in Nature shows that even as Operation Highjump was underway, Pine Island Glacier had already begun melting into the ocean.
Pine Island Glacier, Undermined
As glaciers flow downhill toward the ocean, they push shelves of ice off the land that can permanently float on frigid seawater. And a team led by the British Antarctic Survey drilled cores through some 1,500 feet of ice shelf, as well as 1,500 feet of ocean, to take samples from the seabed below. Their results show that a cavity developed under Pine Island before the mid-1940s, which allowed warmer water to wash under the edges of the ice and lift the entire rock formation that was anchoring it in place. The scientists say that the sediment indicates an unusual trend of warm water, perhaps from El Nino in the tropical Pacific Ocean, reached “half-way across the planet” to influence the Antarctic ice sheet. “A significant implication of our findings is that once an ice sheet retreat is set in motion it can continue for decades, even if what started gets no worse,” says NASA scientist and study co-author Robert Bindschadler. Once the warmer water flows underneath it causes icebergs to calve off the edge into the ocean. As these glaciers shrink back from the water, they can stabilize for a time at key grounding points. These are areas where seafloor ridges rise up beneath the ice and delay the warm water from rushing under. But if there’s no rocky outcrop to stop the water, the retreat will continue.
An underwater robot called Autosub went under Pine Island Glacier’s ice shelf and found the ridge that likely once served as the grounding line. And the earliest satellite image from the region, taken in 1973, also shows a bump along the surface of the ice where the glacier was last grounded. Within a few years, that bump was gone, which leads scientists to suspect this current thinning era was already happening. "Ice loss from this part of West Antarctica is already making a very significant contribution to global sea level rise, and is actually one of the largest uncertainties in global sea-level predictions,” says the British Antarctic Survey’s science director, David Vaughan, a study co-author. “Understanding what initiated the current changes is one major piece of the jigsaw, and now we are already looking for the next — how long will these changes continue and how much ice will Pine Island Glacier and its neighbors lose in the coming century?”