Balmy surface temperatures, not an unstable underbelly, probably prompted the largest ice shelf collapse ever recorded, researchers report in the Sept. 12 Science.
In 2002 a Rhode Island–sized hunk of the Larsen B ice shelf on the Antarctic Peninsula shattered into thousands of icebergs during the peninsula’s hottest summer on record (SN: 3/30/02, p. 197). Radar maps at the time suggested that the point where the shelf floated off the seabed, called the grounding line, had retreated farther inland, triggering the overhanging ice to break off. Now seafloor sediments reveal that Larsen B’s grounding line had remained stable for thousands of years before the ice shelf’s collapse.
The finding demonstrates that the grounding line wasn’t involved in Larsen B’s breakup, says study coauthor Eugene Domack, an earth scientist at the University of South Florida in St. Petersburg. “Up until now the community accepted that grounding line instability is needed for ice shelves to disintegrate,” he says. “We now show that surface warming alone can cause ice shelves to collapse.” He adds that other ice shelves in Antarctica could follow Larsen B’s lead as Antarctic surface temperatures rise under climate change (SN: 7/27/13, p. 18).
Ice shelves line 45 percent of Antarctica’s coast and help stem the flow of the continent’s ice sheets and glaciers into the ocean. Warm seawater can melt an ice shelf’s underside, pushing the grounding line inward until the ice snags on a seafloor formation such as a cliff or hill. Because the grounding line is too far under the ice to observe directly using submersibles or ice drills, scientists glean these deep ice movements from radar data collected by satellites and airplanes.
Larsen B’s destruction therefore presented a unique opportunity, Domack says. After the broken ice floated away, the location of the grounding line before the breakup became open water. That allowed Domack and colleagues to sail into Larsen B’s ruins in 2006 and sample seafloor sediments that had accumulated over thousands of years beneath the ice.
Back in the lab, the team sifted the silt for the sand-sized shells of tiny microbes called foraminifera. Based on the radiocarbon ages of these shells, the team estimated how long ago each layer of sediment formed.
Because distinctive types and amounts of sediment form where an ice shelf meets the seafloor, the researchers could track the location of Larsen B’s grounding line over time. The team determined it hadn’t budged for at least 11,000 years. Domack suggests that scientists in 2002 simply mistook a seafloor trough for a grounding line on the radar maps. Grounding line instability, Domack concludes, did not contribute to the shelf’s collapse.
Domack thinks surface melting, previously considered a secondary mechanism in Larsen B’s collapse, was the prime trigger. During Antarctic summers, a layer of snow usually sits on top of the shelf and acts like a sponge, soaking up meltwater from thawing ice and glaciers and preventing it from forming large pools. During the unusually warm summers that led up to the Larsen B collapse, Domack says, the snow on top of the ice shelf melted, allowing water to collect in large lakes on the surface. The pressure from these lakes probably burst open cracks in the ice, destabilizing the entire shelf.
Glaciologist Eric Rignot of the University of California, Irvine remains unconvinced that surface melting alone was the driving force behind Larsen B’s collapse. He suggests other factors such as warming ocean temperatures thinning the shelf’s underside could have played a role. Rignot adds that no matter the forces involved in the ice shelf’s breakdown, Larsen B demonstrates how unusual the last few decades of warming have been.
“We witnessed the collapse of an ice shelf that had been stable for over 10,000 years,” Rignot says. “This work shows that we still need a much better understanding of ice shelf grounding lines to predict Antarctica’s future.”