Unusual warming played role in 2002 Antarctic ice shelf retreat: Study

A new study has showed that unusual atmospheric and oceanic warming had a role to play in the 2002 collapse of the Antarctic Larsen B ice shelf, when a Rhode-Island-sized area of ice dramatically tore away from it.

The study has revealed that widespread flow acceleration and frequent small-iceberg calving could serve as warning signs for future such ice shelf retreats in the Antarctic, said scientists from The Pennsylvania State University (Penn State), US, who led the study.

The findings have been published in the journal Earth and Planetary Letters.

Ice shelves are floating tongues of ice connected to land but extend out and float on ocean water.

They act as a buttress as they hold back glaciers on land flowing toward the ocean.

Therefore, understanding how they will react to continued warming is important for getting sea-level rise predictions right, the scientists said.

Five calving pulses observed between 1998 to 2002 corresponded with climate anomalies caused by La Nina and the Southern Annular Mode, characterized by strong westerly winds in the Southern hemisphere moving closer to Antarctica, the study said.

Warmer ocean waters may have cut sub-ice-shelf channels, further weakening vulnerable parts of the ice shelf called shear margins. These margins separate flowing ice from stagnant ice or rock, and the areas often have more fractures and softer ice, the scientists said.

”The results suggest that warm climate anomalies control the occurrence of calving, while the extent and speed of calving are governed by ice shelf geometry and mechanical conditions, in particular, the sturdiness of the weakest shear margin,” said Shujie Wang, assistant professor, Penn State and lead author on the study.

Failure of a shear margin in the northern portion of the ice sheet may have triggered the calving pulses, and as the ice retreated, it moved away from rocky islands that had served as buttresses holding the sheet in place, the scientists said.

”When you pin a piece of paper to a wall, the pins prevent the paper from falling to the floor,” Wang said. ”It’s the same with ice flow – these rocky islands serve as ‘pinning points’ that anchor ice and slow down its march to the sea.” The distribution of these pinning points may help determine the vulnerability of an ice sheet, as a weak shear margin with limited buttressing sources played a predominant role in destabilizing the Larsen B ice shelf and starting the small-iceberg calving sequence, the scientists reported.

”The collapse of the Larsen B ice shelf is generally thought of as an independent event,” said Wang.

”Our work shows that it was the last phase in a calving sequence that began in 1998 and was controlled by both atmospheric and oceanic warming anomalies that weakened the ice shelf structure over time,” said Wang.

While scientists have long known that warming air and ocean temperatures melt and weaken ice shelves from the surface and the subsurface, the exact processes leading to collapse are not well understood.

”Ice-shelf loss from environmental warming is the fastest way for Antarctica to drive sea-level rise, but remains very hard to predict in part because we have so few observations,” said Richard Alley, a co-author on the study from Penn State.

”The Larsen B ice shelf was not holding back much land ice, and so its loss was not very important for sea level, but it offers an outstanding laboratory to learn the early warning signs and the processes of ice-shelf loss.

”The new insights gained here should help in the larger effort to project how warming will interact with the ice shelves to control future contributions to sea-level rise,” said Alley.

The scientists gathered data on the ice shelf from as far back as the 1960s and analysed changes over time using satellite observations, modelling experiments and climate reanalysis data, the study said.

Prior to the 2002 collapse, the ice shelf experienced a transition from typical large calving events – when chunks of ice break off into the ocean – to more frequent, smaller calving and to a faster, widespread flow of ice toward the sea.

”Typically, large chunks of ice break off, regrow for decades and break off again,” said Wang.

”Here, many smaller calving events occurred, and the ice did not regrow. And when it retreated from rocky islands that served as a buttress for the ice shelf, that could no longer hold the flow back,” said Wang.

”Those smaller areas matter for the whole region,” said Wang.

”If you think about an ice shelf as a complex system, local areas may have a dominant impact on the whole ice shelf. These fundamentals are important because if we don’t understand the fundamentals, we can’t make the most accurate predictions for the future,” said Wang.