A key region of Antarctica's Ross Ice Shelf – the world’s largest floating sheet of ice at roughly the size of France – is melting 10 times faster than the overall ice shelf. The international team of scientists spent four years collecting data from devices installed under the Ross Ice Shelf to reach the conclusion published in the journal Nature Geoscience. 

In addition to warm deep ocean water, solar heating of ocean surface waters also plays a surprisingly large role – not exactly the curveball you want when the ice shelf is the size of a country at around 500,000 square kilometers (193,000 square miles).

“Previous studies have shown that when ice shelves collapse, the feeding glaciers can speed up by a factor or two or three,” said co-author Dr Poul Christoffersen from Cambridge’s Scott Polar Research Institute, in a statement. “The difference here is the sheer size of Ross Ice Shelf, which is over 100 times larger than the ice shelves we’ve already seen disappear.”

The finding is based on custom-made radar measurements as well as temperature, salinity, melt rates, and ocean current data gathered from a borehole 260 meters (850 feet) deep. On top of that, study authors Dr Craig Stewart and Dr Christoffersen drove more than 1,000 kilometers (620 miles) by snowmobile to measure the ice thicknesses and to map basal melt rates at various locations. 

The rapid melting is happening under a thin yet important part of the ice shelf. The team found that the stabilizing region is near where surface water heated by the Sun flows into a cavity, causing it to melt at three times the usual summer rate. Strong winds and tidal forcing can drive warm water into the cavity. In all, this vulnerable point is melting 10 times faster than the average expected for the whole of the ice shelf. 

“Although some frontal regions are unimportant to the stability of ice shelves, others contain critical pinning points that sustain the location of the front. Ross Island seems to be one such pinning point,” write the authors, “and recent modeling shows that the rapid melting identified here influences a structurally critical region in which ice thickness changes can influence the flow speed of the entire ice shelf.”

The Ross Ice Shelf, which accounts for 32 percent of Antarctica’s total ice shelf, is fairly stable for now, in large part due to ice from feeding glaciers and snow accumulation. However, this balance may change in the future based on the melting of the pinning point. Concentrations of summer sea ice in the Ross Sea are expected to decrease by 56 percent by 2050, with the length of the ice-free period increasing. 

“Climate change is likely to result in less sea ice, and higher surface ocean temperatures in the Ross Sea, suggesting that melt rates in this region will increase in the future,” said Dr Stewart from the National Institute of Water and Atmospheric Research (NIWA).