Floating tongues of ice known as ice shelves function like gatekeepers, holding back massive land-based glaciers that stand to boost global sea levels as they melt. But ice shelves are prone to cracking and breaking, which was spectacularly demonstrated in 2002 when Antarctica’s Larsen B Ice Shelf—larger than the state of Rhode Island—suddenly collapsed.

Scientists have investigated a couple of possible culprits for shelf destabilization: warmer ocean waters that eat away at them from below; light-absorbing debris such as rocks and soil on the shelves’ surfaces that can kick-start melting; and transient lakes of meltwater that absorb more sunlight than ice does and also exert stress on their surfaces. Computer models and laboratory simulations have shown stress from up to a couple million tons of meltwater can cause ice shelves to bend and potentially fracture, but such an effect had never been detected in the field. “That’s why we wanted to go out and collect these measurements,” says Alison Banwell, a glaciologist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder. She and her colleagues have, for the first time, measured an ice shelf flexing up and down as lakes formed on its surface and then drained away a few days or weeks later.

Ian Willis drills a hole to install a metal pole on which to mount a pressure sensor in an empty lake basin at the beginning of the melt season. Credit: Alison Banwell

Banwell’s team snowmobiled out from Antarctica’s McMurdo Station to four roughly kilometer-scale meltwater lakes on the McMurdo Ice Shelf, drilling Global Positioning System stations mounted on poles into the ice at varying distances from the centers of the lakes to measure changes in the ice’s height. They also placed water pressure sensors near some of the GPS stations to measure water levels in the lakes, which were dubbed Peanut, Ring, Rift Tip and Wrong Trousers.

Working from November 2016 to January 2017, Banwell and her colleagues found that the ice underneath the lakes deflected downward as they filled with water during the summer melt season; as the lakes drained, the ice shelf bounced back. The largest bounces—roughly one meter in the case of Ring Lake—were observed near the lakes’ centers. The GPS stations farther away showed weaker deflections, if any. “The flexure is very local,” which could have an impact on where related fractures in the ice occur, Banwell says.

How a floating ice shelf bends (and may also fracture) in response to the filling and draining of a meltwater lake. Credit: Adapted from Fig 1. Banwell et al 2013 GRL

Despite the significant bending they recorded, Banwell and her colleagues did not see any fractures on the McMurdo Ice Shelf. It is possible the lakes on this ice shelf overflow their basins before they become large enough to result in sufficient stresses, the researchers suggest.

These results, published Wednesday in Nature Communications, reveal the effects of a phenomenon that has been under researchers’ noses since the first explorers visited Antarctica, but was not thought to be important and so was ignored, says Ian Willis, a glaciologist at the Scott Polar Research Institute at the University of Cambridge and a member of the research team. “The findings of this paper are exciting,” says Jonathan Kingslake, a glaciologist at Columbia University’s Lamont–Doherty Earth Observatory who was not involved in the research. “It's been suggested that this kind of flexure could have caused rapid collapse of ice shelves in the past and could do so in the future.”

Doug MacAyeal, Grant Macdonald and Becky Goodsell stand by a meltwater stream via which one of the lakes on the McMurdo Ice Shelf drained. Credit: Alison Banwell

Banwell and her team have applied for funding to measure whether other ice shelves are also bouncing up and down. They plan to study ones on the Antarctic Peninsula, one of the fastest-warming places on the continent and where Larsen B was once located. Higher temperatures mean more meltwater—one recent study predicted a twofold increase in melting in Antarctica by 2050—but it is hard to predict whether all of that water will make it into lakes that could trigger flexure, Willis says. Some of the water might refreeze, get redirected down into the ice or flow across a shelf without forming a lake, he says. “These are really big unanswered questions.”