A quarter of West Antarctic ice now unstable: Study

Antarctica's ice sheet has thinned by up to 122 metres in places, with the most rapid changes occurring in West Antarctica where ocean melting has triggered glacier imbalance, scientists say.

The UK Centre for Polar Observation and Modelling (CPOM) tracked changes in snow and ice cover across the continent, by combining 25 years of European Space Agency satellite altimeter measurements and a model of the regional climate.

Researchers said this means that the affected glaciers are unstable as they are losing more mass through melting and iceberg calving than they are gaining through snowfall.

They found that the pattern of glacier thinning has not been static. Since 1992, the thinning has spread across 24 per cent of West Antarctica and over the majority of its largest ice streams -- the Pine Island and Thwaites Glaciers -- which are now losing ice five times faster than they were at the start of the survey.

The study, published in the journal Geophysical Research Letters, used over 800 million measurements of the Antarctic ice sheet height recorded by the ERS-1, ERS-2, Envisat, and CryoSat-2 satellite altimeter missions between 1992 and 2017 and simulations of snowfall over the same period produced by the RACMO regional climate model.

Together, these measurements allow changes in the ice sheet height to be separated into those due to weather patterns, such as less snowfall, and those due to longer-term changes in climate, such as increasing ocean temperatures that eat away ice.

In parts of Antarctica, the ice sheet has thinned by extraordinary amounts, and so we set out to show how much was due to changes in climate and how much was due to weather," said lead author and CPOM Director Professor Andy Shepherd.

The team compared the measured surface height change to the simulated changes in snowfall, and where the discrepancy was greater they attributed its origin to glacier imbalance.

The researchers found that fluctuations in snowfall tend to drive small changes in height over large areas for a few years at a time, but the most pronounced changes in ice thickness are signals of glacier imbalance that have persisted for decades.

"Knowing how much snow has fallen has really helped us to detect the underlying change in glacier ice within the satellite record. We can see clearly now that a wave of thinning has spread rapidly across some of Antarctica's most vulnerable glaciers, and their losses are driving up sea levels around the planet," Shepherd added.

"Altogether, ice losses from East and West Antarctica have contributed 4.6 millimetre (mm) to global sea level rise since 1992," said Shepherd.

"This is an important demonstration of how satellite missions can help us to understand how our planet is changing. The polar regions are hostile environments and are extremely difficult to access from the ground. Because of this, the view from space is an essential tool for tracking the effects of climate change," said Marcus Engdahl of the European Space Agency, a co-author of the study.