ANTARTIC          -           It’s long been suspected but scientists can now show conclusively that thinning in the ring of floating ice around Antarctica is driving mass loss from the interior of the continent.

A new study finds the diminishing thickness of ice shelves is matched almost exactly by an acceleration in the glaciers feeding in behind them.

“The response is essentially instantaneous,” said Prof Hilmar Gudmundsson from Northumbria University, UK.

“If you thin the ice shelves today, the increase in flow of the ice upstream will increase today - not tomorrow, not in 10 or 100 years from now; it will happen immediately,” he told BBC News.

The edge of Antarctica is bounded by thick platforms of floating ice. These “shelves” have formed as the continent’s many glaciers have drained off the land into the sea. On entering the water, their buoyant ice fronts have lifted and joined together to form a single protrusion.

But these shelves are being besieged by the invasion of warm ocean water that’s now eating their undersides. And satellite data over the past 25 years has shown many to be thinning as a consequence. “That’s a problem because the ice shelves act as a kind of architectural buttress, slowing the movement of the ice sheet behind them,” explained Prof Helen Fricker, a satellite expert from the Scripps Institution of Oceanography, US. “So, if you thin an ice shelf, the grounded ice behind can speed up, but what we didn’t know was by how much - and that’s where our work with Hilmar and his modelling comes in.”

Prof Gudmundsson put Scripps’ satellite data of shelf thinning into a numerical ice sheet model to see how the land ice should respond based on the current best understanding of the physics involved. What the UK-US team found was that the predicted changes in the patterns of speed-up tallied precisely with what has been observed in the real world. What was previously just a correlation is now supported by quantifiable evidence.

“If the thinning of the ice shelves is driving the mass loss in the grounded ice, we would expect the pattern in the changes in velocities to match the observations - and that’s exactly what we find,” said Prof Gudmundsson.

The biggest changes are seen in the West of the continent, where huge glaciers such as Pine Island and Thwaites have accelerated in response to their denuded ice shelves. The ice volume contained in just these two ice streams would push up global sea levels by 1-2m - if it were all to melt out. The least change over the 25 years is seen in the East of the continent where shelves and their feeding glaciers have been largely stable.