Vast Antarctic ice shelves shrinking as ocean warms

More than 40 per cent of Antarctica’s ice shelves have shrunk over the past 25 years and released vast amounts of fresh water into the ocean, with major implications for the “conveyor belt” of ocean currents that circulate heat and nutrients around the globe.

About 70 of the 160 ice shelves that surround Antarctica reduced in volume between 1997 and 2021, with a net release of 7.5 trillion tonnes of meltwater into the ocean, scientists from the University of Leeds say in a new paper released on Friday in the Scientific Advances journal.

These ice shelves are a floating extension of the Antarctic ice sheet as it pushes out over the ocean. They act as giant “plugs” at the end of glaciers, slowing the flow of ice, but if they thin or shrink the rate of ice lost to the ocean increases.

Thinning ice shelves release a vast quantity of fresh water into the oceans.Credit: iStock

“We expected most ice shelves to go through cycles of rapid but short-lived shrinking, then to regrow slowly. Instead, we see that almost half of them are shrinking with no sign of recovery,” said Dr Benjamin Davison, an expert in earth observation of polar regions from the University of Leeds.

The researchers looked at more than 100,000 satellite radar images to produce their major assessment of the state of health of the ice shelves.

It found shelves on the western side of Antarctica lost ice, while those on the eastern side remained the same or increased in volume. Over 25 years, some 67 trillion tonnes of ice melted into the ocean, while 59 trillion tonnes formed on the ice shelves, meaning there was a net loss of 7.5 trillion tonnes.

The graphic shows the water temperatures around the Antarctic. On the western side, the temperature at the sea floor is approaching 2 degrees, warm enough to melt the ice that is flowing on top of it. Sea temperatures on the eastern flank are colder.Credit: Dr Benjamin Davison/University of Leeds

The difference in the two sides relates to ocean temperatures and currents around the vast Antarctic continent, which is nearly twice the size of Australia. The western side is exposed to warm water while the east is currently protected by a band of cold water.

Last month, it was reported penguin populations in the Antarctic had suffered catastrophic losses, with no chicks surviving the spring of 2022 in four of five colonies observed in a new study. The loss of the chicks coincides with record low sea ice coverage.

Dr Edward Doddridge, a physical oceanographer at the Australian Antarctic Program Partnership, said it was good news the east Antarctic coastline was not yet showing the retreat or thinning of ice shelves.

“The word ‘yet’ is doing a lot of work there, but it is good news at the moment,” Doddridge said.

But he noted the study covered only until 2021: “We have some indications that warm subsurface water is starting to get up to the ice shelves in Antarctica ... [and] east Antarctica is one of the regions we are worried about.”

Human-induced global warming was likely to be a key factor in the loss of the ice, the study found. If it had been due to natural variation in climate patterns, there would have been some signs of ice regrowth on the western ice shelves.

The Getz ice shelf, in west Antarctica, has had some of the largest ice losses. Only 5 per cent was due to “calving” – where chunks of ice break off – and the rest was due to melting at the base of the ice shelf, the scientists said.

If the ice shelves diminish, or even disappear, this will affect global ocean circulation, the giant deep sea “conveyor belt” that moves nutrients, heat and carbon from this sensitive polar ecosystem, scientists say.

In the Southern Ocean, salty, dense water sinks to the bottom of the ocean and is one of the drivers of the “conveyor belt”. But fresh water released from melting ice dilutes this salty water and this means it takes longer to sink, which can slow the ocean’s circulation system.

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