Over 40% of Antarctica's ice shelves have shrunk in 25 years

An international team of scientists has found that 71 of the 162 ice shelves surrounding Antarctica have reduced in volume over 25 years from 1997 to 2021, with a net release of 7.5 trillion tonnes of meltwater into the oceans. The researchers analysed over 100,000 satellite radar images to produce this major assessment of the ‘state of the health’ of the ice shelves, with their results published in the journal Science Advances.

Ice shelves float on the seas surrounding Antarctica and are extensions to the ice sheet that covers much of the continent. The ice shelves play an important role in buttressing, or slowing the flow of glaciers to the sea, by acting as giant ‘plugs’ at the end of these glaciers. When the ice shelves thin or reduce in size, the plugs weaken — with the result that the rate of ice lost from the glaciers increases.

The scientists found that almost all the ice shelves on the western side of Antarctica experienced ice loss over the 25 years; in contrast, most of the ice shelves on the eastern side stayed the same or increased in volume. Almost 67 trillion tonnes of ice was exported to the ocean, which was offset by 59 trillion tonnes of ice being added to the ice shelves, giving a net loss of 7.5 trillion tonnes.

“There is a mixed picture of ice-shelf deterioration, and this is to do with the ocean temperature and ocean currents around Antarctica,” said study leader Dr Benjamin Davison, from the University of Leeds.

“The western half is exposed to warm water, which can rapidly erode the ice shelves from below, whereas much of East Antarctica is currently protected from nearby warm water by a band of cold water at the coast,” Davison said. The seas on the western side of Antarctica experience different currents and winds than those on the east, and this is driving warmer water underneath the ice shelves on the western flank.

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

“We expected most ice shelves to go through cycles of rapid, but short-lived shrinking, then to regrow slowly,” Davison said. “Instead, we see that almost half of them are shrinking with no sign of recovery.”

Study co-author Professor Anna Hogg, also from the University of Leeds, added, “Many of the ice shelves have deteriorated a lot: 48 lost more than 30% of their initial mass over just 25 years.

“This is further evidence that Antarctica is changing because the climate is warming.

“The study provides a baseline measure from which we can see further changes that may emerge as the climate gets warmer.”

If the ice shelves disappear or even diminish, there will be major knock-on effects for the ice system on Antarctica and for global ocean circulation — the giant ‘conveyor belt’ which moves nutrients as well as heat and carbon from this sensitive polar ecosystem. In the Southern Ocean, dense salty water sinks to the ocean floor as part of the global ocean conveyor belt; this sinking of water acts as one of the engines that drive the ocean conveyor belt.

Fresh water released from the Antarctic ice shelves dilutes the salty ocean water, making it lighter and thus taking longer to sink — this can weaken the ocean circulation system. Over the 25-year study period, the researchers estimated that 66.9 trillion tonnes of fresh water went into the Southern Ocean around Antarctica from the ice shelves alone.

Indeed, a modelling study recently published in Nature Climate Change has shown that future increases in ocean-driven melting of the West Antarctic Ice Sheet over the 21st century may be unavoidable, suggesting that rapid ocean warming in West Antarctica is already committed under a range of emission pathways, and mitigation efforts may only prevent the worst-case scenarios.

The West Antarctic Ice Sheet is Antarctica’s largest contributor to sea-level rise, with previous modelling finding that its loss of ice could be driven by warming of the Southern Ocean, particularly the Amundsen Sea region. Researchers from the British Antarctic Survey used a regional ocean model to understand future changes in ocean heat under different emissions scenarios in the Amundsen Sea, and found that even under a range of mitigation scenarios — including Paris Agreement 1.5°C, Paris Agreement 2°C and Representative Concentration Pathway (RCP) 4.5 — climate change could cause the ocean to warm at three times the historical rate. This indicates that mitigation efforts may have limited power to slow ocean warming in the Amundsen Sea in the coming decades, the researchers suggested.

“It looks like we’ve lost control of melting of the West Antarctic Ice Sheet,” said lead author Dr Kaitlin Naughten. “If we wanted to preserve it in its historical state, we would have needed action on climate change decades ago. The bright side is that by recognising this situation in advance, the world will have more time to adapt to the sea level rise that’s coming.”

The team noted that natural internal climate variability will also play a role in controlling the amount of warming caused by climate change. They project increased melt in areas crucial for buttressing the ice sheet and maintaining stability, with warming concentrated at an intermediate (200–700 m) depth that accesses the ice-shelf cavities and leads to melt.

The authors acknowledged that their work is based on outputs from a single ice–ocean model. They noted that mass loss from the West Antarctic Ice Sheet is just one component of sea-level rise, and other regions of Antarctica are unlikely to lose substantial mass if current emissions targets are met. Since the ice sheet will take centuries or millennia to fully respond to climate change, the choice of emissions scenario could have a larger impact further in the future, beyond the 21st century, the authors concluded.

“We must not stop working to reduce our dependence on fossil fuels,” Naughten said. “What we do now will help to slow the rate of sea level rise in the long term. The slower the sea level changes, the easier it will be for governments and society to adapt to, even if it can’t be stopped.”

Image credit: British Antarctic Survey.