If climate heating continues apace and the planet goes on warming, then up to a third of Antarctica’s ice shelf could tip into the sea.
And tip is the operative word, according to a separate study: at least one Antarctic glacier could be about to tip into rapid and irreversible retreat if temperatures go on rising.
And rise they could: evidence from the past in a third research programme confirms that at the end of the last Ice Age, Greenland’s temperature rose by somewhere between 5°C and 16°C in just decades, in line with a cascade of climate change events.
And ominously a fourth study of climate change 14,600 years ago confirmed that as the ice retreated, sea levels rose at 10 times the current rate, to 3.6 metres in just a century, and up to 18 metres in a 500-year sequence.
Each study is, on its own, an examination of the complexities of the planetary climate machine and the role of the polar ice sheets in climate change. But the message of the four together is a stark one: climate change is happening, could accelerate and could happen at unexpected speeds.
Unstable at 4°C
The Antarctic ice sheet floats on the sea: were it all to melt, sea levels globally would remain much the same. But the ice sheet plays an important role in stabilising the massive reserves of ice on the continental surface.
“Ice shelves are important buffers preventing glaciers on land from flowing freely into the ocean and contributing to sea level rise,” warned Ella Gilbert, a meteorologist at the University of Reading in the UK. “When they collapse, it’s like a giant cork being removed from a bottle, allowing unimaginable amounts of water from glaciers to pour into the sea.”
She and colleagues report in the journal Geophysical Research Letters that their detailed study of the vulnerable platforms of floating ice around the continent revealed that half a million square kilometres of shelf − 34% in total, including two-thirds of all the ice off the Antarctic Peninsula − would become unstable if global temperatures rose by 4°C, under the business-as-usual scenario in which nations went on burning ever-greater quantities of fossil fuel.
If however the world kept to the limit it agreed in Paris in 2015, that would halve the area at risk and perhaps avoid significant sea level rise. But already, just two Antarctic glaciers are responsible for around 10% of sea level rise at the current rate, and researchers have been warning for years that the Pine Island and Thwaites glaciers in West Antarctica could be at risk.
Now researchers in the UK report in the journal The Cryosphere that their computer simulation had identified a series of tipping points for the Pine Island flow.
“Ice shelves are important buffers preventing glaciers on land from contributing to sea level rise. When they collapse, it’s like a giant cork being removed from a bottle, allowing unimaginable amounts of water to pour into the sea”
The third of these, triggered by ocean temperatures that had warmed just 1.2°C, would lead to irretrievable retreat of the entire glacier. Hilmar Gudmundsson, a glaciologist at the UK’s Northumbria University and one of the authors, called the research a “major step forward” in the understanding of the dynamics of the region.
“But the findings of this study also concern me”, he said. “Should the glacier enter unstable irreversible retreat, the impact on sea level could be measured in metres, and as this study shows, once the retreat starts it might be impossible to halt it.”
Rapid polar melt is part of the pattern of climate history. Danish researchers report in Nature Communications that, on the evidence preserved in Greenland ice cores, they identified a series of 30 abrupt climate changes at the close of the Last Ice Age, affecting North Atlantic ocean currents, wind and rainfall patterns and the spread of sea ice: a set of physical processes that changed together, like a row of cascading dominoes.
The precise order of events was difficult to ascertain, but during that sequence the temperature of Greenland soared by 5°C to 16°C in decades to centuries. The question remains open: could such things happen today?
“The results emphasise the importance of trying to limit climate change by, for example, cutting anthropogenic emissions of CO2 and other greenhouse gases, both to reduce the predictable, gradual climate change and to reduce the risk of future abrupt climate change,” said Sune Olander Rasmussen, at the Niels Bohr Institute in Copenhagen, one of the authors.
Greenland’s future role
“If you do not want the dominoes to topple over, you are better off not to push the table they stand on too much.”
And another study in the same journal by British scientists reports on a close study of geological evidence to decipher the pattern of events during the largest and most rapid pulse of sea level rise at the close of the last Ice Age.
Their study suggested that although the sea levels rose 18 metres in about 500 years − a rate of about 3.6 metres a century − it all happened with relatively little help from a melting Antarctica. As the great glaciers retreated from North America, Europe and Asia, so the oceans rose.
“The next big question is to work out what triggered the ice melt, and what impact the massive influx of meltwater had on ocean currents in the North Atlantic,” said Pippa Whitehouse of the University of Durham, one of the researchers.
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