Old ice gives insights into future sea-level rise
Sea levels are already rising as the world heats up, and we know that melting polar ice will be the biggest contributor – but the questions is will most of it come from Greenland or Antarctica?
The IPCC is the international body of scientists tasked by the United Nations with assessing climate change science. In early October 2018, it released a ‘Special Report on Global Warming of 1.5°C'.
The report called on governments around the world to take urgent action to address climate change and keep global warming to 1.5°C. IPCC co-chairJim Skea said at the time that “limiting warming to 1.5°C is possible within the laws of chemistry and physics - but doing so would require unprecedented changes.”
One of the topics discussed in the report was sea-level rise, and how high it might be under different global warming scenarios. Some of the increase in sea level will come from thermal expansion, as the world’s oceans warm. But most of it will come from the melting of the world’s two great ice caps: Greenland and Antarctica.
Greenland is home to the second largest icecap in the world, after Antarctica. The Greenland icecap is up to three kilometres thick, and Professor Dorthe Dahl-Jensen, from the University of Copenhagen in Denmark, says that “if Greenland melted away we would get about seven metres of sea-level rise.”
Recently Dorthe has visited the ARC and gave lectures in Antarctic Studies for S.T. Lee Lecture series.
Professor Andrew Mackintosh, Director of the Antarctic Research Centre, says that in comparison the West Antarctic ice sheet holds about 7 metres equivalent sea rise locked up in its ice, while the giant East Antarctic ice sheet holds 52 metres of equivalent sea-level rise.
Dorthe says that she works “with ice core research. So we try to look back in time to see what happened to the Greenland ice sheet if we look at previous warm climate periods.”
About 130,000 years ago it was about 5°C warmer over Greenland than it is today; at tropical latitudes at the same time it was about 2°C warmer than today.
Global sea level back then was about six to nine metres higher than the present, and evidence from ice cores shows that about two metres came from melting of the Greenland ice cap.
There are no ice cores that go back 130,000 years in Antarctica, and as a result Andrew says it is more difficult to work out exactly where Antarctica’s sea-level rise contribution of four to five metres came from.
Until recently, it was thought that most of the melt would happen to the West Antarctic ice sheet, but recent evidence suggests that more melting than expected happened on the larger East Antarctic ice sheet.
Dorthe and Andrew agree that there are a lot of unknowns in trying to tease out what happened to the great ice sheets in the past, how they might react to different amounts of future warming and what the resulting rise in sea level might be.
“One of the biggest uncertainties is our behaviour in the future,” says Dorthe.
“Because it depends on how we react and how much greenhouse gases we put in the atmosphere, on how strong the temperature change is going to be.”
“We’re sort of playing with fire a little bit here,” says Andrew. “By warming a climate and not knowing quite when we might instigate a massive change in ice sheets and cause even more dramatic sea-level rise.”
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