December 15, 2016, by Tim Radford
Aerial view of the vast Greenland ice sheets that are melting at an alarming rate.
Image: UN Photo/Mark Garten via Flickr
Bedrock drilled from deep under the rapidly-melting Greenland ice sheet contains evidence that the island may once have been almost totally ice-free.
LONDON, 15 December, 2016 – Greenland, which has an icecap that holds enough water to raise global sea levels by at least seven metres, has melted almost completely in the recent geological past, according to new research.
And if it did so once, then perhaps it could be denuded again as the Arctic region warms at an alarming speed.
The conclusion is far from certain – a second paper delivers different evidence and a contrasting conclusion.
But researchers from Columbia University, New York, and others report in Nature journal that the chemistry of the bedrock below Greenland’s ice suggests that it must have been exposed − for entirely natural reasons − to the atmosphere for 280,000 years in the last 1.4 million years.
The researchers cannot be sure whether this chemical signature represents one big event, or whether the ice vanished many times over shorter periods.
“Unfortunately, this makes the Greenland ice sheet look highly unstable,” says Joerg Schaefer, a palaeoclimatologist at Columbia’s Lamont-Doherty Earth Observatory. “If we lost it in periods of natural forcing, we may lose it again.”
Right now, the surface ice of Greenland has been melting at a faster rate, and glaciologists have reported dramatic increases in the flow rate of the island’s giant frozen rivers. There is evidence, too, that both processes could accelerate.
In the last four years, as a result of global warming driven by greenhouse gas emissions from the combustion of fossil fuels, Greenland has shed an estimated trillion tons of ice, and sea levels are creeping up at three millimetres a year.
The study is based on rock samples reached after five summers of drilling at the highest part of the island’s ice sheet.
“This makes the Greenland ice sheet look
highly unstable. If we lost it in periods
of natural forcing, we may lose it again”
The scientists brought their 1.55 metre cores to the surface in 1993, but it has taken another two decades for laboratory techniques to detect and interpret the significance of radioactive particle samples in the rock that could only have come from outer space – which is why scientists think the bedrock must have been exposed, possibly more than once.
But a second study in Nature reports that the story told by rock samples collected off the east coast of Greenland suggests that the ice may not have melted for several million years.
Remnant of ice
Paul Bierman, a geomorphologist and geochemist at the University of Vermont, US, and colleagues say their finding cannot confirm whether a remnant of ice covered only the eastern side of the island, or whether the entire island remained sheathed in ice.
“It’s quite possible that both of these records are right for different places,” he says. “Both of these studies apply a similar innovative technique and let us look much further into the past than we have been able to before.”
The conclusion, drawn by both groups, is that not enough is yet known about the dynamics of the Greenland ice sheet. “And right now, because we’re pumping huge plumes of greenhouse gases into the atmosphere, we really need to know how our world works,” Dr Bierman says.
“But there’s enough sea-level rise tied up in Greenland alone to put a lot of cities and long stretches of coastline under water − including Donald Trump’s property in Florida.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.