Researchers working at the Prudhoe Dome in Greenland
Caleb K. Walcott-George
An ice dome in northern Greenland once melted completely at temperatures the region could experience again this century, a finding that will begin to paint a more accurate picture of how fast the melting ice sheet could raise global sea levels.
Researchers drilled 500 metres down through the centre of the Prudhoe Dome, a bulge of ice the size of Luxembourg in the northwestern corner of Greenland, to collect a 7-metre core of sediment and bedrock. A dating technique using infrared light showed that sand at the surface of the core was bleached by the sun about 7000 years ago. That means the dome was completely melted at that time, when the planet was emerging from the last glacial maximum because of subtle shifts in its orbit around the sun.
Summers in the area then were 3-5°C warmer than today, temperatures they could reach again by 2100 under human-made climate change.
“This is very direct evidence that the ice sheet is as sensitive as we feared to even a relatively small amount of warming that happened in the Holocene,” says Yarrow Axford at Northwestern University in Illinois, who was not involved in the research.
The melting of the Greenland ice sheet could unleash anywhere from tens of centimetres to 1 metre of sea level rise this century. To narrow that prediction, scientists need to better understand how fast different parts of the ice sheet will disappear.
The Prudhoe Dome core is the first of several taken by the GreenDrill project, funded by the National Science Foundation and involving researchers at several US universities. They hope to tease information about past climates from the ground under the ice sheet, which researchers have called the least explored part of earth’s land surface.
Sediment drilled in 1966 from under the ice at Camp Century, a United States nuclear-powered military facility that operated for eight years during the cold war, showed that northwestern Greenland was ice-free about 400,000 years ago. A bedrock core taken in 1993 from under Summit Station, a scientific research facility in the middle of Greenland, proved the entire ice sheet melted away as recently as 1.1 million years ago.
But GreenDrill has taken this under-ice work further by sampling several points near the northern coast.
“This question is, when have the edges of Greenland melted in the past?” says Caleb Walcott-George at the University of Kentucky, the lead author of the new study. “Because this is where … the first foot of sea level rise will come from.”
There has been some disagreement among ice sheet models about whether northern or southern Greenland will melt sooner in the future. This study adds to growing evidence that warming after the last glacial maximum was earlier and more intense in northern Greenland, Axford says.
A possible reason could be feedbacks like the disappearance of Arctic sea ice, which could have released more ocean heat into the atmosphere in the far north.
By proving that Prudhoe Dome melted with 3-5°C of warming, this study will give weight to those ice sheet models that give this result, says Edward Gasson at the University of Exeter in the UK, who wasn’t involved in the research.
“The thing that this will help is tuning surface melt models. When will we really start to lose this ice?” Gasson says.
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