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Greenland ice sheet probably more stable than we thought

  • 24 Jan 2013, 14:30
  • Roz Pidcock

Sepp Kipfstuhl

The ice covering Greenland may be less sensitive to rising temperatures than previously thought, according to a new study which examines the effect of past warming on the ice sheet. But it's not all good news - it could mean ice in Antarctica is more vulnerable. Here's a look at how scientists found all this out, and what it means.

The new research, published today in Nature, is the first time scientists have managed to retrieve a complete ice core from the Greenland ice sheet. From this, they were able to reconstruct a record of temperature changes over Greenland going more than 100,000 years back in time.

The results surprised the scientists for a couple of reasons. First, it had been thought that the temperature during the last interglacial period - between about 115,000 and 130,000 years ago - was just a few degrees Celsius higher than now. The new study suggests a bigger difference - more like eight degrees higher.

Secondly, the ice cores indicated only a "modest ice sheet response" from the Greenland ice sheet to such warming - which means that Greenland melt didn't contribute as much to past sea level rise as previously thought.

Ice cores and air bubbles

Over a period of four years, a team of 300 scientists and students from 14 different countries, led by Professor Dorthe Dahl-Jensen of the Niels Bohr Institute in Denmark, drilled right through the 2.5 km Greenland ice sheet. Earlier attempts to do this have been unsuccessful, but this time the result was a set of ice cylinders - each one 10.2 cm in diameter and 3.5 m long -  formed of layer upon layer of compressed snow.

Like tree rings, the layers of snow and bubbles of gas trapped in the ice tell scientists about climatic conditions when they formed. The ratio of particular forms of oxygen and nitrogen reveal information about ambient temperature, greenhouse gas levels and moisture levels.

Icecore _2009

Lead scientist Dorthe Dahl-Jensen with a section of the ice core. Credit: Sepp Kipfstuhl

Warmer than expected

Before this study, scientists estimated the last interglacial period - known as the Eemian - was between three to five degrees Celsius warmer than present day. The Eemian interglacial was caused by a change in the earth's orbit, altering the amount of solar energy delivered to earth.

But the new ice core suggests temperatures over Greenland peaked at eight degrees Celsius above the average for the past millenium - considerably more than previously thought.

In response to this warming, ice mass in Greenland started to shrink at about six cm per year - but it did not disappear, according to the study. In fact, the data show the ice sheet decreased in thickness by about 400 metres, or less than 25 per cent of its volume. Uncertainty around the measurements, however, means that the loss in thickness could have been as little as 150 m or as much as 650 m.

As Dahl-Jensen explains, this reduction in volume is less than the scientists expected.

"The good news from this study is that the Greenland ice sheet is not as sensitive to temperature increases … in warm climate periods like the Eemian as we thought"

When ice on land melts and runs into the ocean, it causes sea level to rise. So the implication of the new results is that the Greenland ice sheet contributed less to global sea level rise during the Eemian than scientists previously thought - only about two meters.

Not all good news

While it seems like good news that the Greenland ice sheet might not melt as much as previously thought when temperatures rise, it's only half the picture. Scientists estimate sea levels during the Eemian rose to be about four to six metres higher than today. If Greenland's ice sheet only contributed less than half - as the new study suggests - it suggests that ice melt from the other large body of land ice, the Antarctic ice sheet, was responsible for the rest.

As Professor Jim White, the lead US scientist on the project explains:

"When we calculated how much ice melt from Greenland was contributing to global sea rise in the Eemian, we knew a large part of the sea rise back then must have come from Antarctica"

He continues:

"A lot of us were leaning in that direction for some time, but we now have evidence that confirms that the West Antarctic ice sheet was a dynamic and crucial player in global sea level rise during the last interglacial period"

Present day

In summer, about half of the surface of the Greenland ice sheet melts naturally in the warmer temperatures. But in the last ten years, Greenland has experienced intense surface melting similar to the kind seen during the Eemian, say the scientists. Satellite measurements from July 2012 showed 97 per cent of the surface of Greenland melted - more than at any time in the last 30 years of satellite measurements. This could be set to continue, as Dahl-Jensen explains:

"The warming that is predicted to occur over the next 50-100 years will potentially have Eemian-like climatic conditions".

So looking at what happened during the Eemian can help scientists better understand how present day ice sheets may respond. Previously, the IPCC suggested in its 4th Assessment Report that the contribution to sea level rise from the Greenland ice sheet is likely to be more significant than the Antarctic ice sheet, but that could change if ice flows from West Antarctic accelerate.

Scientists are concerned that rising atmospheric and oceanic temperatures around the West Antarctic ice sheet are threatening to melt the floating ice shelves that keep the ice sheet's interior in place. A collapse of the ice shelves and the Western Antarctic ice sheet could raise global sea levels by more than three metres. The possibility that this happened during the Eemian - and could happen again - cannot be discarded, say scientists.

So not only is this new study a scientific first in terms of analysing a complete Greenland ice core, if the Eemian is considered an analog for current and future warming, the new research may also help refine predictions about ice sheet responses and future sea level rise.

 

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