23 January, 2013

Greenland ice core reveals warm past temperatures

British Antarctic Survey scientists have contributed to a new study published in Nature (Thursday 24 January) that provides surprising details on changes in Earth’s climate from more than 100,000 years ago.

Made possible by an international team of researchers’ analysis of a new deep ice core from the Greenland ice-sheet indicates the last interglacial period (the warm period between successive ‘ice ages’) may provide a good picture of where the planet is heading in the face of increasing greenhouse gases and warming temperatures.

The new results from the North Greenland Eemian Ice Drilling project (NEEM) led by the University of Copenhagen shows that 130,000 to 115,000 years ago during the Eemian interglacial, the climate in North Greenland was about eight degrees Celsius warmer than at present.

Despite the strong warming signal during the Eemian — a period when the sea level was roughly four to eight meters higher than today — the surface in the vicinity of NEEM was only about 130 metres lower than its present level, which indicates the Greenland ice-sheet may have contributed less than half of the total increase in sea level at the time.

Project leader Dorthe Dahl-Jensen of the University of Copenhagen said,

“The new findings reveal higher temperatures in Northern Greenland during the Eemian than paleo-climate models have estimated.”

Following the previous glacial period, 128,000 years before present, the surface elevation in the vicinity of NEEM was 200 meters higher than today, but then over the next 6,000 years it reduced rapidly to 130m below the current elevation before levelling off and remaining stable at an ice thickness of 2,400m in the late Eemian 122,000 to 115,000 years before the present.

That rate of surface lowering was 6cm per year, and the research team estimate that the rate of mass loss from the Greenland ice sheet was likely to be on the same order as changes observed during the last ten years.

Intense surface melt in the vicinity of NEEM during the warm Eemian period was seen in the ice core as layers of re-frozen melt-water. Melt-water from surface snow had penetrated the underlying snow, where it re-froze. Such melt events during the past 5,000 years are very rare by comparison, confirming that the surface temperatures at the NEEM site during the Eemian were significantly warmer than today, said the researchers.

The team was in Greenland during the summer of 2012 during a rare modern melt event.

Professor Dorthe Dahl-Jensen, continues,

“We were quite shocked by the warm surface temperatures observed at the NEEM ice camp in July 2012. It was simply raining, and, just as during the Eemian period, meltwater formed subsurface ice layers. While this was an extreme event, the present warming over Greenland makes surface melt more likely, and the predicted warming over Greenland the next 50–100 years will potentially lead to Eemian-like climate conditions.

“The good news from this study is that Greenland is not as sensitive as we thought to temperature increases in terms of disgorging ice into the ocean during interglacial periods.

“The bad news is that if Greenland did not disappear during the Eemian, Antarctica, including the more dynamically unstable West Antarctica, must be responsible for a significant part of the 4–8 m sea level rise.”

The Greenland ice core layers — formed over millennia by compressed snow — are being studied in detail using a wide range of measurements, including stable water isotope analysis that reveals information about temperature and moisture changes back in time. Laser-based instruments are used to measure the water isotopes and atmospheric gas bubbles trapped in the ice cores to better understand past variations in climate on a year-by-year basis — similar in some ways to a tree-ring record.

“It’s a great achievement for science to gather and combine so many measured ice core records to reconstruct the climate history of the past Eemian.” said Dahl-Jensen. “It shows what a great team of researchers we have assembled and how valuable these findings are.”

The NEEM project

Led by the University of Copenhagen and involving 14 nations, the team drilled more than 2.5 km to bedrock in just over two years as part of the North Greenland Eemian Ice Drilling project, or NEEM. The team extracted the first complete ice core record from the Arctic of the last interglacial period known as the Eemian, providing past temperatures, precipitation and levels of atmospheric gases to better understand the current and future warming of Earth that virtually all climate scientists attribute to increases in human-produced greenhouse gases.

Co-author of the Nature paper Dr Robert Mulvaney, an ice core expert at British Antarctic Survey, said,

“For a long time we have said the Eemian paints a picture of how our present climate might develop over the next centuries if climate change continues as we expect. We have also thought that Greenland was a major source of the sea-level rise in the last Inter-Glacial period, but this research shows its contribution was not as much since it appears that less ice melted than we’d believed. But, as we know sea level during this period was higher than today, the implication is that there must have been a significant contribution from melting of the Antarctic, and the obvious place to look there is the West Antarctic Ice Sheet (WAIS). We already know that changes are taking place there today, and this result from Greenland tells us that it imperative to maintain a watchful eye on the state of the WAIS.

“During the drilling operation in Northern Greenland, which was carried out in a covered pit excavated eight meters deep into the ice, scientists set up their instruments in a small, warm cabin within the ice cave where temperatures were around minus 20 degrees Celsius. For 24 hours a day teams of scientists made measurements on the ice core. BAS scientists measured aerosols blown into the site from the surrounding ocean and trapped in the ice. We will use this information to understand the amount of sea ice in the Arctic in the past.”

NEEM is an international ice core research project aimed at retrieving an ice core from North-West Greenland (camp position 77.45°N 51.06°W) reaching back through the previous interglacial, the Eemian. The project logistics is managed by the Centre for Ice and Climate, Denmark, and the air support is carried out by US ski equipped Hercules managed through the US Office of Polar Programs, National Science Foundation.

ENDS

The paper “Eemian interglacial reconstructed from a Greenland folded ice core” by Dahl-Jensen et al is published in Nature.

Press Office Contacts

Please contact the British Antarctic Survey Press Office for a copy of the paper and to arrange an interview with Dr Robert Mulvaney.

Paul Holland – Tel: +44 (0)1223 221226; email: pbmho@bas.ac.uk

Scientists’ contact details

Denmark

Professor Dorthe Dahl-Jensen (Lead Author)
Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen
Email: ddj@gfy.ku.dk

UK

Dr Robert Mulvaney
British Antarctic Survey
Email: rmu@bas.ac.uk; Tel: 01223 221436

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