Drilling triggers rethink on ice
Ice history ... data from complete cores drilled from the sheet goes back more than 120,000 years. Photo: James Balog
THE first complete ice cores drilled from the Greenland ice sheet that extend back more than 120,000 years to the Earth’s last great warming period reveal the ice sheet experienced surface temperatures about 8 degrees hotter than today and was 400 metres thinner.
An international team of scientists have used a 2549-metre long ice core from north Greenland to reconstruct surface temperatures and ice sheet thickness back to the previous interglacial period, when global temperatures averaged about three degrees warmer than today for several thousand years.
This period, known as the Eemian, is thought comparable to the climate conditions the world will likely face in the next century as a result of human-induced climate change.
‘‘The ice is an archive of past climate and analysis of the core is giving us pointers to the future, when the world is likely to be warmer,’’ said a CSIRO ice core scientist, Mauro Rubino, who participated in the study.
As the Greenland ice sheet continues to lose mass faster than ice sheets in Antarctica, scientists have been hoping to measure the contribution it made to global sea levels in the previous interglacial period, according to Dr Rubino.
Researchers estimate Greenland’s ice sheet volume shrank by about 25per cent over 6000 years, because of warming due to changes in the Earth’s orbit.
‘‘The findings show a modest response of the Greenland ice sheet to the significant warming in the early Eemian and lead to the deduction that Antarctica must have contributed significantly to the six-metre higher Eemian sea levels,’’ Dr Rubino said.
To determine Greenland’s surface temperature at different times, the team analysed the ice core’s water isotope levels as a proxy for surface temperature.
‘‘By measuring the variations in isotopes of water at different depths you can reconstruct the temperature back in time for that specific site,’’ Dr Rubino said.
The first two-thirds of the core represented climate conditions for the current interglacial period, known as the Holocene, which began about 11,000 years ago, while the bottom three hundred metres formed during the last interglacial period.
It took the North Greenland Eemian Ice Drilling project team, who publish their results in the journal Nature, several years to drill the ice core.
The last section was particularly difficult to date because the ice layers had folded and moved.
An atmospheric scientist also with CSIRO, David Etheridge, said the study provided insights into how the Earth’s climate system – the interaction between ice sheets, the atmosphere, the oceans and the land – behaved and would behave in the future.
‘‘One of the hardest things to model in climate is the contribution of polar ice sheet melting to sea level rise,’’ said Dr Etheridge, who was not involved in the study.
This study suggested the Greenland ice sheet may be more resilient than previous models had predicted, a finding that could be used to train future models, he said.