News in Science

Greenland ice a benchmark for warming

Core data Greenland was about eight degrees warmer 130,000 years ago than it is today, an analysis of an almost three-kilometre-long ice core in Greenland has revealed.

The finding by an international team of 38 institutions from 14 nations provides an important benchmark for climate change modelling and gives an insight into how the natural world will respond to global warming in the future.

The study, which involves CSIRO researchers, also suggests Antarctica's ice sheets may be more vulnerable to warming than previously thought.

Published in today's Nature journal, the results flow out of a four-year expedition known as the North Greenland Eemian Ice Drilling operation (NEEM).

Dr David Etheridge, principal research scientist with CSIRO Marine and Atmospheric Research who has worked on the project, says the NEEM program is the first to successfully reach down into Greenland's ice core into the Eemian period, which stretched from 130,000 years to 115,000 years ago.

"It has been something of a holy grail for Greenland work to achieve this … we are getting to ice close to the bedrock where you get melting and mixing of the ice layers."

Etheridge says in a process similar to assembling a jigsaw puzzle, scientists used comparisons with gas elements in Antarctica's deep ice core records to re-assemble the layers in their original sequence. Deep ice drilling in the Antarctic has reached as far back as 800,000 years.

Past and future

It is important to understand what happened in Greenland during the Eemian period because the temperatures experienced then are "within the realms of where we are heading", says Etheridge.

However, he says the previous warming was due to the Earth receiving more of the Sun's radiation due to its orbit at the time, while today's warming is being driven by increases in greenhouse gases in the atmosphere.

Nature paper co-author Dr Mauro Rubino, of CSIRO Marine and Atmospheric Research, says it had been previously estimated that Greenland's temperature was about 4°C warmer during the Eemian than now.

But this latest work used analysis of water-stable isotopes to estimate "the temperature 130,000 years ago was up to 8°C warmer [in Greenland] than what it is today", says Rubino.

It also shows sea levels were on average 6 metres higher.

The results provide "important benchmarks for future climate change projections" in temperature and the contribution of the two main ice sheets to sea level rises, Rubino says.

He says the study also reveals the Greenland ice sheet did not melt as much as previously thought so was not the major contributor to sea level at that time.

"It shows the major contribution to sea level rises was not coming from the Greenland ice shelf," he says.

"It was previously believed that Greenland melted entirely [during the Eemian], but in fact the ice sheet was not that much different from what it is now.

Antarctica vulnerable?

"Most of the contribution to sea level rise comes from these two big ice reserves [in Greenland and the Antarctica] so one of the possible interpretations is Antarctica is more susceptible to climate change than we thought."

Etheridge agrees. He says the work shows the Greenland ice sheet survived during the Eemian - although it was about 400 metres thinner.

"From that figure you can deduce how much it contributed to the sea level rise and it is not as much as was thought.

"That throws things back to Antarctica ... previously the thought was Antarctica was too cold and too stable to be impacted."

Etheridge says CSIRO was invited by lead institution, the University of Copenhagen, to be involved in NEEM at its formation because of its expertise in analysing air composition in air bubbles trapped in deep ice.

Rubino says their team began analysis of gas bubbles from the first 80 to 100 metres of ice core down to the final 2540 metre depth.

This helped track changes in climate and temperature on a year-by-year basis.

He says the concentration of greenhouse gases such as carbon dioxide, methane and nitrous oxide in the air bubbles from the Eemian was much lower than what it is today.