Antarctic cores reveal ice history

Antarctica's history can be read in the sediment cores
Image: Cape Roberts science team

By BBC News Online's Kim Griggs

Sediment cores drilled at the edge of Antarctica show global sea levels rose and fell in a dramatic cycle 34 to 15 million years ago.

The research suggests the oceans went up and down by between 50 and 65 metres, as the main ice sheet on the eastern side of the White Continent advanced and retreated in a climate that was 3-4 degrees warmer than today.

"We are seeing the ice margin grounding itself, coming across the drill site, returning again to relatively ice-free, open marine conditions during warm periods, and then coming back again during glacial cold periods," said Dr Tim Naish, who led an international team of scientists working at Cape Roberts.

The study could help climatologists and modellers as they attempt explain future changes in the Earth's climate.

North and south

Scientists already believe that in the past two million years, changes in the Northern Hemisphere, which occurred in tune with small "wobbles" in the Earth's orbit, were likely to have been the main contributor to shifts in global sea level of up to 100 metres.

The ice sheet expanded and retracted with changes in the Earth's orbit
Image: Cape Roberts science team

The changes, called Milankovitch cycles, are known to happen roughly every 20,000, 40,000 and 100,000 years.

"The question is what was Antarctica's role in driving global climate, prior to there being a Northern Hemisphere ice sheet - and during," Dr Naish, a scientist at New Zealand's Institute of Geological and Nuclear Sciences, said.

Now, his team have, in the sediment cores drilled at Cape Roberts, evidence to show that the East Antarctic ice sheet has been unstable in the same way as the northern ice sheet.

Fine detail

For three of the cycles of Antarctic ice sheet change, Naish's team has been able to pin down ages using fossil, chemical and magnetic evidence from the cores.

Milutin Milankovitch (1879 to 1958)

The Serbian scientist showed how small changes, or "wobbles", in the Earth's orbit that influence the amount of solar radiation striking different parts of the planet could cause the advance and retreat of the polar ice caps

"Because we have good control over [the age], we can now see how long it takes for each cycle to occur and they are occurring at Milankovitch frequencies," said team member Dr Michael Hannah, from Victoria University of Wellington, NZ.

But there is more work to be done. "The key is being able to constrain the timing so you know when and how fast, and unfortunately only for certain parts of the core do we have good enough dating to be able to say 'yes, it was happening every 40,000 years' in certain parts, and that the deglaciations could have been as quick as 10,000 years or even quicker," Dr Naish said.

What appears to make the massive East Antarctic ice sheet more responsive to the orbital forcings is the higher temperatures those millions of years ago.

Future change

These would have been about three to four degrees higher than the mean global surface temperature today. Carbon dioxide concentrations would also have been double what they are now.

These are the sort of conditions scientists working for the United Nations' Intergovernmental Panel on Climate Change (IPCC) suggest could be reached again in a hundred years - if their theories and models of global warming are correct.

The Cape Roberts study will therefore provide useful information on how ice coverage in Antarctica might respond in the years to come.

"What we are saying is that if we increase the mean temperature of the Earth by three degrees, we'll go back through a threshold that will take us to a point where the Antarctic ice sheet becomes very responsive to orbital forcings like it was. Currently, it's sort of buffered because the overall temperature is cold enough," Dr Naish said.

The Cape Roberts research is published in the journal Nature.