Ice Cores Unlock Climate Secrets

Global climate patterns stretching back 740,000 years have been confirmed by a three kilometre long ice core drilled from the Antarctic, Nature reports. Analysis of the ice proves our planet has had eight Ice Ages during that period, punctuated by rather brief warm spells - one of which we enjoy today.

If past patterns are followed in the future, we can expect our "mild snap" to last another 15,000 years.

Initial tests on gas trapped in the ice core show that current carbon dioxide (CO2) levels are higher than they have been in 440,000 years.

Nobody quite knows how this will alter our climate, but researchers hope a detailed picture of past fluctuations will give them a better idea.

A cohort of scientists, from 10 different countries, has spent most of the last decade extracting the mammoth column of ice from a location called Dome C, on east Antarctica's plateau.

The European Project for Ice Coring in Antarctica, or Epica, aims to unlock the climatic secrets of our past - and in doing so gain a better understanding of what we can expect in the future.

This is not the first ice core project - but it ventures much further back in time.

Dome C contains 800,000 years worth of snowfall, allowing Epica to obtain a climate record two times longer than its nearest ice core rival.

"We think this project will really change the way we look at climate," said co-author Eric W. Wolff, of the British Antarctic Survey, Cambridge, UK.

For instance, scientists can work out climate by looking at the ratio of hydrogen isotopes.

Deuterium is a heavy isotope - or version - of hydrogen. If a sample of ice has a lot of it, that means the temperature was warmer - and vice versa.

"At very cold temperatures a great deal of the heavy isotopes have rained out," explained Jerry F. McManus, of Woods Hole Oceanographic Institution, US. "So all that is left is what we would call isotopically depleted or lighter. That is how we know how cold it was."

He added: "You might say Antarctica is always cold - and you'd be right. But there is great variation in the degree of cold."

Another important thing that scientists can 'read' in the ice, is the relative concentration of atmospheric gases.

That is because minute bubbles pock mark the core, within which tiny pockets of preserved air lie.

"That is the wonderful thing about ice cores," said Professor McManus. "There is air from three-quarters of a million years ago and it is still locked in these bubbles - it's incredible."

Eric W. Wolff, the British Antarctic Survey, UK Epica is still busy analysing the ice core's atmospheric gases, but preliminary results suggest that present CO2 levels are remarkably high.

"We have never seen greenhouse gases anything like what we have seen today," said Dr Wolff.

Over the last 800,000 years the Earth has, on the whole, been a pretty chilly place. Interglacials - or warm spells - have come every 100,000 years and have generally been short-lived.

Over the last 400,000 years, interglacials have lasted about 10,000 years, with climates similar to this one. Before that they were less warm, but lasted slightly longer.

We have already been in an interglacial for about 10,000 years, so we should - according to that pattern - be heading for an Ice Age.

The Epica team has noticed the interglacial period of 400,000 years ago closely matches our own - because the shape of the Earth's orbit was the same then as it is now.

"The next ice-age is not imminent," said Dr Wolff, "and greenhouse warming makes it even less likely - despite what the Day After Tomorrow says."

Epica scientists hope that after they have fully analysed the ice core's atmospheric gases, they will gain a deeper knowledge of how climate relates to them.

"We will double the timescale over which we can study greenhouse gases," said co-author Thomas F Stocker, of the University of Bern, Switzerland. "We will be able to show what the natural variability is in relation to gases like CO2."

By understanding what greenhouse gases did to global temperature in the past, scientists might be able to predict the effect of man-kind's enthusiastic CO2 belching.

"There is great controversy as to whether human beings have changed the climate, " said Professor McManus. "But there is no doubt about the fact that human beings have changed the Earth's atmosphere. The increased levels of greenhouse gases are geologically incredible."

He added: "It is something of grave concern to someone like me, who sees the strong connection between greenhouse gases and climate in the past."

Record ice core gives fair forecast

18:00 09 June 04
NewScientist.com news service

As long as humans do not mess it up, the Earth's climate is set at fair for the next 15,000 years. That is according to information extracted from the oldest ice core ever drilled.

The Antarctic core is the first to reach as far back as a warm period with characteristics similar to our own interglacial. So it should help make more accurate predictions about when to expect the next deep freeze.

The ice core, drilled from a feature in central Antarctica called Dome C, is around 3 kilometres long and 10 centimetres wide. Changes in the relative proportions of hydrogen isotopes in the ice layers allow scientists to compile a complete record of Antarctic temperatures going back 740,000 years.

The core shows the waxing and waning of eight ice ages. Most critically for making predictions about our climate, it is the first core to record a period known as Termination V, around 430,000 years ago.

Warming pattern

At this point, the world moved from a glacial period into a long, warm interglacial, similar to this era. The previous longest ice-core record, drilled by the Soviet Union at Vostok in Antarctica between 1980 and 1988, went back only 420,000 years.

"All interglacials are slightly different, but we believe Termination V is the most similar to our own," says chief author of the new study, Eric Wolff, at the British Antarctic Survey in Cambridge, UK. It mirrors the pattern of solar warming between seasons and at different latitudes that are caused by fluctuations in the Earth's orbit known as the Milankovitch cycles.

It shows that the Termination V interglacial was unusually long, lasting 28,000 years. The current interglacial is now 12,000 years old, and some scientists feared that we might be heading for an ice age soon since at least one post-Termination V interglacial lasted just 10,000 years.

But the new findings suggest that even without the human hand in global warming, a new ice age would be unlikely for perhaps another 15,000 years, Wolff says.

Ice blanket

The core also sheds light on how ice ages have changed over the past million years. Since Termination V, ice ages have been very intense, with periods of cold weather that blanketed much of the northern hemisphere in ice for 80,000 years punctuated by short interglacials lasting typically 20,000 years.

But the new core shows that, prior to Termination V, the cold and warm periods of the glacial cycle each lasted around 50,000 years but were much less intense.

"Marine deposits suggested some of this, but it stands out much more clearly in the ice record," Wolff says.

Meanwhile, European and US scientists are discussing plans to survey for a site in Antarctica that will extend the record still further. "We want to go back at least 1.2 million years next time," Wolff says. "But we have to find somewhere that we can do it."

Journal reference: Nature (vol 429, p 623)

Fred Pearce

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