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The ice man cometh - ice cores reveal past climates

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Over the past 40 years, ice cores have revealed more about climate change than any other scientific technique. We talk to ice core scientist Dr Robert Mulvaney of British Antarctic Survey about how he mines the tiny bubbles of ancient air trapped in ice thousands of metres beneath the Antarctic ice cap for information about what the Earth's climate was like in the past, and what this tells us about our future.

If the title belongs to any one man, then the Danish climatologist Willi Daansgard is the father of modern ice core science. Two chance events in the 1950s - one a spell of unseasonably wet summer weather and the other a meeting with a Norwegian zoologist - resulted in the development of ice coring as one of science's most powerful tools for studying global climate change.

Daansgard's Eureka! moment came on a dismally damp midsummer's day in 1952. It was, according to Daansgard "a minor - but to me fateful - miracle". Watching the rain fall, he wondered whether its isotopic composition changed from one shower to the next. So, deciding to analyse the rain he began collecting it, first in empty beer bottles fitted with a funnel and then - when he ran out of beer bottles - in any pots and pans he could lay his hands on.

According to Daansgard: "It turned out to be an unusually well-developed front system. When the rain began in western Jutland, it had not stopped raining in Wales, 1,000 km to the west. I have never seen anything like it, before or after. The miracle consisted in my starting the collection accidentally under these unusually favourable conditions."

Message in a bottle

The results were remarkable. Daansgaard discovered that the ratio between two isotopes of oxygen - oxygen-16 and oxygen-18 - depended on the temperature at which the rain was formed within the clouds: the lower the temperature, the lower the ratio, or so-called "delta value" of the rain. But what really made the findings special was Daansgard's idea that the relationship between temperature and delta value might also hold going back in time: "In other words, delta in old water might reflect the climate at the time of formation of the water," he recalls.

"The most significant thing ice cores have told us is that greenhouse gases are so well-related to climate, and that at no point in the last three-quarters of a million years have carbon dioxide and methane reached anywhere near the levels they are at today."
Dr. Robert Mulvaney
British Antarctic Survey

The second chance event occurred four years later, in 1956, when Norwegian zoologist Per Scholander turned up in Daansgard's lab with a new idea: that within the minute bubbles trapped in ancient ice might be samples of the atmosphere thousands of years old. Together, these two discoveries opened a window onto the past - allowing scientists eventually to reconstruct a detailed history of the Earth's past atmosphere and temperature stretching back 800,000 years from nothing more than a piece of ice.

According to Dr Robert Mulvaney of British Antarctic Survey (BAS), who has spent 24 years drilling for ice in both the Arctic and Antarctic: "We didn't realise 15 or 20 years ago that ice coring would become such a powerful technique. The biggest impact of ice cores on our understanding of climate change is the detail we've been able to see. We've also been able to confirm the link between our climate and changes in the orbit of the Sun. But perhaps the most significant thing ice cores have told us is that greenhouse gases are so well-related to climate, and that at no point in the last three-quarters of a million years have carbon dioxide and methane reached anywhere near the levels they are at today."

Much of that knowledge has come from a 3,270 metre-long ice core completed in 2004 from Dome C in Antarctica. Mulvaney was part of the European Project for Ice Coring in Antarctica (EPICA) team that drilled the core, and although not the longest core ever recovered in Antarctica (that record goes to a 3,650 metre core from Vostok), the EPICA core is the oldest, stretching back 800,000 years.

Fact File: Dating an ice core
  • Antarctica has been covered in ice for around the past 30 million years.
  • "Dating an ice core is a bit like counting tree rings," says Dr Robert Mulvaney of British Antarctic Survey. Near the surface of an ice core, annual layers are usually visible. From these, scientists can work out how rapidly snow accumulated at the drill site and therefore calculate how old the ice is at the bottom of the core.
  • Scientists can also use "marker" events such as volcanic eruptions – which leave tell tale changes in the chemistry of the ice core – to help date ice cores.
  • The world's oldest ice core so far was recovered from Dome C in Antarctica. The ice at the base of this core is made from snow which fell 800,000 years ago.

"Although the EPICA core is by far the furthest back in time we've obtained, we'd like to go further back, to something like 1.4 million years, but not just because we want to set new records," Mulvaney says.

Scientists believe that locked in the oldest ice on the planet remain important clues about global climate change. Data from both ice cores and marine sediments - another place in which global climate is recorded - show that the Earth moves between ice ages and warm periods (interglacials) with a consistent cycle. Ice cores show that between today and 800,000 years ago that cycle lasted around 100,000 years, yet marine sediments show that more than a million years ago, that cycle was just 40,000 years. So why the change?

"Given that the main driving force for long-term climate change is the orbit round the Sun, it's not obvious that the Earth's orbit has changed enough to cause this 40,000 year cycle to shift to a 100,000 year cycle. At the moment the change is difficult to understand, but we hope that a 1.4 million year ice core will help us," says Mulvaney.

 

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Images © British Antarctic Survey / Words © Becky Allen.

 

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