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The ice man cometh - ice cores reveal past climates

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Ice work if you can get it

Recovering a 1.4 million year old ice core will be a huge challenge, so once again it's being approached via an international team known as IPICS. Crucial to the project's success will be selecting the right place to drill. But older won't necessarily mean deeper. According to Mulvaney: "It's not just a case of drilling deeper, because an ice sheet is at its coldest at the surface and its warmest at the bed. The ice sits on the Earth's mantle like a blanket, and the thicker the blanket, the harder it is for the heat to escape. If the ice sheet is too thick, the oldest ice at the bottom of the core will have melted."

"Deciding where to drill depends on the question you want to answer, but generally, ice core scientists tend to choose somewhere that's a dome, where the bottom of the ice sheet is relatively flat and where there is little ice flow, because as you drill deeper, you don't want to be drilling into ice that has flowed from somewhere else. We need to find somewhere where the ice isn't much thicker than at Dome C, but where it's colder at the surface. That will mean that it's also colder at the base, so the oldest ice will have stayed frozen. We also want a place with a low accumulation rate so that the ice ages faster as you get deeper," Mulvaney explains.

Slice of ice core from Berkner Island, depth 120m.  Trapped air bubbles (an archive of the past atmosphere) are visible in the ice.
This image is associated with the 2005-2010 BAS science programme: CACHE- Climate and Chemistry: Forcings, Feedbacks and Phasings in the Earth System.
Bubbles trapped in a slice of ice core

Given these criteria, the best bet in the hunt for a 1.4 million year ice core looks like Dome A, more than 4,000 metres above sea level and over 1,000 kilometres from the nearest research station. Before drilling begins, aircraft will survey the area to discover the shape of the bed and how much ice lies above it, and select the best possible site.

Even then, drilling will be tough. "The challenge of working at Dome A will be the high altitude - it's at more than 4,000 metres and because the Earth's atmosphere is thinner in the polar regions, that's like working at between 5,000 and 6,000 metres above sea level. Even working at Dome C - which is at 3,300 metres - had me gasping until I acclimatised," says Mulvaney.

As well as their huge scientific potential, ice cores play an important role in communicating the science of climate change. Few weeks go by when Mulvaney does not have visitors - including students, teachers, politicians and business leaders - to show around BAS's cold room and explain how a simple piece of ice can tell us so much: "What really amazes them is seeing the little tiny bubbles and knowing that in these is the actual atmosphere that was floating around in the past. They are surprised that I can hand them a piece of ice that's remained frozen for tens of thousands of years, and which is melting away as we speak."

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Images © British Antarctic Survey / Words © Becky Allen.


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