By Thomas L. Friedman Jorgen Peder Steffensen made me an offer I couldn't refuse: "If you come to Copenhagen, I will show you a real Christmas snow, the snow that fell between 1 BC and 1 AD." Now that's an offer you don't get every day! But then I don't go to the Arctic Circle every day. "I can also show you a sample of the very last snow that fell right at the end of the last ice age, which was 11,700 years ago," said Steffensen. Steffensen is an ice specialist and curator of the world's most comprehensive collection of ice core samples, a kind of atmospheric DNA drilled out of the glaciers of Greenland and now preserved in refrigerated vaults in the Danish capital. The deeper scientists can drill the ice, the better the picture they can give of the climate in previous eras. Each layer of ice contains water and air bubbles that were trapped in the snow, which reveals the temperature, the amount of greenhouse gases in the atmosphere, and even the amount of sea salt in the air and therefore how close the glacier was to the ocean. Imagine for a moment a freezer filled with such revealing ice cubes. Each ice cube represents one year's atmospheric data beginning 150,000 years ago, which is how far back the current Greenland icecap dates. Well, Steffensen, his wife, Dorthe Dahl-Jensen, both of the Centre for Ice and Climate at the Niels Bohr Institute of the University of Copenhagen, and a team of international experts are assembling precisely that kind of freezer from ice cores drilled here in the far north of Greenland in the Arctic Circle. I travelled to their newest camp with a group of experts led by Denmark's minister of climate and energy, Connie Hedegaard, and including Rajendra Pachauri, the chairman of the UN's Intergovernmental Panel on Climate Change. This is one of the most remarkable and isolated research stations in the world. Everywhere you look, you see a perfectly flat expanse of snow and ice stretching to the horizon. The camp consists of a heated geodesic dome where the scientists eat, a dozen barely heated tents where they (and guests) sleep in insulated sleeping bags and an underground research laboratory, carved out of the ice, where they are installing the drill and ice lab equipment. Over the next three "summers," they will unearth ice core samples all the way down to Greenland's bedrock. Their objective is to project a complete picture of the Greenland climate, from the ice age that lasted from 200,000 to 130,000 years ago, through the warming period known as the Eemian that lasted from 130,000 to 115,000 years ago, through the last ice age from 115,000 to 11,703 years ago, right up to the present warming period we've been in since. Their last drilling project here, which was completed in 2004, focused on the layers 14,500 to 11,000 years ago. That project is already causing a stir in the climate community. In an article published in the journal Science Express, Dahl-Jensen's team wrote about how it had discovered from the ice cores that the atmospheric circulation in the Northern Hemisphere over Greenland "changed abruptly" just as the last ice age ended around 11,700 years ago. It seems to have been driven by a sudden change in monsoons in the tropics. The change was so abrupt that it warmed the Northern Hemisphere over Greenland by 10 degrees Celsius in just 50 years. "It shows that our climate system has the ability to make very abrupt changes all by itself," said Dahl-Jensen. The climate is always changing, so the last thing that mankind should be doing is adding its own forcing actions. Because you never know what will tip the balance and send us hurdling into another abrupt change... and into another era.