Under a White Sky: The Nature of the Future

by Elizabeth Kolbert

Like all glaciers, the Greenland ice sheet is made up entirely of accumulated snow. The most recent layers are thick and airy, while the older layers are thin and dense, which means that to drill down through the ice is to descend backward in time, at first gradually and then much more rapidly. About a hundred and forty feet down, there’s snow dating from the American Civil War; some twenty-five hundred feet down, snow from the time of Plato; and at a depth of five thousand three hundred and fifty feet, snow from when prehistoric painters were decorating the caves at Lascaux. As the snow is compressed, its crystal structure changes to ice. But in most other respects, it remains unchanged, a relic of the moment it formed. In the Greenland ice, there’s volcanic ash from Tambora, lead pollution from Roman smelters, and dust blown in from Mongolia on ice age winds. Every layer contains tiny bubbles of trapped air, each a sample of a past atmosphere. To someone who knows how to read them, the layers are an archive of the sky.