Ice core dating methods
The season turns cold and dark again, and more snow falls, forming the next layers of snow.Each layer gives scientists a treasure trove of information about the climate each year.Scientists recover this climate history by drilling cores in the ice, some of them over 3,500 meters (11,000 feet) deep.These photographs show experimental drilling on the Greenland Ice Cap in summer 2005.Essentially a sharpened pipe rotating on a long, loose cable, the drill pulled up cores of ice from which Alley and others would glean climate information.
As with marine fossils, the ratio of oxygen isotopes in the snow reveals temperature, though in this case, the ratio tells how cold the air was at the time the snow fell.“I have stood in snow pits with dozens of people—drillers, journalists, and others—and so far, every visitor has been impressed.The snow is blue, something like the blue seen by deep sea divers, an indescribable, almost achingly beautiful blue,” writes Alley.In the wall of the pit, dark and light bands of slowly compacted snow distinguish snow deposited in the winter from snow deposited in the summer.(Photograph courtesy Christopher Shuman, NASA GSFC)To pry climate clues out of the ice, scientists began to drill long cores out of the ice sheets in Greenland and Antarctica in the late 1960s.
Scientists have also taken cores from thick mountain glaciers in places such as the Andes Mountains in Peru and Bolivia, Mount Kilimanjaro in Tanzania, and the Himalayas in Asia.