An ice core is a cylindrical sample of ice that is typically removed from an ice sheet or a high mountain glacier. The ice in the core forms from the incremental buildup of annual layers of snow, with lower layers being older than upper ones. As a result, an ice core contains ice formed over a range of years. Ice cores are drilled with hand augers or powered drills, and they can reach depths of over two miles (3.2 km), containing ice up to 800,000 years old.
As snow accumulates, each layer presses on lower layers, making them denser until they turn into firn. Firn is not dense enough to prevent air from escaping, but at a density of about 830 kg/m3, it turns to ice, and the air within is sealed into bubbles that capture the composition of the atmosphere at the time the ice formed. Ice cores are collected by cutting around a cylinder of ice in a way that enables it to be brought to the surface. Early cores were often collected with hand augers, and they are still used for short holes. An auger is essentially a cylinder with helical metal ribs wrapped around the outside, at the lower end of which are cutting blades.
Ice cores are essentially frozen time capsules that allow scientists to reconstruct climate far into the past. Layers in ice cores correspond to years and seasons, with the youngest ice at the top and the oldest ice at the bottom of the core. Ice cores are drilled in glaciers and on ice sheets on all of Earths continents, but most ice cores come from Antarctica and Greenland, where the longest ice cores extend to 3 kilometers or more in depth. Ice cores from the cold interior regions of polar ice sheets provide exceptionally well-preserved and detailed climate records. This is because the lack of melt at these locations does not corrupt the record of trapped gases or blur the record of other impurities.
Ice cores can reveal information about past temperatures, greenhouse gases, precipitation changes, and relationships between the composition of the atmosphere and changing climate. They can even offer glimpses into changing seasons and weather events, and ice-core analysis can reveal temperature changes. Ice cores are stored in specialized facilities, and scientists digitally record the ice cores’ characteristics, such as the presence of volcanic ash or the appearance of bubbles in the ice, in a controlled cold laboratory.
