But a surprisingly robust chronicle is stored in the long-lived glaciers found in the polar regions and a few high mountain ranges in the Americas and Asia. Glaciers form when more snow falls each year than disappears by melting or sublimation, direct conversion of ice to water vapor. Over time, the annual layers of snow are compressed into large ice sheets by their own weight. In the polar regions, these glaciers have been accumulating for thousands of years. But there is more than old snow in a glacial ice core, according to Cravens.
Fresh snow is fluffy, with lots of air space between the ice crystals.
As the snow is compacted, the ice forms sturdy walls, trapping the air in minuscule bubbles. Moreover, the snow is full of dust and soot scrubbed from the air. Once the snow has solidified into ice, the air bubbles and particulates trapped in the snow can no longer intermix. So the snow acts like a time capsule, preserving a record of the atmospheric conditions at the time the snow fell. Scientists pierce the glaciers to retrieve these time capsules, Cravens explained.