How does a glacier hold records of Earth's Climate?

The Greenland Ice Sheet is a group of glaciers formed together that look like a huge dome. This "dome" covers most of Greenland. The ice sheet dome forms as snow falls from the sky, especially at higher elevations in the middle of the dome. Over time the snow becomes compacted into glacial ice and the glaciers begins to flow outward from the center of the ice sheet similar to a lump of soft cookie dough placed on a baking sheet. Every year a new layer of snow is added to the ice sheet. The snowfall brings with it some of the chemicals and gasses that are present in the atmosphere at that time.