Ice cores are cylinders of ice, sometimes miles deep, drilled from ice sheets and glaciers around the world. Scientists analyze these ice cores to get information about the atmospheric conditions that existed in each location up to hundreds of thousands of years ago.
Air trapped by snow crystals as they are deposited on the ground contains the proportional amount of carbon dioxide and other gases present in the atmosphere at that time. As more snow is added with each passing year, the air samples are trapped farther below the surface.
Higher carbon dioxide levels in the atmosphere cause more heat to be trapped near the surface of the earth, which raises atmospheric temperatures, and vice versa. Therefore, carbon dioxide levels in ice cores are used as an indication of atmospheric temperatures at the time the carbon dioxide was trapped.