Climate scientists use "proxy data", such as the information stored in ice cores, to study climates of the past.
Climate scientists use "proxy data" to study climates of the past, before humans with thermometers began keeping temperature records. These "proxies" include tree rings, layers within ice cores pulled from glaciers and ice sheets, growth layers in coral, and layers of sediments from the bottoms of lakes and oceans.
Gases, dust, and other materials trapped in ice can provide clues about climates of the past. Scientists dig ice cores from the Antarctic Polar Cap, the Greenland Ice Sheet, and glaciers around the world. Air bubbles in the ice trap miniscule samples of Earth's atmosphere from the past, providing scientists a way to measure levels of carbon dioxide and other gases in prehistoric times. The thickness of ice bands tells us about past snowfall rates. Darkened layers of ice contain dust, providing further clues about past climates. Some ice core data from Antarctica provides information about climate to more than 700,000 years in the past, a period which spans eight ice age cycles!
The video clips in this video are from the "To What Degree? What Science is Telling Us about Climate Change" web site developed by the National Science Foundation.