Ice cores are cylindrical samples of ice removed from ice sheets and glaciers. Because ice cores are taken from regions which remain frozen year-round, they contain detailed information about the history of the Earth's climate, for those who know how to look. Paleoclimatologists often study ice cores extensively to gather data about major climate events and to piece together patterns in the Earth's meteorological history. Ice cores can be found in storage in numerous research facilities and archives.
In order to take a core sample from ice, researchers must find an area with significant ice deposits, ideally an area where ice has been present for thousands of years. The polar ice caps are a prime location for taking ice cores, as are some permanent glaciers. The researchers drill into the ice with specialized equipment, using a liquid to maintain pressure so that the hole does not collapse, and they remove samples of ice from the hole and bag them for further study.
These samples must be handled with care, to ensure that they are not contaminated by the modern climate. Because ice cores often experience radical pressure changes when they are pulled to the surface, researchers must first allow them to “relax” at extremely low temperatures so that they do not shatter. At all times, the ice cores must be kept scrupulously clean, and when the cores are finally ready for study, they are handled in a clean room to reduce the risk of contamination.
Viewed in cross-section, an ice core has a series of layers representing decades of snowfall. Each layer can be used to gather information about that year's climate. Ice can trap particulate materials like pollutants and ash, along with radioactive isotopes, and various levels of dissolved chemicals. Using ice cores, researchers can see what the oxygen and carbon dioxide levels were like historically. They can also find clues like pollen and dust in ice cores which could be used to estimate the prevailing wind direction in any given year, and to learn more about what was happening on other parts of Earth.
When examining ice cores, one of the key issues is accurate dating. Without a solid date to work with, the data is not terribly helpful. Dating can be accomplished by physically counting back layers, much like one does with tree rings. It can also be done by analyzing levels of isotopes in the ice and comparing the levels to known ice samples, or by looking for key layers in the ice which could be used to extrapolate. For example, when Krakatoa erupted in 1883, it distributed volcanic ash all over the world, leaving a tell-tale trace in ice cores from this era.