Ice cores hold secrets of long ago climate. Cores drilled through glaciers in the polar and high mountain regions of the world preserve information that scientists use to reconstruct past changes in climate and air composition:

(1) The structure of the ice itself (crystal size, orientation, density, amount of air trapped) call tell a lot about the conditions under it which it was formed. For example, ice formed from snow that fell under cold and dry conditions contains more trapped air bubbles than ice formed under milder conditions, when summer melting can occur.

(2) The frozen water molecules in the ice contain different types (isotopes) of oxygen and hydrogen atoms. Their proportions are determined by the source of the water vapor (from example: warm seawater from tropical latitudes vs. cold seawater from polar latitudes) and by the air temperature at the time the snow fell. Scientists can measure the isotope abundances in ancient ice layers from a core using a mass spectrometer, and in this way can reconstruct past changes in air temperature, water vapor source, or both.

(3) Ice cores also contains trace amounts of various air constituents. These include:
  • trapped air bubbles, which can be analyzed to find out how much carbon dioxide or methane was in the atmosphere thousands of years ago;
  • mineral glass particles from volcanic eruptions (called "tephra");
  • pollen grains from distant plants and trees;
  • ash from forest fires;
  • dust blown from distant deserts or from micro-meteors entering the atmosphere
  • particles produced by cosmic rays in the upper atmosphere;
  • sea spray;
  • soot and metal particles from coal-burning power plants, metallurgical smelters and vehicle exhaust;
  • radioactive particles from surface nuclear tests and accidental emissions from nuclear power plants;
  • bacteria.
In the past 30 years, ice cores have become very important tools for informing and guiding environmental policy in international affairs. This is because they provide critical evidence of the human influence on the Earth's climate and atmospheric composition. For example air extracted from Antarctic ice cores showed that the present rise of CO2 levels in the Earth's atmosphere far exceeded levels experienced for thousands of years. Likewise, ice cores from Greenland and Canada have been used to verify if legislation on air pollution (such as the Clean Air act in the United States) have effectively reduced pollution worldwide.