Information stored in ice thousands of years old may be used to reconstruct climates thousands of years ago. Ice cores have been collected from Antarctica in the Southern Hemisphere and Greenland in the Northern Hemisphere. Some of these ice cores are several kilometres in length.
As snow accumulates on ice caps and sheets where temperatures usually remain below freezing year round, it lays down a record of the environmental conditions at the time of its formation. Over time the snow, buried under further accumulations, is compacted to ice, preserving the climatic information. Layers of ice form a palaeoclimatic record through time. Information concerning these conditions can be extracted from ice that has survived the summer melt by physical and chemical means. Palaeoclimate information during the Ice Age and previous warmer interglacial period (the last 120,000 years) has been obtained from ice cores by three main approaches. These involve the analysis of the (isotopic) composition of the water in the ice for the reconstruction of temperature, the dissolved and particulate matter in the ice, and the physical characteristics of the ice, and of air bubbles trapped in the ice, for the reconstruction of atmospheric composition.