Researchers around the world are taking ice core samples from mountain glaciers and polar ice sheets. Studying trends in local areas contribute to the big picture of global climate.
An ice core can be taken from a 3,500 meter borehole, providing a record from 110,000 to 750,000 years. Snow presses down on layers of compressed snow that become ice. Layers of compressed ice are easily identified by the naked eye.
These layers provide information on annual atmospheric conditions. That is, temperature, precipitation, atmospheric composition. Temperature can be directly measured. The balance of oxygen isotopes is measured, that is the percentage of light oxygen to heavy oxygen, resulting from past condensation and evaporation.
In the the layers of ice, are particulates of dust, volcanic ash, pollen and small pockets of air. Air is ”trapped” in between ice crystals making up the layers of ice. These air bubbles hold the past atmospheric conditions.