An ice core is a tube of ice removed from an ice sheet. The ice is older the further down it gets, so an ice core contains ice formed over a range of years.
Ice cores contain an abundance of climate information, more so than any other natural recorder of climate such as tree rings or sediment layers. Although their record is short (in geologic terms), it can be highly detailed.
Each layer of ice in a core corresponds to a single year, sometimes even a single season and almost everything that fell in the snow that year remains behind, including wind-blown dust, ash, atmospheric gases, even radioactivity.
An ice core from the right site can contain an uninterrupted, detailed climate record extending back hundreds of thousands of years. This record can include temperature, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity and a variety of other climate indicators.
It is the simultaneity of these properties recorded in the ice that makes ice cores such a powerful tool in paleoclimate research.