Ice core

Snow accumulates on glaciers and ice sheets, compresses under later snowfall, and gradually becomes ice. Drilling extracts a long cylindrical core that preserves a sequence of layers. The age represented by a given depth depends on the site's snowfall, ice flow and deformation; deeper layers are not automatically evenly spaced in time. Field teams record the core's depth and condition, then transport and store sections under controlled cold conditions for analysis.

Ice cores contain several kinds of climate evidence. Ratios of stable oxygen and hydrogen isotopes in the ice can be used as indicators of past temperature, with interpretation shaped by location and climate processes. Dust, volcanic particles and chemical ions provide further evidence about atmospheric conditions and events. Air is enclosed in bubbles as porous snow densifies into ice, preserving samples of past atmospheric gases, including carbon dioxide and methane. The gas is generally younger than the surrounding ice because air remains able to move through the snow before pores close.

Researchers establish an age-depth chronology using visible or measured annual layers where these can be resolved, and may use volcanic horizons, chemical patterns, other dating evidence and ice-flow models. Annual layers become compressed and harder to distinguish at depth. Measurements are therefore interpreted together rather than as a simple thermometer or direct record of every aspect of climate. Uncertainty in dating, local conditions and the physical processes that alter ice or gas must be taken into account.

Ice-core records help scientists investigate changes in temperature, atmospheric composition, dust and volcanic influence across past climate intervals. Comparing records from different places and with other evidence helps test explanations of climate variability. A core is a local archive rather than a complete global snapshot; its resolution and time span depend on the site and drilling depth. The air bubbles also record atmospheric composition at the time air became sealed, not necessarily the exact age of the adjacent ice.