As snow and ice accumulates on polar and alpine ice caps and sheets, it lays down a record of the environmental conditions at the time of its formation. Information concerning these conditions can be extracted from ice and snow that has survived the summer melt by physical and chemical means. When melting does occur, the refreezing of meltwater can provide a measure of the summer conditions. Palaeoclimate information has been obtained from ice cores by three main approaches (see Table 3.1). These involve the analysis of: a) stable isotopes of water; b) dissolved and particulate matter in the firnFirn is the term given to snow that has been converted to ice by compaction from overlying layers of subsequent snowfall. and ice; and c) the physical characteristics of the firn and ice, and of air bubbles trapped in the ice. Each approach has also provided a means of dating the ice at particular depths in the ice core.