In Polar Regions and on high mountains, snow precipitates to form annual layers that accumulate over many years. As time goes on, the bulk of the snow at the top presses down and compacts the snow at lower levels, eventually turning it into ice. In Antarctica and Greenland, in particular, there thus exist extensive ice sheets that extend down to bedrock. These polar ice cores are used to study traces of the past history of variations in the earth's climate. Knowledge of past climate change has a bearing on models constructed to predict global warming over the next one hundred years and finding appropriate measures to slow down the enhanced greenhouse effect by reducing emissions of carbon dioxide and other greenhouse gases into the atmosphere, as well as finding ways to adapt to warmer climates in the future. An ice core is a long cylindrical plug that is pulled out of an ice sheet.