Wikipedia:Today's featured article/October 18, 2017

An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Cores are drilled with hand augers (for shallow holes) or powered drills; they can reach depths of over two miles (three kilometres), and contain ice up to 800,000 years old. The physical properties of the ice and of material trapped in it can be used to reconstruct the climate over the age range of the core. The ratio of oxygen and hydrogen isotopes provides information about ancient temperatures, and the air trapped in tiny bubbles can indicate the level of atmospheric gases such as carbon dioxide. Greenland ice cores contain layers of wind-blown dust that correlate with cold, dry periods in the past. Radioactive elements, either of natural origin or created by nuclear testing, can be used to date the layers of ice. Some volcanic events that were sufficiently powerful to send material around the globe have left a signature in many different cores that can be used to synchronise their time scales. Climate models can be constructed by piecing together all the available data.