Glaciers and ice sheets form as layers of snow accumulate on top of each other. Over time, the buried snow compresses under the weight of the snow above it, forming ice. Bubbles of trapped air become part of the ice as layers of ice accumulate year after year. The air bubbles act as time capsules, providing a direct measurement into the composition of the atmosphere when the ice formed.

Ice cores are cylinders of ice drilled from ice sheets. Layers in ice cores correspond to years, with the youngest ice at the top and the oldest ice at the bottom of the core. In Antarctica the ice sheet averages 2160 meters, with a maximum thickness of 4776 meters. By analyzing the chemical composition of bubbles trapped in kilometers-long ice cores, scientists can reconstruct the concentration of CO₂ in the atmosphere going back hundreds of thousands of years.

The graph below shows the concentration of CO₂ in the atmosphere going back two millennia. In addition to direct CO₂ concentration measurements from Mauna Loa and the South Pole, this graph shows the concentration of CO₂ in bubbles trapped in the Law Dome (MacFarling et al., 2006; Rubino et al., 2013; 119.87 m) and Siple (Neftel et al., 1985; Friedli et al., 1986; 200 m) ice cores. Notice the close correspondence between the direct atmospheric CO₂ measurements and the ice core data.

The graph below takes us further in time. It includes the Taylor ice core (Indermuhle et al., 2000; 471 m) and the much deeper Vostok (Petit et al., 1999; 3623 m) and Dome C (Monnin et al., 2001; Siegenthaler et al., 2005; Luthi et al., 2008; 3,270 m) ice cores from the interior regions of East Antarctica.

The map below shows the locations of the Antarctic ice cores as well as the stations where atmosphere CO₂ was directly measured since the late 1950s.

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