Analysis of Ice Cores

The ice sheets of Antarctica and Greenland have built up from snow over hundreds of thousands of years. Scientists drill out long cylinders of ice called ice cores. Each year's snowfall forms a layer, so deeper ice is older. The longest Antarctic cores cover about 800,000 years.Carbon dioxide: as snow is compressed into ice, small bubbles of air are trapped. Extracting and analysing this air gives the concentrations of CO₂ and methane in the atmosphere at the time.Temperature: the ratio of isotopes in the water molecules of the ice depends on the temperature when the snow fell. Water containing the heavier isotope oxygen-18 evaporates less easily and condenses more readily, so in colder periods snow contains less of it. A lower ¹⁸O:¹⁶O ratio therefore means a colder climate.The records show CO₂ and temperature rising and falling together through repeated ice ages and warmer interglacials. CO₂ varied naturally between about 180 ppm in glacial periods and 300 ppm in interglacials. Today's value of over 420 ppm is far outside this range.Limitations:
• air takes time to be sealed off in compacting snow, so the bubbles are younger than the ice around them;
• layers near the bottom are compressed and harder to date;
• a core records conditions at one place, which may not represent the whole globe.