Ice Core Climate Data

How Ice Cores Record Climate

Snow falling on ice sheets gradually compresses under the weight of subsequent snowfall, transforming into firn (compacted granular snow) and eventually solid ice. During this compaction, air between snow grains becomes trapped in sealed bubbles at depths of roughly 60 to 120 meters, depending on local temperature and accumulation rate. These bubbles preserve samples of the atmosphere at the time of trapping, providing direct measurements of past CO2, methane, and nitrous oxide concentrations.

Temperature information comes from the ratio of heavy to light isotopes of oxygen (O-18 to O-16) and hydrogen (deuterium to hydrogen) in the ice itself. Water molecules containing heavier isotopes condense preferentially at warmer temperatures, so the isotope ratio in precipitation correlates with local temperature. This relationship has been calibrated using modern observations and borehole temperature measurements, providing a thermometer for past climate with typical uncertainties of 1 to 2 degrees Celsius.

Annual layers are visible in ice cores from high-accumulation sites through seasonal variations in dust content, isotope ratios, and chemistry. These layers allow precise dating of recent sections. For deeper ice where annual layers are too thin to resolve, dating relies on ice flow models, matching volcanic markers to independently dated eruptions, and orbital tuning.

Major Ice Core Records

The EPICA Dome C core from Antarctica provides the longest continuous record, extending 800,000 years with 3,270 meters of ice. This record covers eight complete glacial-interglacial cycles, showing CO2 oscillating between roughly 180 ppm during ice ages and 280 ppm during warm interglacials. Temperature variations of 8 to 10 degrees Celsius accompanied these CO2 changes.

The Vostok core revealed the tight coupling between CO2 and temperature over 420,000 years when published in 1999. The GRIP and GISP2 cores from Greenland provide higher-resolution records revealing abrupt Dansgaard-Oeschger events in which temperatures swung by 8 to 16 degrees Celsius within decades. The WAIS Divide core provides high-resolution Antarctic data for the past 68,000 years.

CO2 and Temperature Coupling

Ice core data show CO2 and temperature closely correlated over 800,000 years, but the relationship involves complex timing. During transitions from ice ages to warm periods, orbital changes initiated warming at high southern latitudes, causing the Southern Ocean to release dissolved CO2. This CO2 increase then amplified warming globally through the greenhouse effect, with full transitions taking 5,000 to 8,000 years.

The observation that temperature sometimes changed before CO2 during past transitions demonstrates CO2 acted as an amplifying feedback. Today the situation is reversed: CO2 is rising first due to emissions and temperature is following. Current CO2 of over 425 ppm is roughly 50 percent higher than the maximum value recorded in any ice core, and rising 100 times faster than the fastest natural increases during deglaciations.

Additional Information from Ice Cores

Beyond greenhouse gases and temperature, ice cores record volcanic eruptions through sulfate layers, dust deposition indicating aridity and circulation patterns, sea salt related to sea ice extent, and cosmogenic isotopes tracking solar activity. Biological markers including pollen and bacteria provide information about past vegetation. The combination of all these proxies makes ice cores one of the most information-rich paleoclimate archives available.