Ice cores and climate change
Slices of ice core, drilled from the depths of the Earth’s ice sheets reveal details of the planet’s past climate
Ice cores are cylinders of ice drilled out of an ice sheet or glacier. Most ice core records come from Antarctica and Greenland, and the longest ice cores extend to 3km in depth. The oldest continuous ice core records to date extend 123,000 years in Greenland and 800,000 years in Antarctica. Ice cores contain information about past temperature, and about many other aspects of the environment. Crucially, the ice encloses small bubbles of air that contain a sample of the atmosphere — from these it is possible to measure directly the past concentration of gases (including carbon dioxide and methane) in the atmosphere.
Direct and continuous measurements of carbon dioxide (CO2) in the atmosphere extend back only to the 1950s. Ice core measurements allow us to extend this way back into the past. In an Antarctic core (Law Dome) with a very high snowfall rate, it has been possible to measure concentrations in air from as recently as the 1980s that is already enclosed in bubbles within the ice. Comparison with measurements made at South Pole station show that the ice core acts as a faithful recorder of atmospheric concentrations (see Fig. 1), although we do have to be cautious, as artefacts can arise at sites with high concentrations of other impurities.
Antarctic ice cores show us that the concentration of CO2 was stable over the last millennium until the early 19th century. It then started to rise, and its concentration is now nearly 40% higher than it was before the industrial revolution (see Fig. 2). Other measurements (e.g. isotopic data) confirm that the increase must be due to emissions of CO2 from fossil fuel usage and deforestation. Measurements from older ice cores (discussed below) confirm that both the magnitude and rate of the recent increase are almost certainly unprecedented over the last 800,000 years.
Natural climate changes: glacial-interglacial cycles
By measuring the ratios of different water isotopes in polar ice cores, we can determine how temperature in Antarctica and Greenland has changed in the past. The oldest ice core we have was drilled by the European Project for Ice Coring in Antarctica (EPICA) from Dome C on the Antarctic plateau. It extends back 800,000 years and shows a succession of long cold ‘glacial’ periods, interspersed roughly every 100,000 years by warm ‘interglacial’ periods (of which the last 11,000 years is the most recent). This succession of events is well-known from other records, and the coldest periods in Antarctica are the times when we had ice ages. Ice sheets extended over North America as far south as Wisconsin, and over Britain to south of The Wash.
The role of greenhouse gases in glacial-interglacial cycles
Abrupt climate changes
The climate changes described above were huge, but relatively gradual. However, ice cores have provided us with evidence that abrupt changes are also possible. During the last glacial period, Greenland experienced a sequence of very fast warmings (see Fig. 5).
Ice cores provide direct information about how greenhouse gas concentrations have changed in the past, and they also provide direct evidence that the climate can change abruptly under some circumstances. However, they provide no direct analogue for the future because the ice core era contains no periods with concentrations of CO2 comparable to those of the next century.
- Ice core. Cylinder of ice drilled out of an ice sheet or glacier. Most ice core records come from Antarctica and Greenland.
- Ice cores contain information about past temperature, and about many other aspects of the environment.
- Atmospheric carbon dioxide levels are now nearly 40% higher than before the industrial revolution. This increase is due to fossil fuel usage and deforestation.
- The magnitude and rate of the recent increase are almost certainly unprecedented over the last 800,000 years.
- Methane also shows a huge and unprecedented increase in concentration over the last two centuries.
- MacFarling Meure, C., and others: Law Dome CO2, CH4 and N2O ice core records extended to 2,000 years BP, Geophys. Res. Lett., 33, L14810, doi:10.1029/2006GL026152, 2006
- Atmospheric data supplied by NOAA/ESRL
- Friedli, H., and others: Ice core record of the 13C/12C ratio of atmospheric CO2 in the past two centuries, Nature, 324, 237–238, 1986
- Siegenthaler, U., and others: Supporting evidence from the EPICA Dronning Maud Land ice core for atmospheric CO2 changes during the past millennium, Tellus Ser. B-Chem. Phys. Meteorol., 57, 51–57, 2005
- Jouzel, J., and others: Orbital and millennial Antarctic climate variability over the last 800,000 years, Science, 317, 793–796, 2007
- Lüthi, D., and others: High-resolution carbon dioxide concentration record 650,000–800,000 years before present, Nature, 453, 379–382, 2008
- Monnin, E., and others: Atmospheric CO2 concentrations over the last glacial termination, Science, 291, 112–114, 2001 with data converted to the age scale of Parrenin, F., and others: The EDC3 chronology for the EPICA Dome C ice core, Climate of the Past, 3, 485–497, 2007
- North Greenland Ice Core Project Members: High-resolution record of Northern Hemisphere climate extending into the last interglacial period, Nature, 431, 147–151, 2004