Polar regions and some alpine areas are sufficiently cold that snowfall accumulates from year to year, building up as glaciers. As snow at the surface gets buried with time it gets compressed to form solid ice and this ice carries with it information about the climate when the snow originally fell.
By drilling down into a glacier and recovering this old ice, the information can be extracted to help understand past climate.
The information obtained from ice cores can be divided into three types. The first of these types of information comes from the solid and dissolved impurities in the snow. Generally snow that falls in such remote places is almost pure water, but it still contains traces of dust, salts from the ocean, and pollutants from human activities, volcanoes and forest fires, to name some examples.
This sort of information can be used to detect major environmental changes as well as variations in the chemistry and circulation of the atmosphere.
The second type of information obtained from ice cores comes from bubbles in the glacier ice. These bubbles are formed as snow becomes compressed and the air between the flakes gets trapped.
This process occurs during burial, and trapping is generally not complete until the bubbles are around 60 metres or more below the surface. Once the bubbles have formed, they act as time-capsules of the atmosphere when they were trapped. By extracting the air from the bubbles in ice cores, changes in the past atmosphere can be studied. Studies of these changes show the dramatic increase in greenhouse gases caused by human use of fossil fuels as well as natural changes that accompany glacial/ interglacial climate shifts.
The third type of information obtained from ice cores comes from the frozen water itself. Naturally occurring hydrogen and oxygen both come in rare heavier forms called isotopes. In the oceans, one in about every 500 oxygen atoms is the heavy isotope, while one in about 70 hydrogen atoms is heavy. However as the water evaporates and is transported to polar regions, the mix of the heavy isotopes changes. These changes are mostly influenced by temperature and it turns out that by measuring water isotopes in ice cores researchers can infer temperatures when the snow originally fell.
In the past 30 years, many ice cores have been drilled to study past climate. Ice cores have been retrieved from almost all glaciated areas of the earth from the tropics to the poles. The longest records come from the large ice sheets of Greenland and Antarctica, which are over 3km thick and produce records stretching back several hundred thousand years.
Such deep drilling projects use self-contained drills suspended from a cable at the surface. These drills produce cores with diameters of typically 10cm to 13cm and can retrieve only a short section (2m to 6m long) in each drilling run. After each drilling run, the drill is raised to the surface, the core section is removed from the drill and the whole process repeated. A major core, thousands of metres deep requires many months to a year or more of drilling. This is often conducted over several summer field seasons and so the duration of a project can be several years.
Once the core is drilled, some analysis may be undertaken in the field. After this is complete, the bulk of the core is generally returned from the field in frozen storage so that it can be further studied in major laboratories.