Glacier ice, particularly the ice in the inland ice sheets of Antarctica and Greenland, holds a treasure trove of information about climate in ancient times. The snow that once fell here contains information about ambient climate hundreds of millennia back in time. Tiny air bubbles trapped in the ice allow scientists to study how the composition of the atmosphere has changed with temperature over time.
One of the most important sources of information in these icy archives is cryptically called δ18O or dD. This is a measure of the relative concentration of different stable isotopes of oxygen in the water the ice crystals are made of. In simple terms, every time water evaporates from the ocean or falls as precipitation, the molecules of water (H2O) that contain certain stable isotopes are more likely to be involved. The exact fraction is temperature-dependent, so if we analyse the snow on the glaciers, we can create a time-line that tells us how temperatures in that area have varied. When this information is stored over long time spans, it becomes a climate archive.
As in all archives, accurate dating is important. Many different methods can be used to calculate the age of an ice core, and several are usually used in parallel. Horizons (layers) formed in conjunction with historic events are important in this context. Volcanic eruptions provide another important way of dating ice cores. Read more.
The Norwegian Polar Institute helps secure information about ancient climate by studying ice cores from both Svalbard og Antarctica.