Reconstructing climate history using ice cores
The climate has clearly changed in the past, affecting the planet and living things generally. For over two million years, the climate has been dominated by extended periods of globally colder temperatures than present (several degrees or more depending on location). These ice ages (or “glacial periods”) are interrupted by briefer warm (or “interglacial”) periods like the present one, which has lasted for approximately 10 000 years.
While the climate is complex it is clear from past history that it has at least these two semi-stable states. Exactly what triggers a change between these states is not completely understood. We know, for example, that some of the glacial/interglacial cycle is explained by changes in the earth's orbit, which influence the amount and distribution of heat input from the sun.
The precise timing of changes in orbital variations, temperature and other global indicators (e.g. greenhouse gas concentrations) is an important clue to advancing understanding.
These large climate changes have been accompanied by dramatic variations in vegetation and sea level, which have driven and controlled human migration in pre-history.
Indeed the warm, stable climate of the present interglacial, which has lasted some 10 000 years, has been the period in which modern agriculture and civilisations have developed. Such stability is very likely a necessary requirement for this development.
Besides the large climate changes, however, there are smaller changes in climate that may affect regions by up to a few degrees for decades to centuries.
One such change was the “Little Ice Age” which had a dramatic impact on European agriculture in approximately the 15th to 19th centuries. Other changes in say the severity and regularity of phenomena like El Nino, may also be linked to climate change and could have widespread impact on society.
The dramatic changes in greenhouse gas concentrations as a result of human activity exceed anything seen in the natural record. The complexity and even chaotic behaviour of the climate system make it difficult to predict its response to human and other forcing factors such as volcanic activity and solar variations. In an increasingly populated planet, such predictions are ever more important.
To meet this prediction challenge, complex computer models of the climate have been developed. These must be able to reproduce this past climate behaviour if they are to be useful in predicting future climate change.
Analysis of the air bubbles trapped in the ice cores allows us to see increases of greenhouse gases in the atmosphere from fossil fuel use. We can also detect the climate changes that occurred through this period.
We work mostly on ice cores from near the coast of Antarctica where snowfall is highest. This provides thick layers of snow which give very detailed climate records. Our longest ice core comes from near the summit of Law Dome, near Casey station. The ‘Dome Summit South’ (DSS) core is around 1200 m long, from surface to bedrock, and covers around 90 000 years of climate history. We are now working towards extracting a one million year old ice core, which will add to our knowledge of observed climate change in Australia, Antarctica and the Southern Hemisphere.
The ice core climate history work is also part of the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC). This centre brings together researchers studying other aspects of climate change, including climate history studies using sediments laid down on the ocean floor and in lakes. We work with these and other international groups studying past climate from ice, tree rings and sediments.