Climate change: past and future
How has Antarctica’s climate changed in the past? Is Antarctica being affected by ‘global warming’ and how might we expect its climate to change in the future? What evidence is needed to address these questions?
Past climate change in Antarctica: the evidence in ice
Antarctica hasn’t always had the same climate that it has today. In the geological past (many millions of years ago), Antarctica has been much warmer than present, and fossils found in rocks indicate that at various times even trees have covered much of the continent. These warmer periods in Antarctica were the result of different tectonic configurations and patterns of ocean circulation many millions of years ago (see Key factors behind Antarctica’s climate). However, for at least the last 6 million years, ice has covered most of the continent.
To look at climate changes over the last approximately 2 million years, ice cores are used instead of the geological record. Ice cores are obtained by drilling into an ice sheet or glacier and extracting cylindrical sections of ice. These sections of ice represent many thousands of years of snow accumulation: the addition of snow each year buries underlying layers of snow, and over time these layers become compressed into glacier ice. Each layer of an ice core is derived from snow that fell at a certain time in the past, and each layer is like a time capsule, containing information about what the atmosphere was like at the time the snow fell.
Recently, a new ice core from Antarctica’s eastern ice sheet has been found to record changes in the Earth’s atmosphere for the last 2.7 million years. This is the oldest ice core every recorded. Over this period, both landmasses and oceans have been in the same position as they are today. Therefore, the changes recorded in Antarctic ice are more informative about how the climate varies under the present tectonic configuration and how it may change in the near future.
The evidence in ice cores
The beauty of ice cores is that they contain a variety of forms of evidence that can tell us so many different things about the past climate and environment. The evidence is also continuous through time and can be dated with a high degree of precision. Some important types of evidence and information are:
- Stable isotope ratios of the oxygen and hydrogen making up layers of ice are used to reconstruct the air temperature at the time that the snow fell.
- Where annual layers of ice can be identified, the thickness of layers indicates the yearly quantity of snowfall in the past.
- The concentration of dust preserved in layers of ice gives an indication of how much wind erosion was occurring on continents in the past (related to both the strength of winds and the levels of vegetation cover over the land in the past).
- Tephra shards and levels of acidity of the ice show when major volcanic eruptions happened in the past.
- Air trapped inside the ice can be analysed for its concentration of carbon dioxide and methane, indicating how concentrations of these greenhouse gases in the atmosphere have fluctuated through time.