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 climate that it has now. In the geological past Antarctica has been much warmer, and fossils indicate that at various times trees have covered much of the continent. These warm times for 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). For at least the last 6 million years, ice has covered most of the continent.
In contrast with the much older geological evidence, the ice core record extends from the present day back to about 800 000 years ago. Over this time span the distribution of land and sea across the world has been essentially the same as today; and 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.
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.
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.
There are three main areas of Antarctica that have supplied long ice cores. The longest ice core record (extending back approximately 800 000 years) was taken from the Dome C area of East Antarctica by the EPICA team (European Project for Ice Coring in Antarctica). The coring was completed in 2004 to a depth of 3270m in the ice. Another important core, analysed in the 1990s, was taken from the Vostok station, also in East Antarctica. The Vostok ice core provides climate information extending back to about 400 000 years ago. West Antarctica is represented by the Byrd ice core record that was drilled to a depth of 2037m.
Antarctic ice cores have contributed a great deal to our understanding of climate change, particularly in the following ways:
- Over the past several hundred thousand years, fluctuations in mean annual surface air temperature over Antarctica have exceeded 10°C.
- The long timescale Antarctic temperature fluctuations correlate with the pattern of glacials and interglacials that affected the Northern Hemisphere. (Today we live during an interglacial (warm) time called the Holocene in which there is relatively little ice in the Northern Hemisphere compared to what has been usual over the past 2.6 million years (the time span of the Quaternary Period)).
- The levels of carbon dioxide and methane in Antarctic ice cores correlate with the pattern of cooling and warming associated with glacials and interglacials.
- Analyses of carbon dioxide and methane in the ice cores show that the present concentrations of these greenhouse gases in the atmosphere are higher than at any time spanned by the ice core record.
The diagram below summarise these major insights: