Viewing Antarctica on the scale of geological time
Figur 4 s. 6 i Mayewski et al. 2009 – The inland ice sheet Photo: Changes in global temperature over the past 80 million years, illustrated with changes in the Antarctic ice sheet.
The ice that currently covers most of Antarctica formed about 34 million years ago (which is a relatively short time from a geological perspective), probably owing to a decrease in atmospheric levels of CO2.
This decrease was caused by a combination of reduced release of CO2 from sub-oceanic mountain ranges and volcanos, and increased uptake of carbon. The atmospheric CO2 decrease lowered global average temperature – although it was nonetheless about 4°C warmer than today’s average. At that time the ice reached to the edge of the continent, but was probably warmer and thinner than it is today.
About 14 million years ago, the Antarctic climate suddenly grew colder, probably reinforced by the continent’s increasing geographic isolation rather than by a change in CO2 levels. The other continents drifted ever farther away from Antarctica and at the same time the Antarctic Circumpolar Current developed. At that time the inland ice sheet grew to approximately the size it has now, and it is believed to have maintained this size ever since.
The temperature difference between ice ages and interglacials in Antarctica has been about 9°C. The ice expanded in both the Arctic and Antarctic during the ice ages, and one consequence was that sea level fell by about 120 metres. Ice cores from both Greenland and Antarctica show that interglacial periods over the past 400 000 years have had temperatures 2-5°C and sea levels 4-6 metres above those we see today.
Studies of ancient climate show that the Antarctic Circumpolar Current developed in the transition between Eocene and Oligocene (34 million years ago), when Tasmania parted ways with Antarctica (geologically speaking), and Drake Strait opened up for a circumpolar ocean current around the continent. This gave the Southern Ocean its role of linking the world’s oceans through the deep ocean circulation. At the same time, the current isolated Antarctica by preventing heat transport to higher latitudes.
Ice cores from Antarctica also provide an archive of ancient climate. The cores show that the ice has been in a constant state of change owing to changes in solar radiation (variations in the shape of the earth’s orbit and thus its distance from the sun), and also reveal a strong link between atmospheric levels of greenhouse gases and air temperature.
About 98% of Antarctica is covered by an ice cap (the inland ice sheet) with an average thickness of at least 2.1 kilometres. This ice contains 90% of the world’s fresh water. Along with the sea ice that surrounds the continent, this ice plays a crucial role in the radiation budget at high southerly latitudes and is an important driver of atmospheric circulation. The inland ice sheet, which is over 4000 m thick in some places, keeps air temperatures low in the southern hemisphere and stabilises the cyclone belt around the continent.
Ice accumulates on the inland ice sheet through precipitation that falls as snow. This snow is compressed to form glacier ice that flows toward the coast impelled by gravity – sometimes moving rapidly in ice streams. When the ice reaches the coast, it flows out onto the sea, creating massive floating ice shelves. This movement coastward movement of ice gives the inland ice sheet a significant role in maintaining the regional climate system; changes in the balance between accumulation and loss of ice will have implications for climate and global sea level.
Considerable effort is going toward increasing our knowledge about the dynamics of the Antarctic ice sheet and its importance for the climate system. In this way, global climate models can be improved. However, much work is needed to obtain adequate models. The Norwegian Polar Institute has organised several large research projects to contribute fundamental new knowledge that can help refine the models. Examples include such projects as ICE Fimbulisen og ICE Rises.
The inland ice sheet of Antarctica is an important indicator for ongoing climate change, and changes in this ice can have far-reaching implications. Antarctica has lost ice mass over the past two decades. These losses have chiefly affected a restricted area, namely the Antarctic Peninsula and the part of West Antarctica that lies south of the Amundsen Sea. Many knowledge gaps remain to be filled to create a firm basis for adequate predictions of what will happen to the inland ice sheet in the future. Current models predict that over the next century, the ice balance will be positive for the inland ice sheet, owing to increased precipitation, but that overall mass balance will be negative: more ice will be lost along the coast (through calving and melting) than accumulates in the inland regions.
The ice sheet as a climate archive
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.
The Norwegian Polar Institute helps secure information about ancient climate by studying ice cores from both Svalbard og Antarctica.