Climate components in Antarctica

Antarctica produces cold air, cold water and ice. These processes are fundamental for the climate system of the southern hemisphere, and influence that of the northern hemisphere as well.

The inland ice

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.

Photo: 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.

Viewing Antarctica on the scale of geological time

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.[1] 

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.[1]

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.[2] 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.[3] 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.[3]

Circulation

Den globale havsirkulasjonen

Photo: AMAP (ACIA-rapporten, Key finding 1)

The Southern Ocean is the largest ocean in the world. The Antarctic Circumpolar Current connects the Pacific, Atlantic and Indian Oceans. The Southern Ocean produces the coldest, densest water that forms the bottom water in the global ocean circulation. The Southern Ocean and its physical characteristics are crucial for the earth’s climate.

The Southern Ocean is one of the least studied marine areas in the world. For large parts of this ocean, data from before 1950 are rare or non-existent. Data availability increased only when satellite measurements began in the 1970s and 1980s. Logistic challenges and weather conditions still impede data collection, particularly in winter. Automatic data loggers deployed in the ocean and on marine mammals are now supplementing our knowledge. Nonetheless, understanding of the role the Southern Ocean plays in the global climate system is still limited owing to lack of data.

The Southern Ocean ventilates the world’s oceans and regulates the climate system by taking up and storing heat, fresh water, oxygen and atmospheric CO2.

The strong westerly wind belt that surrounds the Antarctic Continent drives the world’s largest and most powerful system of currents: the Antarctic Circumpolar Current. It is also assumed to be the most important driver for ocean circulation worldwide.

The Southern Annular Mode (SAM), also known as the Antarctic Oscillation, is the main driving force creating variations in atmospheric circulation at high latitudes in the southern hemisphere. SAM accounts for about 35% of the climatic variability seen in the southern hemisphere, and is probably the driving force behind large-scale circulation in the Southern Ocean.[2] In the past 50 years, SAM has shifted to a more positive phase, where air pressure is lower along the Antarctic coast and higher in mid-latitudes. This phase shift has resulted in 15-20% stronger winds in the westerly wind belt over the Southern Ocean since the 1970s.[1]

Sea ice

During the Antarctic winter, up to 18 million square kilometres of sea ice forms around the Antarctic continent, but unlike Arctic sea ice, nearly all the ice that forms in winter melts again the following summer. At the end of summer only about 3 million square kilometres of sea ice remains. This is mainly caused by the open seas. In an open sea, sea ice moves more freely; drift speeds are higher, and because of the land masses that limit drift to the south, the ice moves more easily toward warmer waters where it melts. At its maximum extent, sea ice is distributed fairly symmetrically around the Antarctic continent. Ice extent in Antarctica is characterised by large year-to-year variability.

Given that Antarctic sea ice is not particularly long-lived, it does not have an opportunity to become as thick as sea ice in the Arctic. The thickness varies considerably, but is usually 1-2 metres.

Satellite data from Antarctica show that the average extent of sea ice has increased somewhat overall (1.2-1.8% per decade between 1979 and 2012), but that there are large regional differences. It is not clear whether this increase is a sign of a substantive change, however, because the ice extent around Antarctica varies considerably from year to year. Nonetheless, available knowledge suggests that it is reasonable to assume that the sea ice around Antarctica will gradually decrease in extent and thickness.

[3] Just as in the Arctic, this may in the long run perturb the radiation balance of the global climate system through the albedo effect. Warming may also influence bottom water formation owing to rising surface temperatures and increased fresh water runoff; this would perturb the force that drives global ocean circulation, which in turn defines the framework for the world’s climate.

Ice shelves

About 10% of the dry land in Antarctica is ice shelves. Fresh meltwater from the underside of these ice shelves is an important factor in sea circulation and ice cover around Antarctica. The water that runs out to sea creates a surface layer with low temperature and low salinity. The condition of ice shelves is thus an important component in the climate system.

Ice shelves are by nature dynamic. The lose mass through both calving at the seaward edge and melting from below. About half the ice loss from the Antarctic ice sheet occurs through melting from the underside of ice shelves. Around the Antarctic Peninsula, however, the past couple decades have seen ice shelves disintegrating to a degree unprecedented in the last 10 000 years. At the end of the 1990s, parts of the Larsen ice shelf at the Antarctic Peninsula collapsed; in 2009 the Wilkins ice shelf in West Antarctica collapsed. All told, seven of twelve ice shelves around the Peninsula have retreated, losing a total area of 28 000 km2. The ice is currently retreating about 6 000 km per decade.[3]

Studies in recent years show that melting in the sea-to-ice interface (under the shelves) is more important than previously assumed, and suggest that this melting accounts for as much as 55% of the loss of mass from Antarctic ice shelves. Another crucial factor is that the ice shelves serve as retaining walls for glaciers and the ice sheet farther inland. If the ice shelves collapse in the future, it would affect the seaward flow of land-based ice and thus also sea level. Observed collapses of individual ice shelves on the Antarctic Peninsula have led to 300-800% increases in the flow rates of glaciers inland of where the shelves have disappeared.[3]

Studies in recent years show that melting in the sea-to-ice interface (under the shelves) is more important than previously assumed, and suggest that this melting accounts for as much as 55% of the loss of mass from Antarctic ice shelves. Another crucial factor is that the ice shelves serve as retaining walls for glaciers and the ice sheet farther inland. If the ice shelves collapse in the future, it would affect the seaward flow of land-based ice and thus also sea level. Observed collapses of individual ice shelves on the Antarctic Peninsula have led to 300-800% increases in the flow rates of glaciers inland of where the shelves have disappeared. This includes research at the Norwegian Polar Institute.

References

  1. A.J. Turner et al. 2009. Antarctic climate change and the environment. Antarctic Science. DOI:10.1017/S0954102009990642.
  2. P.A. Mayewski et al. 2009. State of the Antarctic and Southern Ocean climate system. Reviews of Geophysics (47): 1. DOI: 10.1029/2007RG000231
  3. Intergovernmental Panel on Climate Change (IPCC) 2013. Fifth assessment report contribution.
  4. Arnold L. Gordon et al. 2010. A seasonal cycle in the export of bottom water from the Weddell Sea. Nature Geoscience 3, 551 - 556. DOI:10.1038/ngeo916