Status and prospects
Over the past couple decades, significant warming has been registered in some parts of Antarctica and great changes are likely in the years to come.
Status
Temperature
Since the early 1950s, considerable warming has been measured over the Antarctic Peninsula and to some extent the rest of West Antarctica; over the same time, little change has been seen in the rest of the continent. The greatest temperature increase is seen in the western and northern parts of the Antarctic Peninsula, where the rise is 0.53°C per decade between 1951 and 2006. On the west of the Peninsula, the increase is greatest in the winter season: 1.03°C per decade in 1951-2006. In the eastern parts of the Antarctic Peninsula, temperatures have increased most in summer and autumn, rising 0.41°C per decade between 1946 and 2006. Many recent studies based on available ground measurements from research stations and automatic weather stations, as well as satellite observations, indicate that West Antarctica has also seen a temperature increase in the past few decades, calculated to about 0.1°C per decade since 1950. Sipleregionen er identifisert som ett av de områdene i verden hvor temperaturen øker raskest i dag. No statistically significant changes in surface temperature have been registered elsewhere in Antarctica.
Precipitation
Measurements conducted since 1957 show no statistically significant changes in total precipitation in Antarctica. However, precipitation trends vary from region to region. Increased precipitation has been registered on the west of the Antarctic Peninsula. In East Antarctica, there is as yet no conclusive evidence of an increase, but in recent years, relatively large amounts of precipitation have been observed regionally in Dronning Maud Land. It is not clear whether this can be ascribed to natural variation, or marks the beginning of a long-term increase in precipitation.
Climate over the ocean
The temperature of the Antarctic Circumpolar Current in the Southern Ocean increased by 0.17°C at 700 to 1100 metres depth between the 1950s and the 1980s. This is above the global average. The temperature increase is related to the fact that this ocean current has shifted to the south owing to a southward shift of the westerly winds in the same time span.
The bottom water that forms in Antarctica and is exported to the South Atlantic has become warmer, for reasons that remain poorly understood. The water in the Indian and Pacific sectors of the Southern Ocean has become fresher – and the same is true of the bottom water that forms here. Along the Antarctic Peninsula, remarkably strong warming has been registered in the upper water masses: over 1°C during the Antarctic summer in the period from 1995 to 1998; this is probably related to reduction of sea ice in the area.
Interpreting the observations
Studies suggest that the hole in the ozone layer? above Antarctica over the past 30 years has helped limit the effects of global warming over the continent. The temperature increases on the Antarctic Peninsula are caused by warm air transported to the Peninsula with strong west winds. The increase over West Antarctica is related to higher surface temperature in the tropical part of the Pacific. Model studies show that the strong temperature increase in the Antarctic Circumpolar Current is probably due to increased emission of greenhouse gases, whereas the speed of the increase is attenuated by the cooling effect of aerosols (particles in the atmosphere).However, because of the limited amount of observational data, it is not clear how much of the rest of the warming in Antarctica can be attributed to human activities.
Future climate change
The information we have concerning future climate change in Antarctica is based on circulation models that consider atmosphere, ice and oceans as a whole. The models are not yet able to simulate with acceptable accuracy the changes we have already observed in Antarctica over the past couple decades, so the model results remain highly uncertain, particularly on the regional scale.
Despite the uncertainty inherent in the models, there is general agreement in the research community, based on model projections, that if the emission of greenhouse gases to the atmosphere continues at the current rate, the temperature over the Antarctic continent will increase several degrees during this century. Model projections consistently show that the surface temperature over land in Antarctica will increase through the year 2100, at a rate of between 0.14 and 0.5°C per decade. The largest increase is predicted at high altitudes inland in East Antarctica. Nonetheless, the surface temperature in 2100 is expected to remain well below the freezing point over most of Antarctica; thus the temperature increase will not contribute to melting of the Antarctic ice sheet. The models show the greatest temperature increase above sea ice in winter (0.51 ± 0.26°C per decade off the coast of East Antarctica) because of the expected retreat of sea ice, exposing the open ocean.
Climate models generally perform poorly when calculating the amount of precipitation during the 20th century owing to the difficulty of creating accurate mathematical descriptions of important processes involved in determining precipitation. Because of this, model projections into the future are also uncertain. However, most climate models predict a general increase in the intensity of the Circumpolar Current in response to an expected southward shift and intensification of the westerly winds over the Southern Ocean. The observed warming in the Southern Ocean is expected to continue, and involve all depths, although the warming in the uppermost water layers will be weaker there than in other oceans. Model calculations imply a rise in bottom water temperature of 0.25°C before 2100. This will have impact on water density and thus the circulation in the water masses.
The ozone layer
The ozone layer is a part of the atmosphere with a considerable concentration of ozone, and where this gas plays an important role in radiation balance. Ozone absorbs ultraviolet light. Radiation in the ultraviolet part of the spectrum has both positive and negative effects on biological systems.
The term ”ozone hole” describes a strong depletion of the ozone layer over polar regions. It was first observed over Antarctica, in measurements done at the British research station Halley. The ozone hole is almost certainly due to anthropogenic emission of chlorinated gases. When this became clear, many countries started implementing restrictions on these gases. More specific targets and emission limits were put in place in the Montreal Protocol of 1987. Model calculations of how the ozone layer will evolve show that the ozone layer should be back at 1980 values sometime between 2025 and 2050.
Impacts
Ongoing and expected climate changes over the next few decades influence – and will continue to influence – large-scale process with impact on the climate system within and around Antarctica. These climate changes in Antarctica will have multiple consequences. In the long run the sea ice is expected to retreat, and melting glaciers will contribute strongly to a rise in sea level. Last but not least, living conditions for plants and animals will change.
Research on climate change
The Polar Institute performs research on the past climate as well as present-day physical processes in the sea, the sea ice and the ice on land. In Antarctica, our scientists have helped to acquire information on the climate by drilling ice cores from the thick inland ice sheet in Dronning Maud Land. The ice is a climate archive which “traps” gases in the atmosphere and provides knowledge that stretches 900 000 years back in time. Through its wide-ranging research programme, ICE Fimbulisen, ICE (the Centre for Ice, Climate and Ecosystems) supplies knowledge about the climate in Antarctica and the impact it may have on the rest of the planet. The formation of deep water and the sea ice is being studied in the Fram Strait, and the size (mass balance) of glaciers is being studied in Svalbard.
Climate components
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
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.[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.
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.
Circulation
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.
Photo: AMAP (ACIA-rapporten, Key finding 1)
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.
Weddell Sea Bottom Water
On the continental shelf around Antarctica, the water becomes cold and dense. As a result, it sinks to the bottom and flows beyond the continental shelf to the deep ocean, where it constitutes the Antarctic Bottom Water. The coldest and largest contribution to this bottom water comes from the Weddell Sea. Seasonal variations in bottom water from the Weddell Sea are linked to seasonal variability in the wind patterns in the Sea’s western reaches, whereas annual variations are linked to the cyclonic gyre? in the Weddell Sea and thus also to large-scale climate phenomena such as the Southern Annular Mode and El Niño/Southern Oscillation
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.
Just as in the Arctic, this may in the long run perturb the radiation balance of the global climate system through the albedo effect? 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.
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.
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.
Climate indicators
Both sea ice and ice on land can warn us of changes in global or regional climate and are therefore important climate indicators.
Sea ice
The extent of Antarctic sea ice is stable or increasing slightly, albeit with large regional differences. The ice extent in Antarctica reached a record high in 2013, when it was greater than at any time for which measurements from satellites are available. Satellite data from Antarctica show that the average ice extent has increased slightly in the area as a whole (ranging from 1.2 to 1.8% per decade between 1979 and 2012), but that the regional differences are large. Around the Antarctic Peninsula, the sea ice cover is shrinking. In the Bellingshausen Sea, the ice cover has retreated, with about 5.3% less ice cover per decade in recent years, an effect that correlates closely with rising temperature over the Antarctic Peninsula. West of the Antarctic Peninsula, the ice season is now almost 90 days shorter, and multiyear ice has disappeared. These changes are accelerating. The trends in sea ice in the south can be linked to ice movement patterns; wind-driven changes in the advection of ice appear to be the most important factor. It has also been suggested that fresh water from the melting ice sheet plays a role.
Future scenarios
Available knowledge indicates that is it is reasonable to assume that sea ice around Antarctica will gradually decrease in extent and thickness.
Central processes in sea ice
The amount and the properties of snow on top of sea ice, the occurrence of soot and meltwater ponds on the ice are all factors that affect albedo?.
Much of the research done today focuses on quantifying how these factors contribute to melting in the Arctic. The role of clouds in global warming is also a focus of research interest, as are attempts to understand how a reduction of sea ice in the Arctic will affect cloud formation. These research results will make climate models more robust and accurate, which is crucial for projecting future developments in sea ice.
Ice sheets, ice caps and glaciers
On the other side of the globe there are also changes – Antarctica has lost ice mass over the past two decades. Most of this loss is from the Antarctic Peninsula and the area south of the Amundsen Sea in West Antarctica. The ice shelves around the Antarctic Peninsula are changing: the ice has retreated, become fissured, and large areas have collapsed on both sides of the Peninsula. Over the last 50 years, the total area of the ice shelves has decreased by an estimated 28 000 km2. The ice is now retreating about 6 000 kilometres per decade.
Ice shelves retreat when the ice cracks and the cracks are filled with meltwater. Relatively warm seawater also penetrates in under the shelves. When parts of an ice shelf disappear, the inland glaciers that feed into the shelf increase their flow rate. Collapse of some ice shelves on the Antarctic Peninsula has led to a 300-800% increase in the flow rate of glaciers in the areas where the ice shelves have disappeared.
The area south of the Amundsen Sea is the part of Antarctica where the glaciers of inland ice sheet are retreating fastest. For example, the retreat of the Pine Island glacier has sped up 40% since 1970. Thwaite Glacier and four other glaciers in this sector are thinning at an accelerating rate. The flow rate of Smith Glacier has increased 83% since 1992.
At present, this area is losing 50-137 gigatonnes of ice per year, which is approximately the same amount lost from the entire Greenland ice sheet. This is a significant contribution to global sea level rise.
In East Antarctica, the changes in the inland ice cap are less dramatic, with the greatest changes near the coast. The mass of the inland parts of the ice cap is growing moderately because of increased precipitation. Ice shelves differ: some are thickening slightly, others are thinning substantially. Data from passive microwave instruments suggest increased melting of ice shelves.