One of the main impacts of climate change is sea level rise, brought about through melting of the world’s ice sheets and glaciers, as well as through thermal expansion of the oceans. The vast majority of world’s fresh water is locked up in the massive ice sheets covering Antarctica and Greenland. Of the two, Antarctica is by far the larger, holding about twelve times as much ice as Greenland. While the antarctic continent has not yet contributed a great deal to rising sea levels, it has the potential to eclipse the contribution from all other sources in the coming decades and centuries.
According to the results of the international Bedmap 2 project, which were published in 2013, the volume of ice covering the antarctic continent is approximately 26.5 million cubic kilometers (6.5 million cubic miles), which would be enough to raise world sea levels by around 58 m (190 feet) were it all to melt. If we exclude the comparatively small Antarctic Peninsula region, Antarctica can be divided into two major ice sheets, which between them cover almost the entire continent. The West Antarctic Ice Sheet, holds enough ice to raise sea levels by approximately 4.3 meters (14 feet) should it all melt. However this is dwarfed by the massive East Antarctic Ice Sheet, which contains enough ice to potentially raise sea levels by 53.3 meters (175 feet). These two ice sheets are separated by the Transantarctic Mountain Range; a chain of mountains which straddles the continent between the Ross and Weddell Seas.
Antarctica’s location means that it is subject to very different climatic influences and weather patterns than those affecting the Arctic. The Antarctic Circumpolar Current is a clockwise flow which occurs in the Southern Ocean. Because there are no significant landmasses to impede its flow, this current circles the continent of Antarctica, and effectively acts to keep colder waters near to the edges of the continent. Associated patterns of atmospheric circulation also help to isolate the Antarctic continent from major weather systems originating at temperate latitudes. The combined effect of these influences causes the climate of Antarctica to be considerably cooler than it would otherwise be.
The coast of the antarctic continent is buffered by a number of floating ice shelves, two of which, the Ross, and the Filchner-Ronne shelves are similar in size to Spain and California respectively. These two massive ice shelves are located between East and West Antarctica, on either side of the continent, and reach ice thicknesses of between 500 and 800 meters (1,650 and 2,600 feet). Other, smaller ice shelves are found all around the coastline of Antarctica. Unlike sea ice, which is seasonal, the ice shelves of Antarctica are semi-permanent features, which play an important role in regulating the transfer of ice from the continental interior to the ocean. Because they are floating, ninety percent of their mass is hidden below the ocean. Sections of the ice shelves often break away to form massive tabular icebergs, many of which are the size of small cities. The largest of these, Iceberg B-15, which broke off from the Ross Ice Shelf in March 2000, was 295 km (183 miles) long and had a surface area of 11,000 km2 (4,200 square miles).
Warming of the Antarctic Peninsula
While ocean and atmospheric circulation patterns provide a buffer between the Antarctic and the southern temperate latitudes, the effects of climate change are now becoming apparent in a number of regions. This is especially true for the Antarctic Peninsula, a narrow sliver of land which extends towards the tip of South America, and which represents the most northerly part of Antarctica. This peninsula is mountainous and is the only part of Antarctica which experiences significant summer snow melt. Temperatures here have warmed by an average 2.8 °C (5 °F) since records began fifty years ago, making it the fastest warming area in the southern hemisphere.
The key vulnerability of the antarctic ice sheet is believed to the response of the floating ice shelves to increased ocean temperatures. Since their ice is already floating, the presence or absence of floating ice shelves does not directly affect sea levels. However ice shelves serve an important role in that they act as a buffer, slowing the flow of ice from the continental interior into the sea. Since this ice originates on land, its melting directly contributes to rising sea levels. Without the damping effect of ice shelves, transportation of ice via ice streams and outlet glaciers would likely be considerably greater, causing sea levels to rise faster.
The effects of ice shelf collapse were dramatically demonstrated in early 2002, when a large part of the Larsen B ice shelf, some 3,250 km2 (1,270 square miles) in area disintegrated over a 35 day period. Larsen B is located on the Antarctic Peninsula, at a more temperate latitude than most other ice shelves in Antarctica. Nonetheless, the almost overnight breakup of such a major ice shelf caused shock amongst scientists. This event graphically illustrated how climate change can cause non-linear responses to occur in natural systems.
Though the exact causes of the breakup of Larsen B are unknown, the most widely accepted theory is that a period of unseasonably warm weather lead to the development of meltwater ponds on surface of the ice shelf. The water in these ponds forced its way down through natural cracks in the ice, and the weight of the water allowed them to propagate to the base of the ice shelf, weakening the entire structure and ultimately leading to the break up. Comparatively warm ocean temperatures likely compounded this effect through enhanced melting of the ice shelf from beneath, helping to weaken the overall structure. In the wake of the Larsen B collapse, satellite observations showed that the speeds of glaciers flowing into the affected area were between two and eight times faster than they had been prior to the collapse.
The Stability of the West Antarctic Ice Sheet
While increased melting in the Antarctic peninsula region shows how regional temperatures are changing, the potential contribution from this region to increased sea levels is comparatively small. Of more concern to scientists is the West Antarctic Ice Sheet. If this ice sheet did not exist West Antarctica would consist of a number of islands, with much of the region being below sea level. This is in marked contrast to the East Antarctic Ice sheet, which is mostly land based. It is this fact that makes scientists believe that the destabilization of the West Antarctic Ice Sheet is a distinct possibility.
The key area of concern are six major glaciers flowing into the Amundsen Sea, and particularly the massive Pine Island and Thwaites Glaciers. The Pine Island Glacier drains some ten percent of West Antarctica, and satellite observations have determined that its drainage basin provides a greater net contribution of ice to the ocean than any other glacial drainage basin anywhere in the world. Pine Island Glacier is approximately 250 km (156 miles) long and is over 2 km (1.25 miles) thick in places. It is extremely remote, and it is difficult for scientists to access this region, since the glacier is heavily crevassed and there are no research bases located anywhere within 1,000 km (625 miles).
Unlike most other major glaciers in Antarctica, the Pine Island Glacier is not constrained by a large ice shelf. There is a comparatively small ice shelf into which the glacier flows, which extends some 50 km (30 miles) into the ocean. This does provide a certain amount of buffering, however flow rates are still considerably faster than those of most other major antarctic glaciers, which flow into much larger and more stable ice shelf regions. Close to the point where it enters the ocean the Pine Island Glacier flows at over 4 km (2.5 miles) per annum, making it one of the fastest-moving glaciers in Antarctica.
As with glaciers in Greenland, one of the main concerns scientists have for glacier stability is the retreat of the grounding line; the point where a glacier flowing into the ocean loses contact with its bed and starts to float. This is a response to warmer ocean temperatures at depth, which causes melting of the glacier from underneath. As the grounding line retreats, warm water from the deep ocean can penetrate further under the glacier, thereby extending the melting of the base further up the glacier. Flow rates increase as the ice thins from beneath, since the glacier is no longer held back by contact with its bed. Faster flow rates in turn further increase the thinning of the glacier. As the thickness of a marine-terminating glacier decreases, there is less weight of ice pressing down on the glacier bed, so an increased area of the glacier begin to float, leading to further retreat of the grounding line. This cycle is a classic example of a positive feedback loop, which arises as a result of warmer ocean temperatures at the base of the glacier.
In spite of the influence of the Antarctic Circumpolar Current, warmer ocean temperatures are starting to affect some coastal regions of the Antarctic. The Amundsen Sea is one area which has seen significant ocean warming over the last few years, and this has caused the floating section of the Pine Island Glacier to thin by an estimated 5-7 meters (16-23 feet) per year in recent years, mainly as a result of melting at the base of the glacier.
The glaciers flowing into the Amundsen Sea share a number of characteristics which make them particularly vulnerable to increased ocean temperatures. Unlike most other antarctic glaciers, glacier beds in this region are below sea level over the entire catchment, and tend to slope inward towards the continent, which means that the maximum depth of each glacier bed occurs many kilometers upstream of current grounding lines. This renders these glaciers particularly vulnerable to the effects of grounding line retreat. All six affected glaciers also have very shallow surface inclinations, meaning that if warm ocean water is able to penetrate far under them, it could potentially cause large areas of the glacier to start floating.
A recently released study has concluded that the major glaciers flowing into the Amundsen Sea are in the initial phases of an unstoppable retreat, which will eventually lead to the destabilization of this region of West Antarctica. These glaciers collectively contain enough ice to raise sea levels by at least 1.2 m (4 feet), and their loss could potentially affect the rest of the West Antarctic Ice Sheet, ultimately resulting in sea level rises of more than 3 m (10 feet). The study is based on twenty years of radar satellite observations, which were used to determine the changing positions for the grounding lines of all six glaciers. This information was combined with Bedmap 2 data, which provided details of the topography of the glacier beds. A parallel study, which used computer modelling, estimated the potential time frame of destabilization at between 200 and 500 years into the future. Although destabilization of this region is now believed to be all but inevitable over the longer term, this study suggests that the contribution to sea level from these glaciers over the next century will be no more than a few centimeters, with the bulk of the sea level rise occurring after this period.
Potential Problems in East Antarctica
So far the East Antarctic Ice Sheet has shown little sign of change, and scientists believe that it is generally stable at present. This is likely due to the fact that the base of this ice sheet lies above sea level in most areas, and also because ocean temperatures are generally cooler than those around West Antarctica. The East Antarctic Ice Sheet is estimated to be between thirty and forty million years old. It is believed to have formed under warmer climatic conditions than those of the current day, when atmospheric CO2 levels were much higher than at present. This makes it likely that the majority of ice currently locked up in Antarctica will remain there for the foreseeable future, even under more extreme warming scenarios. The elevation of the continental interior, and its location over the south pole mean that temperatures remain at many degrees below the freezing point year round, and this is unlikely to change over any timeframe meaningful to us. Nonetheless, some coastal regions of East Antarctic are vulnerable to the effects of increased ocean temperatures, and these have the potential to impact sea levels in a significant way.
There are parts of East Antarctica which do lie below sea level, and these potentially contain enough ice to raise sea levels by 19 m (63 feet) should they all melt. While this possibility is considered unlikely, a recent study identified an area known as the Wilkes Basin as potentially being vulnerable to the effects of ocean warming. This area is currently protected from destabilization by a number of small ice plugs, which prevent ocean water from reaching the basin. If warmer ocean temperatures were to cause these plugs to disappear, destabilization of the Wilkes Basin could potentially contribute 3-4 m (10-13 feet) to global sea levels. This is not seen as an immediate threat, but the authors suggest that it is a distinct possibility over timescales of more than a century. Historical records suggest that this may have happened during the Pliocene, around four million years ago. There are other threats too, such as the potential collapse of coastal ice shelves, and the corresponding increase in ice transport to the ocean.
Awakening the Sleeping Giant
Melting of ice from Antarctica currently contributes only about a tenth of the annual increase to global sea levels, less than the amount of melt from the world’s glaciers, and also less than the amount from the Greenland ice sheet. Antarctica’s current contribution to sea level is about 0.25 mm (0.01 inches) out of an average rise of about 3 mm (0.1 inches) per year. However the potential for widespread melting of the West Antarctic Ice Sheet, and also of parts of East Antarctica make it likely that this picture will change in the future. Predicted rises in sea level over the 21st century range between 0.5 m and 2 m (1.6 – 6.6 feet), and it appears likely that Antarctica will become an increasingly important contributor over this period.
However it is the medium-term picture which is of particular concern. Because of its size, the glacial dynamics of Antarctica have a high degree of built-in inertia. Changes which appear small at present will likely cause non-linear responses in the future. The probable destabilization of the West Antarctic Ice Sheet, and the possible destabilization of the Wilkes Basin region of East Antarctica, are likely to raise sea levels by several meters over the coming centuries. Much of this future sea level rise is already locked in, and cannot be avoided even if we manage to keep the rise in global temperature below the often talked about two degree threshold.
Though slow to wake up, Antarctica is truly a sleeping giant. In the coming centuries it is likely that ice melt from the antarctic continent will significantly exceed that from all other sources, including Greenland. Increased sea levels may not affect our lives dramatically at present, but our descendants will have to live with consequences we can only begin to imagine. Once started, unstable melting of the Antarctic is likely to continue for hundreds, or even thousands of years, until a new equilibrium is reached. This is the consequence of our carbon intensive way of life, and even if we were to stop emitting CO2 tomorrow our legacy will endure for thousands of years into the future.
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photo credit: Polargeo via Wikimedia Commons