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The continent of Antarctica is the coldest, driest, and windiest place on Earth. Located almost entirely south of the Antarctic Circle, about 98% of Antarctica is covered in ice (Wikipedia). In fact, the continent contains about 90% of all the ice in the world, enough to cause the land underneath to sink beneath the weight. Parts of the West Antarctic Ice Sheet are estimated to be up to 2.5 km below sea level because of the weight of the ice (NSIDC).
Antarctica is divided by the Transantarctic Mountain range. The Western Antarctic ice sheet is the portion of the continent west of the Weddell Sea and east of the Ross Sea (See Figure 1). Everything east of this point is the Eastern Antarctica ice sheet. The Antarctic ice sheet is at least 40 million years old (NSIDC). Antarctic ice has an average thickness of about 1.6 kilometres (1.0 mi) (Wikipedia) but is over 4,200 meters thick in some areas (NSIDC).
Over the previous decades, Antarctica, like Greenland, has been experiencing a number of changes in response to climatic changes that can best be described as changes in mass balance. According to Overland et al. (2008), "Although both the Arctic and Antarctic are subject to a similar annual cycle of solar radiation and the same increasing greenhouse gas concentrations, over the previous two decades the two regions have experienced dramatically different changes in sea ice extent, temperature, and other climatic indicators. While these differing responses suggest a paradox, they are largely consistent with known climate dynamics." Like Greenland, the small number of research and reporting stations compared to the massive size of the continent make it difficult to ascertain what is actually happening across the entirety of Antarctica. However, advances in technology have allowed scientists to decipher, observe, and record data from which regional trends are quite apparent.
Building on the results of a number of studies employing various methods and techniques, the IPCC suggests overall Antarctic Ice Sheet mass balance ranged from growth of 50 Gt per year to shrinkage of 200 Gt per year from 1993 to 2003 (IPCC 2007). Despite that wide range, the IPCC finds that acceleration of mass loss is "likely to have occurred, but not so dramatically as in Greenland" (IPCC 2007). Other studies have found that "the ice sheet as a whole was certainly losing mass [over the time period of the study], and the mass loss increased by 75% in 10 years" (Rignot et al., 2008). Part of this has to do with the differences in regional mass balance trends.
The interior and coastal areas of Antarctica receive vastly differing amounts of precipitation due to the fact that weather fronts often are not able to reach the center of the continent, leaving it cold and dry. Although the South Pole receives roughly 10 cm (4 inches) of precipitation each year, precipitation in the interior averages closer to 20 mm (0.8 inches) annually, making Antarctica home to the largest desert in the world (Wikipedia). Despite the low levels of precipitation, ice lasts for long periods of time in the interior. However, in some areas, including the McMurdo Dry Valleys, the precipitation that does fall is not enough to offset sublimation, resulting in negative mass balances of glaciers and ice sheets. The coastal areas, on the other hand, typically receive quite a bit of snowfall as they are warmer, allowing for more precipitation. Essentially, the East Antarctic Ic Sheet is observed to be thickening while the West Antarctic Ice Sheet is thinning (See Figure 2).
The East Antarctic Ice Sheet has been thickening at a rate of about 2.2 cm per year since the mid-1990's, more than most global computer models predicted. This is primarily attributed to increased snowfall (Davis et al., 2005) – although other studies (Monaghan et al., 2006; Rignot et al., 2008) did not find the strong increase in snow accumulation suggested by Davis et al. (2005). The changes in Western Antarctica, on the other hand, are considered to be related to ice and glacier dynamics.
As the Antarctic ice sheets spread under their own weight, mass is transferred from the interior to the margins where it may break off, forming icebergs or become an ice shelf, floating on the water but remaining attached to the ice sheet. As an ice shelf advances, it receives new material from both snowfall on its surface and ice from the interior. The majority of ice streams and outlet glaciers on the Antarctic continent flow into ice shelves.
Over recent years, significant changes in thickness have been observed in ice shelves (permanent ice floating on the ocean surrounding Antarctica), which cover roughly 11% of the entire Antarctic ice sheet, an area of about 1.5 × 106 km2 (IPCC 2007). A 2005 study found there to be "a clear regime of [glacial] retreat, which now exists across the Antarctic Peninsula." The authors of the 2005 study found that of 244 marine glaciers under observation, 87% have retreated since the middle of the last century, and the boundary between advancing and retreating glaciers has moved further and further southward (Cook et al. 2005). Even the glaciers that were advancing were small compared to the scale of retreats observed in the other study areas. Additionally, in 2006, a study found that ice shelves fed by West Antarctic glaciers were losing mass by 95 ± 11 Gt per year while ice shelves fed by East Antarctic glaciers were gaining mass at a rate of 142 ± 10 Gt per year. Overall, faster rates of change were observed on smaller ice shelves (Zwally et al., 2006).
Rates of mass loss are being watched closely, as they can indicate potential ice shelf instability. The 2007 IPCC report notes, "Thinning of about 1 m yr–1 (Shepherd et al., 2003; Zwally et al., 2006) preceded the fragmentation of almost all (3,300 km2) of the Larsen B Ice Shelf along the Antarctic Peninsula in fewer than five weeks in early 2002 (Scambos et al., 2003)."
Changes in the mass of ice shelves doesn't directly impact sea level because the material contained in the ice shelf is already floating. However, ice shelves do affect sea level indirectly in that they impact the flow of glaciers and other nearby ice. Both the hydrostatic force of the ocean and friction at the side of the ice shelves and ocean floor slows the flow of the ice shelf and its discharge. When the ice shelf is no longer there to regulate flow from the interior, it continues unimpeded into the sea. This phenomenon was observed after the 2002 breakup of the Larsen B ice shelf, when glaciers flowing into the ice shelf started moving at eight times their normal speed. Despite this large increase, there was very little change in the velocity of ice still supported by the intact portions of the ice shelf, which has been observed to be undergoing widespread glacial retreat (Rignot et al., 2004; Scambos et al., 2004; Cook et al., 2005). Across Antarctica, glacier flow seems to be accelerating primarily due to reductions in the ice shelves, whereas in Greenland increased glacial flow results mainly from dynamic thinning.
A number of studies have observed increased flow velocity of glaciers along the Antarctic Peninsula (De Angelis and Skvarca, 2003; Scambos et al., 2004; Rignot et al., 2004, 2005, 2008; Zwally et al., 2006; Shepherd et al., 2004; Thomas et al., 2004 are some). One recent study found the flow rate of these glaciers has increased by about 12% over the past 12 years alone (Pritchard and Vaughan, 2007). A study of the glaciers in the Amundsen Sea area showed faster thinning during 2002-03 than in the 1990s extending 300 km (190 miles) inland. The increased velocity of the tributary glaciers in this area has led to ice shelf thinning (Shepherd et al., 2002, 2004; Joughin et al., 2003; Thomas et al, 2004). According to Thomas et al. (2004), if these shelves break up like Larsen B the resulting glacial drainage would raise sea levels by 130 cm (51 inches). In other locations in Western Antarctica, such as the Whillans and Bindschadler Ice Streams, however, ice flow deceleration is occurring (Joughin and Tulaczyk, 2002).
Coastal Antarctica is also experiencing fluctuations in sea ice coverage. Sea ice off of West Antarctic is especially vulnerable to warming since it is exposed to both increasing air and ocean temperatures. A 2008 study found that from 1979–2004, sea ice in the Antarctic Peninsula region retreated 31 ± 10 days earlier and advanced 54 ± 9 days later, resulting in a decrease of the ice season by 85 ± 20 days. Conversely, in the western Ross Sea region, sea ice retreated 29 ± 6 days later and advanced 31 ± 6 days earlier.an increase in the ice season of 60 ± 10 days. According to the study's authors, "Sea ice advance appeared to be more sensitive to climate variability than sea ice retreat, perhaps owing to its unconstrained equatorward expansion and ability to quickly respond to changing atmospheric conditions. Sea ice extent determined in part by circulation patterns. Continued greenhouse gas increases would suggest…a continuation, and perhaps spatial expansion, of the regional sea ice trends described here. If the trends in the [Antarctic Peninsula region] continue, then thinning of the West Antarctic ice sheet will likely continue, exacerbating sea level rise" (Stammerjohn et al., 2008).
The current trends in Antarctica are due to changes in temperature and responses to forcing. Once these changes occur, they can lead to positive feedback loops that will have other, sometimes unforeseen effects on the region's climate.Changes in ocean temperatures
The average temperature of oceans across the globe has been increasing. Over the period 1961 to 2003, global ocean temperature has risen by 0.10°C from the surface to a depth of 700 m. Despite a slight cooling since 2003, the IPCC states "…even if all radiative forcing agents were held constant at year 2000 levels, a further warming trend would occur in the next two decades at a rate of about 0.1°C per decade, due mainly to the slow response of the oceans" (IPCC 2007).
The Southern Ocean, which surrounds Antarctica, has also been warming. Gille (2002) found the largest increases in ocean warming occur in subantarctic regions, while a slight cooling occurred to the north (See Figure 3). The upper ocean next to the West Antarctic Peninsula warmed by more than 1°C from 1951 to 1994 (Meredith and King, 2005). The increasing temperature is likely to have resulted from "large regional atmospheric warming," as well as diminishing winter sea ice (Vaughan et al., 2003; IPCC 2007). In the Amundsen Sea region, increased melt is consistent with an observed 0.2°C increase in ocean temperature nearby (Jacobs et al., 2002; Robertson et al., 2002; IPCC 2007). It is estimated that a 1°C increase in water temperature under an ice shelf increases basal melt rate by about 10 m per year, which can lead to thinning and structural instability (Shepherd et al., 2004; IPCC 2007).Changes in surface and air temperature
In addition to being one of the driest areas on the planet, Antarctica is the coldest place on Earth. Scientists at Vostok Station recorded the world's lowest temperature of –89°C (–129°F) (Wikipedia). During the winter, the temperatures in the interior can drop as low as –90°C (–130°F) while summer maximums along the coast can average as low as 5°C and as high as 15°C (41°F and 59°F). Of the two halves of the continent, Eastern Antarctica is colder because it has a higher elevation than the West.
Over the past five decades, however, the Antarctic Peninsula has seen temperature increases of 0.5°C (0.9°F) per decade, making it one of the fastest warming places on Earth, according to the National Snow and Ice Data Center (Scott, 2008). According to the 2007 IPCC report and satellite imagery, surface temperature changes show regional patterns—strong warming in the Antarctic Peninsula region has been observed, while the interior has experienced a slight cooling if any change at all (van den Broeke, 2000; Vaughan et al., 2001; Thompson and Solomon, 2002; Doran et al., 2002; Schneider et al., 2004; Turner et al., 2005; NASA Earth Observatory) (See Figure 4).
Scientists believe the cooler temperatures observed in the interior of the continent have two possible explanations. The first is related to warmer ocean temperatures in the region. As ocean temperatures increase, more precipitation is produced over the interior of the continent. This cools the region around the South Pole. Despite a 2005 study that showed thickening across the interior of the continent, as mentioned earlier, other studies (Monaghan et al., 2006; Rignot et al., 2008) have not found evidence of this trend.
The second possible explanation is the affect of the ozone hole on the polar vortex, a tight band of winds spinning around the South Pole. Ozone absorbs ultraviolet radiation in the Earth's stratosphere, which consequentially warms. The loss of ozone therefore may have caused the stratosphere above Antarctica to cool, which in turn would strengthen the polar vortex. The cooling trend over the interior is a result of the vortex acting as a more effective atmospheric barrier to the penetration of warmer air from the coastal areas into the center of the continent. According to Overland et al., (2008), the effect of lower ozone levels has masked the true impacts greenhouse gases are having on temperatures and sea ice loss.
As the ozone hole becomes smaller, the stratosphere will continue to warm, leading to a marked increase in temperatures across the whole of Antarctica. With this relatively warmer weather, more precipitation will be able to fall, leading to a projected mass gain over the interior of the continent such as that already being experienced in Greenland, however this has not yet been conclusively observed. However, the troposphere, the layer of the atmosphere between the surface and the stratosphere, is already showing signs of warming. In a 2006 study, air temperatures 5.5 km above sea level have increased by 1.5–2.1°C (2.7–3.9°F) since the 1970's.
Unlike the interior, coastal areas and the western portion of the Antarctic continent have undergone the most rapid average annual temperature rise of anywhere on the planet—a rate, according to scientists, unprecedented over the past two millennia (Vaughan et al., 2003; Fox and Cziferszky, 2008). The biggest increase in surface temperatures is occurring in the Antarctic Penisula, where there has been a warming of mean annual temperatures of more than 2.5°C (4.5°F) since the 1950s, 2°C of which have happened since 1980 alone (King et al., 2003; Overland et al., 2008). The Antarctic Peninsula is one of the few places on the continent that melts and thaws each year. However, according to Vaughan (2006), the number of days per year on which melting of ice can occur has increased by up to 74% in the same time period.
Surface melting is less of an issue for overall mass balance in Antarctica than it is in Greenland because while surface melt runoff is large for parts of the Antarctic Peninsula, it has generally small or zero elsewhere on the ice sheet (IPCC 2007). However, there is evidence that the cooling trend that dominated throughout the end of the last century is over, and that warming 2004–2005 summer seasons in Antarctica marked the beginning of a warming trend.
According to NASA, temperatures were so warm in January 2005, an area the size of California melted. Although the area had refrozen only a week later, there were a number of interesting things about this occurrence. The team found "the observed melting occurred in multiple distinct regions, including far inland, at high latitudes and at high elevations, where melt had been considered unlikely. Evidence of melting was found up to 900 kilometers (560 miles) inland from the open ocean, farther than 85 degrees south (about 500 kilometers, or 310 miles, from the South Pole) and higher than 2,000 meters (6,600 feet) above sea level. Maximum air temperatures at the time of the melting were unusually high, reaching more than five degrees Celsius (41°F) in one of the affected areas." With warming surface temperatures, it seems likely that surface melt will continue to increase.
According to the IPCC, "…the pattern of observed temperature trends in the last half of the 20th century (warming over the Antarctic Peninsula, little change over the rest of the continent) is not projected to continue throughout the 21st century." Instead, the rest of the continent will begin warming as well (see Figure 5). The science suggests that ice shelf changes have resulted from environmental warming—oceanic and atmospheric. The effects of changing patterns of oceanic circulation are also being explored, as they likely have significant impacts on the region.Subglacial Topography
The underlying landscape impacts the formation and stability of the ice and snow above it. The composition of bedrock and bed sediments impacts the speed at which the glacier flows. The slope of land beneath glaciers can also have significant impact on the stability of glaciers. The Pine Island Glacier, for example, is one of Antarctica's thinning glaciers. A recent study of Pine Island subglacial topography confirms the potential instability of the glacier's lower basin, which holds enough ice to raise global sea by about 24 cm (Vaughan et al., 2006). Another place under watch is the Ross Ice Shelf, which scientists regard as a barometer of the Western Antarctic ice sheet's health. The Ross Ice Shelf's bed is actually below sea level, and therefore at increased risk of deterioration from rising and warming seas (Fox, 2008). The ice sheets that survived the last period of deglaciation are on land, not over the sea.
Water also plays a role in glacial flow and ice sheet health. Below a glacier or ice sheet, water can act as a lubricant and cause glacial acceleration and dynamic thinning. Lakes have recently been found underneath East Antarctica's Recovery Glacier. Scientists have linked these subglacial lakes to increased flow in the ice stream. In addition to accelerated melting due to these underground lakes, the authors of the study warn that glacial lake outburst floods (GLOFs) are possible in subglacial lakes such as these (Bell et al., 2007). These GLOFs would not only add to sea level rise, but also would lead to additional thinning of the ice sheet.Responses to Forcing
Ice sheets, shelves, and glaciers respond to environmental forcing over numerous time scales. According to the IPCC, surface warming needs more than 10,000 years to affect ground temperatures, while a moulin allows water to penetrate to the bed and change temperatures within minutes. Likewise, changes in velocity can occur over long time periods, or, as scientists are discovering after observing events like the Larsen B collapse, very quickly. These processes may begin cycles of positive feedbacks, wherein a thaw can lead to accelerated movement, leading to dynamic thinning, further increasing surface melt, etc.
Evidence from the paleo-record suggests that during previous ice ages ice sheets respond to warming by shrinking, and conversely, grow in response to cooling. Additionally, there is evidence that warming-induced shrinkage can be far faster than growth (Paterson, 1994; Clark et al., 1999). The IPCC states, "Despite competition from stabilizing feedbacks, warming-induced changes have led to rapid shrinkage and loss of ice sheets in the past, with possible implications for the future" (IPCC 2007).
Many of the trends we've discussed so far have been almost contradictory for the East and West Antarctic Ice sheets. The two ice sheets that make up 95% of what we know as Antarctica have, according to a growing number of scientists, experienced vastly different climate histories. Recently publicized findings of algae and plant material from the Dry Valleys, as well as analysis of the 2006 and 2007 ANDRILL cores show the Western Antarctic ice sheet experienced a climate as much as 20°C warmer than it does currently (Fox, 2008). Temperatures in Western Antarctica fluctuated in response to changes in the temperature of the ocean, which underlies most of the region's ice sheet.
These findings reaffirm the vulnerability of the West Antarctic Ice Sheet to climate changes. One of the researchers pointed out that "[we are] getting this sense of extended periods when the West Antarctic Ice Sheet was very small, if not gone altogether" (Fox, 2008). One of these times was 4 million years ago, when records from other regions indicate global temperatures 3 or 4°C warmer than today and CO2 levels were around 400 parts per million. These conditions have been projected by the IPCC in their latest assessment as within the range of expected levels by 2100. The melting of the West Antarctic Ice Sheet would lead to sea level rise of as much as 20 feet. The central East Antarctic Ice Sheet, on the other hand, is stable. While some of it fringes lie above the ocean, the majority of the sheet is high, dry, and cold, making it unlikely that it will melt significantly.
These climatic changes and trends are already having an impact on the environment of the region, as well as further abroad. Antarctica is surrounded by the Southern Ocean, which is the region south of 30°S where the Atlantic, Indian, and Pacific Oceans flow together. Here, where the waters from the different oceans mix, lies one of the densest parts of the Meridional Overturning Circulation (MOC). Because the bottom waters of the MOC form in the Southern Ocean, changes observed in the waters surrounding Antarctica will have impacts on regions across the world. Environmental changes will, and in many cases, already are impacting societies and wildlife.Sea Level Rise
Because Antarctica holds about 90% of the world's ice, rising temperatures and increased mass loss bring with them rising concerns about the impacts on sea level rise. If the Antarctic ice shelves melted completely, the sea would rise by over 73m (USGS). Even the Antarctic Peninsula alone is estimated to hold enough ice to raise global sea level by 5 to 10 cm (Fox, 2008). If the shelves melted, it would drastically change the shapes of the continents as we know them. For an idea of what impact this amount of melt would have on coastlines across the world, click here.
Like the Greenland ice sheet, Antarctica is not going to completely melt into the sea immediately. Current models predict that most of Antarctica's ice will remain in the interior for many years. Numerous papers, however, show a rate of sea level rise of 3.1 ± 0.7 mm per year over 1993 to 2003, a significant portion of which are due to changes in the Southern Ocean surrounding Antarctica (Cabanes et al., 2001; Cazenave and Nerem, 2004; Leuliette et al., 2004; IPCC 2007). According to the IPCC's latest report, Antarctica contributed 0.2 ± 0.35 mm per year to sea level rise over the period 1993 to 2003, with accelerated loss through 2005.
Although an increase in precipitation over Antarctica is projected to contribute negatively to sea level rise relative to the present day, it does not take into account accelerated surface melting, nor accelerated ice flow. Likewise, many aspects of ice shelf and glacier dynamics are still not understood well enough to accurately model. Currently, the IPCC (2007) believes that thickening of East Antarctica has been more than offset by thinning related to increased ice outflow in coastal regions of West Antarctica.Freshwater flux
Because Antarctica has about 90% of the world's ice it has about 70% of the world's fresh water. Freshening can result from a combination of factors, including increased precipitation, reduced sea ice production, melting, and glacial flow. As this freshwater is released into the sea, it affects both the salinity and density of ocean water. Small, long-term contributions do not have much effect. On the other hand, large, quick inputs are enough to significantly impact ocean dynamics. One of the big concerns is that massive inputs of freshwater from large-scale melting in Greenland or Antarctica will slow or shut down the MOC, which, in turn, as significant effects on the climates of Europe and North America.
According to the IPCC, the oceans in subpolar latitudes have been freshening since 1955 (IPCC 2007). For example, in the Ross Sea area, decreases in salinity of 0.003 psu per year have been observed over the last four decades (Jacobs et al., 2002). Freshening in the Southern Ocean has already been linked to changes in the South Pacific, where the deeper waters originate from Antarctica. This area has seen a freshening of 0.01 psu from 1968 to 1991 as well as reduced bottom transport (Johnson and Orsi, 1997).Impact on flora and fauna
Warming temperatures and melting ice in Antarctica are having an effect on the continent's wildlife. Grasses and birds not common to the Antarctic Peninsula have been colonizing the area as it warms and loses ice. Like the polar bear in the Arctic, the U.S. Fish and Wildlife Service is currently evaluating 12 species of penguins, including the Emperor Penguin, for listing under the Endangered Species Act. Many of these species have declined by as much as 50 percent over the past 50 years, due to a reduction in habitat and food sources brought about by the changes taking place across the continent (Mullen, 2007) (See Figure 6). A number of bird species are observed to have undergone changes in nesting and laying habits. A study of nine sea birds found that on average, the birds arrive at their nesting colonies up to 9.1 days later than in the 1950's, and lay eggs 2.1 days later (Barbraud and Weimerskirch, 2006).Economic impact: Mineral extraction
Although the climatic changes taking place across the Antarctic continent will have economic impacts felt throughout the world, melting ice and warmer temperatures will open Antarctica itself up to exploitation. The main mineral resources known on the continent are coal and iron ore. However, the most valuable resources of Antarctica likely lie offshore in the form of oil and natural gas. Fields were initially explored in the Ross Sea in 1973, but there are more extensive areas to explore under the ice shelves. Currently, the Antarctic Treaty bans all exploitation of mineral resources. Despite the protection afforded by the Treaty, it will be interesting to see if the ban on resource extraction will be challenged as those resources become more accessible and countries attempt to establish proprietary rights to them such as in Greenland and the Arctic.
According to the IPCC, "Comprehensive model runs for ice sheet behaviour over the last century… match overall ice sheet trends rather well (Huybrechts et al., 2004) but fail to show these rapid marginal thinning events," suggesting these changes are responses to processes or stimuli not currently included in the models. There are many processes, such as those linked to sub-ice bathymetry, that are not well understood yet. Additionally, the knowledge that outlet glacier and ice stream speeds can change as rapidly as they have is recent. Furthermore, Antarctica's subglacial topography is still being uncovered and its potential impacts studied. The roles of the Southern Annular Mode (SAM), the southern hemisphere's pattern of atmospheric and climactic variability, and radiative forcing must be further clarified in order to understand precipitation, humidity, and temperature trends. Therefore, it is likely that model predictions and current assessments underestimate the causes, impact, and effects of thinning ice sheets.
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