Thesub-Antarctic zone[1][2] is aphysiographic region in theSouthern Hemisphere, located immediately north of theAntarctic region. This translates roughly to alatitude of between46° and60° south of theEquator. The subantarctic region includes manyislands in the southern parts of theAtlantic,Indian, andPacific oceans, especially those situated north of theAntarctic Convergence.Subantarctic glaciers are, by definition, located on islands within the subantarctic region. Allglaciers located on thecontinent ofAntarctica are by definition considered to beAntarctic glaciers.
The subantarctic region comprises two geographic zones and three distinctfronts. The northernmost boundary of the subantarctic region is the rather ill-definedSubtropical Front (STF), also referred to as the Subtropical Convergence. To the south of the STF is a geographic zone, the Subantarctic Zone (SAZ). South of the SAZ is the Subantarctic Front (SAF). South of the SAF is another marine zone, called the Polar Frontal Zone (PFZ). The SAZ and the PFZ together form the subantarctic region. The southernmost boundary of the PFZ (and hence, the southern border of the subantarctic region) is the Antarctic Convergence, located approximately 200 kilometers south of theAntarctic Polar Front (APF).[3]
The subantarctic Front, found between48°S and58°S in the Indian and Pacific Ocean and between42°S and 48°S in the Atlantic Ocean, defines the northern boundary of theAntarctic Circumpolar Current (or ACC).[3] The ACC is the most importantocean current in theSouthern Ocean, and the only current that flows completely around the Earth. Flowing eastward through the southern portions of the Atlantic, Indian, and Pacific Oceans, the ACC links these three otherwise separateoceanic basins. Extending from the sea surface to depths of 2000–4000 meters, and with a width of as great as 2000 kilometers, the ACC transports more water than any other ocean current.[4] The ACC carries up to 150Sverdrups (150 million cubic meters per second), equivalent to 150 times the volume of water flowing in all the world's rivers.[5] The ACC and the globalthermohaline circulation strongly influence regional andglobal climate as well as underwaterbiodiversity.[6]
Another factor that contributes to the climate of the subantarctic region, though to a much lesser extent than the thermohaline circulation, is the formation ofAntarctic Bottom Water (ABW) byhalothermal dynamics. The halothermal circulation is that portion of the global ocean circulation that is driven by globaldensity gradients created by surface heat andevaporation.
Several distinctwater masses converge in the immediate vicinity of the APF or Antarctic Convergence (in particular theSubantarctic Surface Water (Subantarctic Mode Water or SAMW),Antarctic Surface Water, and theAntarctic Intermediate Water). This convergence creates a unique environment, noted for its very highmarine productivity, especially forantarctic krill. Because of this, all lands and waters situated south of the Antarctic Convergence are considered to belong to the Antarctic from aclimatological,biological andhydrological standpoint.[citation needed] However, the text of theAntarctic Treaty, article VI ("Area covered by Treaty") states: "The provisions of the present Treaty shall apply to the area south of60° South latitude".[7] Therefore, Antarctica is defined from apolitical standpoint as all land andice shelves south of 60°S latitude.
At between about46°–50° south of the Equator, in the region often referred to as theRoaring Forties, are theCrozet Islands, thePrince Edward Islands,Wager Island, theBounty Islands, theSnares Islands, theKerguelen Islands, theAntipodes Islands, and theAuckland Islands. The geography of these islands is characterized by tundra, with some trees on the Snares Islands and the Auckland Islands. These islands are all located near the Antarctic Convergence (with the Kerguelen Islands south of the Convergence) and are properly considered to be subantarctic islands.
At between51°–56° south of the Equator, theFalkland Islands,Isla de los Estados,Ildefonso Islands,Diego Ramírez Islands, and other islands associated withTierra del Fuego andCape Horn, lie north of the Antarctic Convergence in the region often referred to as theFurious Fifties. Unlike other subantarctic islands, these islands havetrees,temperate grasslands (mostlytussac grass), and evenarable land. They also lacktundra andpermanent snow and ice at their lowest elevations. Despite their more southerly location, it is debatable whether these islands should be considered as such because their climate and geography differs significantly from other subantarctic islands.
At between52°–57° south of the Equator, theCampbell Island Group,Heard Island and McDonald Islands,Bouvet Island, theSouth Georgia Group,Macquarie Island, and theSouth Sandwich Islands are also located in the Furious Fifties. The geography of these islands is characterized by tundra,permafrost, andvolcanoes. These islands are situated close to or south of the Antarctic Convergence, but north of60° S latitude (the continental limit according to the Antarctic Treaty).[7] Therefore, although some are located south of the Antarctic Convergence, they should still be considered as subantarctic islands by virtue of their location north of 60° S.[citation needed]
At between60°–69° south of the Equator, in the region often referred to as theShrieking Sixties, theSouth Orkney Islands,South Shetland Islands,Balleny Islands,Scott Island, andPeter I Island are all properly considered to be Antarctic islands for the following three reasons:
In light of the above considerations, the following should be considered to besubantarctic islands:
This is alist ofglaciers in the subantarctic. This list includes onesnow field (Murray Snowfield). Snow fields are not glaciers in the strict sense of the word, but they are commonly found at theaccumulation zone or head of a glacier.[10] For the purposes of this list, Antarctica is defined as any latitude further south than 60° (the continental limit according to the Antarctic Treaty).[7]
Together, the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) act as acarbon sink, absorbingatmospheric carbon dioxide and storing it in solution. If the SAMW temperature increases as a result ofclimate change, the SAMW will have less capacity to store dissolved carbon dioxide. Research using a computerized climate system model suggests that if atmospheric carbon dioxide concentration were to increase to 860 ppm by the year 2100 (roughly double today's concentration), the SAMW will decrease in density and salinity. The resulting reductions in the subduction and transport capacity of SAMW and AAIW water masses could potentially decrease the absorption and storage of CO2 in the Southern Ocean.[11]
TheAntarctic realm andAntarctic Floristic Kingdom include most of the subantarctic islands nativebiota, with manyendemicgenera andspecies offlora andfauna.
The physical landscape and biota communities ofHeard Island and McDonald Islands are constantly changing due tovolcanism, strong winds and waves, andclimate change. Volcanic activity has been observed in this area since the mid-1980s, with freshlava flows on the southwest flanks of Heard Island.Satellite imagery shows that McDonald Island increased in size from about 1 to 2.5 square kilometers between 1994 and 2004, as a result of volcanic activity.[12]
In addition to new land being produced by volcanism,global warming of the climate is causing theretreat of glaciers on the islands (see section below). These combined processes produce new ice-freeterrestrial andfreshwaterecoregions, such asmoraines andlagoons, which are now available for colonization by plants and animals.[12]
Heard Island has vast colonies ofpenguins andpetrels, and large harems of land-based marine predators such aselephant seals andfur seals. Due to the very high numbers ofseabirds andmarine mammals on Heard Island, the area is considered a "biological hot spot".[12] The marine environment surrounding the islands features diverse and distinctivebenthic habitats that support a range of species includingcorals,sponges,barnacles andechinoderms. This marine environment also serves as a nursery area for a range of fishes, including some species of commercial interest.[12]
Glaciers are currentlyretreating at significant rates throughout theSouthern Hemisphere. With respect to glaciers of theAndes Mountains inSouth America, abundant evidence has been collected from ongoing research atNevado del Ruiz inColombia,[13][14]Quelccaya Ice Cap andQori Kalis Glacier inPeru,[15][16]Zongo,Chacaltaya andCharquini glaciers inBolivia,[17] theAconcagua River Basin in thecentral Chilean Andes,[18] and theNorthern Patagonian andSouthern Patagonianice fields.[19][20][21] Retreat of glaciers inNew Zealand[22] and Antarctica is also well documented.[citation needed]
Many subantarctic glaciers are also in retreat.Mass balance is significantly negative on many glaciers on Kergeulen Island, Heard Island, South Georgia and Bouvet Island.[23][24]
Heard Island is a heavily glacierized, subantarcticvolcanic island located in theSouthern Ocean, roughly 4000 kilometers southwest ofAustralia. 80% of the island is covered in ice, with glaciers descending from 2400 meters tosea level.[23] Due to the steeptopography of Heard Island, most of its glaciers are relatively thin (averaging only about 55 meters in depth).[24] The presence of glaciers on Heard Island provides an excellent opportunity to measure the rate of glacial retreat as an indicator of climate change.[12]
Available records show no apparent change inglacier mass balance between 1874 and 1929. Between 1949 and 1954, marked changes were observed to have occurred in the ice formations above 5,000 feet (1,500 m) on the southwestern slopes ofBig Ben, possibly as a result ofvolcanic activity. By 1963, major recession was obvious below 2,000 feet (610 m) on almost all glaciers, and minor recession was evident as high as 5,000 feet (1,500 m).[25]
Retreat of glacier fronts across Heard Island is evident when comparingaerial photographs taken in December 1947 with those taken on a return visit in early 1980.[23][26] Retreat of Heard Island glaciers is most dramatic on the eastern section of the island, where thetermini of formertidewater glaciers are now located inland.[23] Glaciers on the northern and western coasts have narrowed significantly, while the area of glaciers andice caps onLaurens Peninsula have shrunk by 30% – 65%.[23][24]
During the time period between 1947 and 1988, the total area of Heard Island's glaciers decreased by 11%, from 288 km2 (roughly 79% of the total area of Heard Island) to only 257 km2.[24] A visit to the island in the spring of 2000 found that theStephenson,Brown andBaudissin glaciers, among others, had retreated even further.[24][26] The terminus of Brown Glacier has retreated approximately 1.1 kilometres since 1950.[12] The total ice-covered area of Brown Glacier is estimated to have decreased by roughly 29% between 1947 and 2004.[26] This degree of loss of glacier mass is consistent with the measured increase in temperature of +0.9 °C over that time span.[26]
The coastal ice cliffs of Brown Glacier and Stephenson Glacier, which in 1954 were over 50 feet (15 m) high, had disappeared by 1963 when the glaciers terminated as much as 100 yards (91 m) inland.[25] Baudissin Glacier on the north coast has lost at least 100 vertical feet (30 vertical m), andVahsel Glacier on the west coast has lost at least 200 vertical feet (61 vertical m).[25]Winston Glacier, which retreated approximately one mile (1.6 km) between 1947 and 1963, appears to be a very sensitive indicator of glacier change on the island. The young moraines flankingWinston Lagoon show that Winston Glacier has lost at least 300 vertical feet (91 vertical m) of ice within a recent time period.[25]
The glaciers of Laurens Peninsula, whose maximum elevation is only 500 m above sea level, are smaller and shorter than most of the other Heard Island glaciers, and therefore much more sensitive to temperature effects. Accordingly, their total area has decreased by over 30 percent.Jacka Glacier on the east coast of Laurens Peninsula has also demonstrated marked recession since 1955.[25] In the early 1950s, Jacka Glacier had receded only slightly from its position in the late 1920s, but by 1997 it had receded about 700 m back from the coastline.[23][24][27][28]
Possible causes of glacier recession on Heard Island include:
TheAustralian Antarctic Division conducted an expedition to Heard Island during theaustral summer of 2003–04. A small team of scientists spent two months on the island, conducting studies onavian and terrestrial biology and glaciology. Glaciologists conducted further research on the Brown Glacier, in an effort to determine whether glacial retreat is rapid or punctuated. Using a portableecho sounder, the team took measurements of the volume of the glacier. Monitoring of climatic conditions continued, with an emphasis on the impact ofFoehn winds on glacier mass balance.[29] Based on the findings of that expedition, the rate of loss of glacier ice on Heard Island appears to be accelerating. Between 2000 and 2003, repeatGPSsurface surveys revealed that the rate of loss of ice in both theablation zone and theaccumulation zone of Brown Glacier was more than double average rate measured from 1947 to 2003. The increase in the rate of ice loss suggests that the glaciers of Heard Island are reacting to ongoing climate change, rather than approachingdynamic equilibrium.[26] The retreat of Heard Island's glaciers is expected to continue for the foreseeable future.[23]
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