Australia: The Land Where Time Began
Antarctic Climate Change and Environment - Changes During the Instrumental Period
In Antarctica the instrumental period began with the International Geophysical Year, about 50 years ago.
The large scale atmospheric circulation
In the atmospheric circulation of the high southern latitudes the major variability mode is the Southern Hemisphere Annular Mode (SAM), which is a circumpolar pattern of atmospheric mass displacement in which intensity and location of air pressure gradient between mid-latitudes (high pressure) and the coast of Antarctica (low pressure) changes in a non-periodic manner over widely ranging time scales. The SAM became more positive over the last 50 years, as around the Antarctic coast pressure dropped and increased at mid-latitudes. Westerly winds have been increased over the Southern Ocean by this change by 15-20 % since the 1970s. The Amundsen Seas Low has deepened as a result of the combination of stronger westerlies around Antarctica with off-pole displacement of Antarctica, with consequent effects on temperature and sea ice in the coastal region of West Antarctica.
Increases in atmospheric greenhouse gasses and the development of the Antarctic ozone hole caused the SAM to change, and it was the loss of ozone in the stratosphere that had by far the greatest influence. The loss of stratospheric ozone, which occurs in the Austral spring, cools the stratosphere, and this increases the strength of the polar vortex – a large cyclonic circulation at high altitude, in the middle and upper troposphere and stratosphere in winter over the Southern Ocean around Antarctica. The effects of the ozone hole propagate down through the atmosphere in summer and autumn, which increases the atmospheric circulation around Antarctica at lower levels. The greatest change in the SAM resulting from this is indicative of conditions at the surface in autumn.
A decrease in the annual and seasonal numbers of cyclones south of 40oS was a result of these changes in the SAM from 1958-2007. In the coastal zone of Antarctica between 60-70oS there are now fewer though more intense cyclones, with the exception of the region of the Bellingshausen Sea.
There have been more El Niño events that are more intense in recent decades. A signal of the El Niño cycle can be seen in the Antarctic during some of them. To date no evidence has been found indicating that long term climate trends in Antarctica have been affected.
Surface temperature trends show there has been significant warming across the Antarctic Peninsula, and in West Antarctica, to a lesser extent, since the early 1950s, though there has been little change across the remainder of the Antarctic continent. It is in the northern and western parts of the Antarctic Peninsula that the largest warming trends have occurred. It is there, at the Faraday/Vernadsky Station, that the largest statistically significant (<5%) trend of +0.53oC per decade for the period 1951-2006 has occurred. A warming trend of +0.20oC/decade is shown by the 100-year record from Orcadas on Laurie Island, South Orkney Islands. Warming on the western part of the Peninsula has been largest during the winter, at which time the temperatures at the Faraday/Vernadsky Station have been increasing by +1.03oC/decade from 1950 -2006. During the winter there is a high correlation between the extent of sea ice and surface temperatures, which suggests more sea ice during the 1950s-1969s which has been reducing since then. Natural variability may be reflected by this warming.
Temperatures have risen most during summer and autumn on the eastern side of the Antarctic Peninsula (at +0.41oC/decade from 1946-2006 at Esperanza), linked to the strengthening westerlies that occurred as the SAM shifted into a positive phase, which is primarily a result of the ozone hole. On the eastern side if the Peninsula warm, maritime air masses are brought across by the stronger westerly winds.
West Antarctica has warmed by about 0.1oC/decade, based on satellite data and data from automated weather stations, especially in winter and spring. This warming is suggested to have begun about 1800 by ice core data from the Siple Dome. Elsewhere in Antarctica there have not been many changes that are statistically significant during the instrumental period.
A statistically significant cooling over recent decades, which is interpreted as being due to fewer maritime air masses penetrating into the interior of the continent, has been shown on the plateau, Amundsen-Scott Station at the South Pole.
Large interannual to decadal variability is shown by temperatures that were reconstructed from ice cores, with anti-phase anomalies between the continent and the peninsula being the dominant pattern, which is the classic signature of the SAM. Antarctic temperatures are suggested to have increased on average by about 0.2oC since the late 19th century.
Temperature profiles from Antarctic radiosonde data show that the troposphere has warmed at 5 km above sea level, while the stratosphere above it has cooled over the last 30 years. This is the sort of pattern that would be expected to result from increasing atmospheric greenhouse gasses. At this level the tropospheric warming in winter is the largest on Earth. It may be in part resulting from the insulating effect of greater amounts of polar stratospheric cloud during the winter. The formation of these clouds is a response to cooling of the stratosphere which related to the ozone hole.
About 6 mm of global sea level equivalent on average falls as snow in Antarctica per year, though there has been no statistically significant increase since 1957, and snowfall trends vary from region to region. On the western side of the Peninsula snowfall has increased, where it has been linked to decreasing populations of Adélie penguins, which prefer habitat that is snow free.
Antarctic ozone hole
In the 1970s a decline began in levels of stratospheric ozone, following the release of CFCs into the atmosphere with the result that virtually all the ozone was destroyed between altitudes of 14 and 22 km above Antarctica. Due to the success of the Montreal Protocol the amounts of ozone-depleting substances in the stratosphere are decreasing by about 1% per year. This has resulted in the stabilisation of the size and depth of the ozone hole, though neither is reducing so far.
The flowering plants that were native to Antarctica (Deschampsia Antarctica and Colobanthus quintensis) in maritime regions of Antarctica, which have become more abundant at some sites, are the clearest evidence of Antarctic terrestrial organisms responding to climate change. The growth and spread of established plants and increased establishment of seedlings are encouraged by warming. Biological production in lakes has increased due to changes of temperature and precipitation, the main reason being decreases in the duration and extent of ice cover on the lake. As a result of drier conditions some lakes have become more saline.
On most of the sub-Antarctic islands, as well as some parts of the continent, alien microbes, fungi, plants and animals have been introduced through human activity. The structure and functioning of native ecosystems and their biota have been impacted by these introduced organisms. Rates of establishment through anthropogenic introduction outweigh those from natural colonisation processes by 2 orders of magnitude on Marion Island and Gough Island.
In the Antarctic Peninsula ice shelves have changed rapidly in recent decades, with warming leading to ice shelf retreat on both sides of the permafrost. Warm air being brought over the Peninsula by stronger wester9 ies, that are forced by changes in the SAM, driven ultimately by the development of the ozone hole. Increased fracturing by infilling with meltwater of crevasses that are pre-existing, and the penetration of warm masses of ocean water beneath the ice are resulting in ice shelf retreat. The speeding up of the flow of glaciers from the interior has resulted from the loss of ice shelves.
There are some islands that are now covered by snow only in winter that were formerly snow covered throughout the year. Since the 1940s Heard Island glaciers have been reduced by 11%, and there are now several coastal lagoons that have formed there. 28 of the 36 glaciers on South Georgia that have been surveyed are retreating, 2 are advancing and 6 are stable. Ice cover has reduced by about 40 % on Signy Island.
212 (87 %) of the 244 marine glaciers draining the ice sheet and associated islands of the Antarctic Peninsula have shown overall retreat since 1953. Small advances have been shown by the other 32 glaciers.
· The Amundsen Sea sector - the most rapidly changing region of the Antarctic ice sheet. The current rate of mass loss from Amundsen Sea embayment ranges from 50-137 Gt/year, equivalent to the current mass loss rate from the entire Greenland ice sheet, and it makes a significant contribution to rising sea levels
· The Pine Island glacier – grounding line has retreated and the glacier is now moving 60 % faster than it did in the 1970s.
· The Pine Island glacier – as well as adjacent glacier systems, are more than 40 % out of balance, discharge = 280 ± 9 Gt/year of ice, receive = 177 ± 25 Gt/year of snowfall
· The Thwaites Glacier – as well as 4 other glaciers in this region, are thinning at an accelerated rate,
· The Smith Glacier – flow speed increased by 83 % since 1972
The changes are the result of warming of the sea beneath the ice shelves that are connected to the glaciers. The stronger winds associated with the more positive SAM are driving warm Circumpolar Deep Water up against the western coast of the Peninsula and the coast of the Amundsen Sea.
Across most of the East Antarctic ice sheet the changes are less dramatic, the most significant changes being close to the coast. There is moderate thickening in the interior of the ice sheet and a mixture of modest thickening and strong thinning among the fringe ice shelves. Recent passive microwave data suggest there is increasing coastal melt.
Changes in sea level
It is suggested by data from tide gauges and satellite altimeters that in the 1990s-2000s global sea level rose at a rate of 3 mm/year or more, which is higher than expected from IPCC projections, though since then the rate has slowed to about 2.5 mm/year. There is concern about the possibility those large contributions that result from the dynamic instability if ice sheets during the 21st century. At around 2005 the Antarctic Peninsula was estimated to have been contributing at a rate of 0.16 ± 0.06 mm/yr to global rise in sea level.
The Southern Ocean
An intensive seasonal cycle can induce large errors when there are only a few samples, which can make it difficult to detect change in surface waters. There are, however, observations from around South Georgia going back to 1925 that are frequent enough to resolve the annual cycle and reveal a significant warming that averages 2.3oC over 81 years in the upper 150 m, and being about twice as strong in winter than in summer.
The temperature of the waters of the ACC has increased 0.06oC/decade at depths of 300-2,000 m between the 1960s and the 2000s, and by 0.09oC/decade since the 1980s, which means it has warmed more rapidly than the global ocean as a whole. On the southern side of the ACC the warming has been more intense than the waters to the north of it, the Upper Circumpolar Deep Water at depths of 150-500 m on the poleward side of the Polar Front reaching a maximum increase of 0.17oC/decade. These changes are consistent with the ACC shifting southward in response to the southward shift of the westerly winds that has been driven by enhanced greenhouse forcing. Since the 1980s a significant freshening of -.0.01 salinity units/decade has been observed on the northern side of the ACC. No evidence has been that would suggest the ACC transport has been increasing. It is suggested by recent studies that there has been an increase in wind forcing that causes an increase in the meridional transport of heat and salt, though increased transport has been by eddies and not by a change in zonal transport by the ACC.
Changes are evident in the character of Ross Sea and the Weddell Sea, though neither of them has shown major trends in the long term, with the exception of a small degree of freshening. The Weddell Polynya, a persistent gap in the sea ice, occurred for several winters during the 1970s, which had a significant impact on the properties of deep water, as it was a place where the ocean lost a large amount of heat to the atmosphere. Significant decadal and regional change has been observed in deep water masses in the Weddell Sea which make it difficult to detect long-term trends. Bottom water in the central Weddell Sea is warming and increasingly more saline over decadal time scales, while in the western Weddell Sea and the Australian sector, which includes the Ross Sea and Adélie Land, is cooling and freshening. These changes are of interest because it is in these places where the Antarctic Bottom Water originates and changes in these locations will spread into the world ocean.
The global oceans are ventilated by the Southern Ocean and it also regulates the climate system by absorbing and storing heat, freshwater, O2 and atmospheric CO2. The CO2 concentration in the ocean increased to the south of 20oS in the southern Indian Ocean from 1991-2007. CO2 increased faster in the ocean than it did in the atmosphere at latitudes polewards of 40oS, which suggests the ocean became a less effective sink for atmosphere CO2. Stronger westerly winds lead to the oceanic water at the surface being mixed with deeper water rich in CO2, and this saturates the carbon reservoir of the surface water, which therefore limits it ability to absorb atmospheric CO2. These changes appear to be linked to the increasing wind strength which is driven by the more positive SAM. The ocean becomes more acidic as a result of the increase of CO2 concentration in the ocean.
Ship observations over the first half of the 20th century suggest the extent of sea ice was greater than had been seen in recent decades, though the validity of such observations has been questioned. The data concerning the extent of sea ice from satellites for 1979-2006 show a positive trend of around 1 % per decade. The trend is positive in all sectors with the exception of the Bellingshausen Sea, where the extent of sea ice has been reduced significantly. At about 4.5 % per decade, the greatest increase is in the Ross Sea. The deepening of the Amundsen Sea Low, which is linked to the ozone hole, has led to the reduction of the extent of sea ice in the Bellingshausen Sea and an increase of extent of sea ice in the Ross Sea.
Not much is known about Antarctic permafrost. Between 1963-1990 the active layer, the layer that freezes and thaws according to season, on Signy Island increased in thickness by 30 cm, when Signy Island was warming, than from 1990-2001, when Signy Island cooled, the thickness decreased by 30 cm. The permafrost in McMurdo Sound the temperature at -360 cm has remained stable, in spite of the small decrease in atmospheric temperature of 0.1oC/year.
The ecosystem of the Southern Ocean was disturbed by sealing and then in the early part of the 20th century by whaling. Within a period of a few decades about 300,000 blue whales were killed, which is equivalent to 30 million tonnes of biomass. Most of those killed were in their feeding areas in the southwest Atlantic in an area of 2 million km2, which is a density of 1 blue whale/6 km2. The krill stock was expected to increase following the near-extinction of some of whale populations as there was a release from grazing pressure, but it didn’t. There was an increase in the predation by birds and seals, the total biomass of birds and seals remain only a fraction of the former whale population. On a regional scale benthic habitats have been devastated by bottom fishing; it is expected that fish stocks and invertebrates will recover only very slowly.
The Antarctic marine ecosystem has been affected by climate change for the past 50 years, especially on the western side of the Antarctic Peninsula, where the water is warming and the extent of sea ice is declining. The krill in this warming water is declining, in some areas there has been a decrease of phytoplankton while in other areas they are increasing and a shift to the south of the population of gelatinous salps. A decline in phytoplankton may reflect a decrease in the input of iron from the continental margin that in turn is related to the reduction of sea ice formation in this region and hence to climate change. All components of the pelagic Antarctic ecosystem that are related to sea ice are impacted by the consequences of the regional decrease of sea ice to the west of the Antarctic Peninsula, and there are only hints, though not yet evidence, of the impacts of climate change are present for the benthic fauna.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|