The physical environment

2011

The physical environment includes both the nonliving factors that characterise an ecosystem (e.g. weather patterns, ice coverage, atmosphere), and the processes that drive them (e.g. weathering of rocks, ozone depletion of the atmosphere).

The atmosphere-climate and weather patterns

The climate of our polar regions and their dominant weather patterns are due to the shape of the planet. As Earth is spherical, the angle of incidence of solar radiation is shallower at the poles than at the equator. Thus, the same amount of sunlight is distributed over a larger area in the polar regions than at lower latitudes. The interior of Antarctica, where the ice sheet is 2-4 kilometres thick and hence high above sea level, remains very cold as it is generally well shielded from the warmer air masses found at the mid-latitudes (Figure 7.3). In contrast, the equatorial regions, where seasonal change is barely apparent, remain warm all year round. This latitudinal pressure difference causes circulation patterns in the atmosphere that create cyclonic systems between 40°S and 70°S. The clockwise movement of the cyclones transports heat from the equator towards the Antarctic continent.36 With the high elevation of the Antarctic ice sheet (average about 2200 metres),37 the air above the continent cools significantly, becoming much denser than the air at the coast, and results in gravity-driven, strong katabatic winds (caused by local downward motion of cool air) in the coastal regions, where they are particularly prevalent during winter.

Although the atmosphere above Antarctica has been studied since the early 20th century, it is only in the past 30 years that increasingly detailed measurements have been available-albeit sparsely spaced. The Australian Bureau of Meteorology gathers year-round detailed weather information at all of Australia's Antarctic and subantarctic stations. Similar data are collected by other countries. In addition, weather data are gathered by automatic weather stations at more than 20 remote locations in East Antarctica, as well as by drifting buoys, balloons, and various satellite and ground-based remote sensing techniques.2 While there is significant inter-annual variability in Antarctic weather due to various large-scale processes associated with the global movement of heat, trends are apparent in the historical and palaeoclimate records. In 2009, the Scientific Committee for Antarctic Research published a detailed assessment of the impact of climate change on the Antarctic environment, and a summary of the outlook for the continent and the Southern Ocean over the next century.37

Over the past half-century, the western and northern parts of the Antarctic Peninsula have warmed faster and to a greater extent than anywhere else on Earth; at the Vernadsky Station a statistically significant annual trend of +0.53°C per decade occurred from 1951 to 2006. Winter temperatures at this site have an even stronger trend of +1.03°C per decade. West Antarctica has warmed by approximately +0.1°C per decade, particularly during winter and spring.16,38 On the high plateau at the South Pole, a statistically significant cooling trend has been observed, which may be due to reduced penetration of weather systems to the pole. Coastal East Antarctica is warming less than West Antarctica.16,37

Balloon and satellite measurements indicate that the Antarctic lower troposphere (surface to 10-kilometre height) has warmed over the past 50 years,37 while the lower stratosphere (10-30 kilometres) has cooled.39 These temperature changes are very likely due to the effects of increased greenhouse gas concentrations in the atmosphere and, particularly in the case of the stratosphere, to decreases in ozone concentrations.40

The main variability of the atmosphere at southern mid and high latitudes is associated with the southern annular mode (SAM). This is a see-sawing of pressure levels between mid and high latitudes that operates on seasonal and inter-annual timescales around the entire Southern Hemisphere. Since the 1960s, the surface air pressure has decreased around the Antarctic coast but increased at southern mid-latitudes, producing a long-term trend in SAM. The trend has deepened the meteorological feature known as the Amundsen Sea Low, which resulted in the warming of the Antarctic Peninsula and to a lesser extent West Antarctica, and to a reduction of sea ice in these regions. Additionally, changes in SAM have led to fewer but more intense cyclonic weather systems in the Antarctic coastal region (60°S-70°S), except in the region of the Amundsen and Bellingshausen seas.37

Recent findings37, 40-41 show that the trend in SAM is largely due to atmospheric circulation changes that have been brought about by the 'ozone hole'- the anomalous reduction of the amount of ozone in the lower stratosphere (12-20 kilometres in altitude) above Antarctica that has occurred each spring since around 1980 (Figure 7.4). The destruction of ozone is caused by chemical processes involving human-made halon gases, particularly chlorofluorocarbons (CFCs), which are promoted by the extreme cold and special circulation conditions in the Antarctic stratosphere during winter. The ozone hole led to an increase in the damaging ultraviolet radiation received by Earth and also a cooling of the stratosphere. Each year, ozone levels are depleted in late winter to early spring, reducing temperatures of the Antarctic stratosphere during spring. The consequences of the cooling are changes in the lower atmosphere leading to a polewards shift of about 300 kilometres of the jet stream (strong winds 7-12 kilometres above the Earth's surface), which in turn influences the route of storms in the high to mid latitudes. The wind changes have been linked to regional changes in precipitation,42 increases in sea ice around Antarctica,37 warming of the Southern Ocean,37 a local decrease in the ocean sink of carbon dioxide40 and influences on the circulation in the mesosphere.43

Restrictions on the use of CFCs and other ozone depleting substances were negotiated internationally when the Montreal Protocol was signed in 1987. This treaty has since been ratified by 196 states and has led to a gradual reduction in equivalent effective stratospheric chlorine, which is an estimate of the effective quantity of halogens in the atmosphere. This estimate is used to quantify the depletion of ozone in the stratosphere. The levels of CFCs peaked in the mid-1990s; since then, the use of these chemicals has been greatly reduced and Antarctic ozone levels appear to have increased by approximately 15%.44 Since about 2000, the ozone hole has not increased in size. Although ozone levels are expected to fluctuate from year to year,44 they are expected to recover in the middle of this century,40 and over the remainder of the 21st century the surface changes brought about by ozone loss are expected to gradually relax.44

However, two recent studies report that the effects of the reversal of the ozone hole may be countered by increases in the concentrations of greenhouse gases, at least during the southern summer.45-46 While the jet stream should return to the same latitude where it occurred before the ozone depletion, increasing greenhouse gas concentrations may cancel out the effects of the ozone recovery and the jet stream may stay at its current latitude. The interactions of these two competing events are not yet fully understood and much will depend upon the speed of the ozone recovery and the rate of increase of greenhouse gases.45-46

The El Niño Southern Oscillation (ENSO) is a large-scale mode of atmospheric and oceanic variability that is mainly situated in the low latitude Pacific Ocean region. It is associated with pooling of warm water alternately on the western and eastern Pacific Ocean every few years. ENSO does provide a contribution to climate variability in coastal Antarctica, but there is currently no evidence that this influence is changing in the long term.37

EESC = effective equivalent stratospheric chlorine; km = kilometre; ODS = ozone depleting substance

The cryosphere-Antarctic ice sheet and glaciers

The Antarctic ice sheet consists of three topographically different regions: the Antarctic Peninsula, which reaches further north than any other area in Antarctica; the West Antarctic Ice Sheet (WAIS); and the East Antarctic Ice Sheet (EAIS), which is by far the largest component, extending from about 30°W to about 165°E.

The ice mass budget of the Antarctic continental ice sheet is the balance between mass gained from snow fall and mass lost by melt and discharge as icebergs at the coast. The net mass balance is complex to assess because changes in snow fall and iceberg discharge vary by region and are not uniform across the continent. In some coastal regions abrupt changes have been observed, including the rapid disintegration of floating ice shelves, and this has raised questions about the potential for rapid ice discharge from Antarctica into the sea. The major environmental consequences of changes in the Antarctic ice sheet are to global sea level and to the freshwater input to the Southern Ocean, with possible flow-on effects to global ocean circulation and marine ecosystems.

Based on measurements from the Gravity Recovery and Climate Experiment (GRACE) satellites, which determine the weight of the ice sheet from space, Rignot et al.47 report that the entire Antarctic ice sheet may have a net loss of 150 ± billion tonnes each year (this is equivalent to a sea level rise of 0.4 millimetres each year), and that this rate is accelerating (by 14.5 ± 2 billion tonnes per year each year) (Figure 7.5). They claim that the loss estimate is supported by separate estimates of snow fall and ice discharge - the input - output method. However, the timespan of these observations is short, and there are considerable uncertainties in the observations. It is important to note that the observed net loss in ice mass is despite an overall thickening of the interior of East Antarctica by 1.8 ± 0.3 centimetres per year, measured by satellite radar altimetry from 1992 to 2003, which has been attributed to increased snow fall.48 This increase in snow fall is consistent with a warmer atmosphere, which leads to more evaporation from the ocean.48-49 Hence, increased snow fall on the interior of the ice sheet is consistent with the predicted responses to global warming.

In West Antarctica, GRACE satellite observations indicate that 132 ± 26 billion tonnes of ice per year are lost.51 The WAIS is predominantly resting on bedrock that is far below sea level, and which is widely expected to make it vulnerable to global warming. The WAIS has been thinning during the most recent decades; however, this was due to the increased discharge of ice by a number of glaciers rather than surface melt.52 For example, the flow rate of the Pine Island Glacier in West Antarctica has recently accelerated. The glacier ice is thinning and its grounding line (where the continental ice begins to float) is retreating southward. This is attributed to warmer ocean waters that increase melting from the base of the floating ice. As the bedrock beneath the glacier becomes deeper south of the grounding line, it is argued that the melt could continue to accelerate, due to the effect of pressure further lowering the freezing point, and therefore further contribute to sea level rise.53

In contrast, the EAIS is believed to be relatively stable; it is larger and higher than the WAIS and there are currently not such clear signs of warming in East Antarctica as there are in the west. However, recent results from field measurements show that much of the EAIS is also below sea level, and that the ice in some coastal regions is thinning and losing mass. Measurements by the GRACE satellites indicate the coastal fringes of the EAIS have lost about 60 billion tonnes of ice each year since 2006 (again with considerable uncertainty: 57 ± 52 billion tonnes of ice per year)51 and that the annual loss has increased over the short measurement period.50-51 Satellite measurements of the elevation of the ice sheet surface also show that the ice sheet is thinning near the coastal margins in a few locations in East Antarctica, particularly in the Totten Glacier and Cook Ice Shelf regions, indicating ice discharge is exceeding input. This is in general agreement with the regions of East Antarctic loss indicated by GRACE. The bedrock topography beneath East Antarctica is only recently being revealed from airborne surveys, with recent studies showing that the bedrock in the Totten Glacier catchment is largely below sea level.54 Further inland, as much as 21% of the Aurora Basin, Wilkes Land, region is more than 1000 metres below sea level.55 Further investigations of the depth and the bedrock slopes will determine the response of thinning at the margins to the ice mass balance of the region.

The influences of climate change on Antarctica are also illustrated by events in the Antarctic Peninsula region. The Antarctic Peninsula has experienced one of the highest regional temperature increases on the planet (2.8 °C in 50 years). Several floating ice shelves in that region have recently collapsed abruptly; for example, the Larsen B Ice Shelf collapsed in March 2002, and the Wilkins Ice Shelf started to disintegrate in March 2008.16,56 By 2009, the Antarctic Peninsula had lost about 28 100 km2 from the 152 200 km2 of ice shelves present in the 1950s.14 With the buttressing effect of grounded ice sheets gone, glaciers adjacent to the collapsing ice shelves now flow around three to four times faster into the ocean since the shelves disintegrated.57-58 This increase in the discharge of grounded ice from the ice sheet to the ocean is contributing to sea level rise.

Glaciers at subantarctic Heard Island are also retreating. For example, the areal extent of Brown Glacier decreased from approximately 6.2 million square metres in 1947 to 4.4 million square metres in 2004, a 29% loss of its original area. Measurements in 2000 and 2003 reveal the recent rate of ice loss of this glacier is more than double the 57-year average from 1947-2004.59

2.1.3 The cryosphere-sea ice

Sea ice plays a key role in ocean-atmosphere interactions, global ocean circulation and the global climate system by forming an insulative, high-albedo cover (reflective of solar radiation) over a vast, although seasonally variable, area of the Southern Ocean (of around 3-19 million km2). It strongly influences the ocean and ecosystems through the injection of brine during its formation and fresh water when it melts.60 Being closely associated with patterns of atmospheric and oceanic temperature and circulation, sea ice responds sensitively to climate change and variability, and is also a key modulator of change and variability. It also dominates the seasonal physical and chemical dynamics of the high-latitude Southern Ocean and plays a major role in structuring high-latitude marine ecosystems. It follows that any substantial change in sea ice coverage will have potentially complex and wide-ranging impacts (Box 7.1), although the task of tracking environmental and biological consequences is immense and complex.61

There has been a small increase in the net areal extent of sea ice around Antarctica over the past 30 years (based on satellite data analysis),62 although this result has recently been called into question.63 However, undisputed regional changes are occurring in the Antarctic sea ice cover in response to changing patterns of large-scale atmospheric circulation. Most notable are the strong reductions in the sea ice extent west of the Antarctic Peninsula in the Bellingshausen Sea, and the strong increases in sea ice extent in the Ross Sea.64 In the western Antarctic Peninsula region there is mounting evidence that a decreasing ice season duration has affected multiple levels of the marine food web.65-66 The sea ice signal in the East Antarctic sea ice zone is mixed and complex,64,67 but has shown only minor changes over the satellite record, consistent with natural variability.

Box 7.1 Recent Antarctic sea ice change and variability, and their implications

Antarctic sea ice forms a highly reflective insulating 'skin' over much of the Southern Ocean. It expands from 3-4 million square kilometres each summer to about 19 million square kilometres each September-October.68 Sea ice and its accumulated snow cover are very important for both climatic and biological systems. Sea ice formation is a driver of global ocean circulation and is also a habitat for algae, which forms the base of the Antarctic food web. Consequently, significant changes in the amount of sea ice formed each year will impact the climate, biology and ecology of Antarctica and the Southern Ocean, with potentially significant global consequences.

Over the era for which satellite imagery of sea ice is available (1979 to present), the areal extent of sea ice has increased at a small but statistically significant rate of approximately 1% per decade.62 However, of much greater significance are the regional changes observed. Increases in the Weddell Sea sector have been around 1% per decade, 0.9% per decade in the West Pacific Ocean sector, 2.1% per decade in the Indian Ocean sector,62 and the largest increase has been in the Ross Sea with 5% per decade. In contrast, the Amundsen-Bellingshausen seas have lost sea ice at 7% per decade. Moreover, proxy records derived from analysis of ice sheet core and historical whaling records suggest that sea ice coverage may have declined substantially in certain regions since the late 1950s and early 1960s.69-70 It is apparent from the satellite passive microwave data records that substantial seasonal and decadal variability is superimposed on longer term trends in all sectors.

The seasonality of the sea ice coverage (formation and retreat) has also changed, but again with large differences between regions,64,71 especially in West Antarctica (Figure A). In the north-east and west Antarctic Peninsula and in the Bellingshausen Sea, the sea ice season has shortened by about 85 days from 1979-80 to 2004-05. These changes are probably due to changes in large-scale modes of atmosphere circulation affecting regional winds and temperatures,64 although other factors, such as the recent incursion of relatively warm waters onto the continental shelf, may also have been a factor in the weakening of ice shelves.72

In contrast, sea ice in the western Ross Sea now persists for about 60 days longer than in 1979.68 In East Antarctica, the sea ice zone is narrower than in West Antarctica. The large sea ice area and the longer season are linked to lower surface temperatures 68 and an increase in the occurrence of more southerly winds. Complex patterns of change have been observed in both the timing and extent of the seasonality of sea ice across the region from 1979 to 2009, with sea ice persisting for one to two days more per year (R Massom, University of Tasmania, pers. comm., May 2011). In the Prydz Bay region, sea ice persists for two to three days more per year.

While observations show sea ice extent is increasing slightly over time, current models predict that Antarctic sea ice will decrease in extent by 24%, and 34% by volume, by 2100,73 highlighting the need to understand the current conditions and how sea ice impacts the biological components of the Southern Ocean ecosystem. For example, fast ice is an important habitat for sea ice algae and microorganisms, and a breeding platform for Weddell seals and emperor penguins. Increased fast ice extent or duration can negatively affect breeding success. Pack ice, the area where many top predators forage, is influenced by snow fall and surface flooding. Changes in sea ice and wind regimes, light availability and mixed-layer depth of oceanic waters affect phytoplankton communities, which in turn affect food availability for these predators.

Figure

Fieldwork within the Australian Antarctic Program provides crucial and essential information on sea ice, biological and biogeochemical processes and properties, and is an important means of validating key satellite data products, including sea ice thickness, which remains difficult to measure accurately around Antarctica

km2 = square kilometre; m3 = cubic metre

Wienecke B (2011). Antarctic environment: The physical environment. In: Australia state of the environment 2011, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/science/soe/2011-report/7-antarctic/2-state-and-trends/2-1-physical-environment, DOI 10.4226/94/58b65b2b307c0