Australia: The Land Where Time Began
Antarctic Climate Change and Environment - Next 100 Years
It is a challenge to predict how the Antarctic environment will evolve over the next 100 years, yet it has implications for science and for policy makers. The use of coupled atmosphere-ocean-ice models is necessary to predict climate evolution with some degree of confidence. As the models take a large number of parameters into consideration the model outputs are an improvement on simple projections on current trends. They are the only means of providing synoptic views of the future behaviour of environments, albeit crudely and at a coarse resolution. The output of the models do not quite accurately simulate the changes that have been observed over the last few decades, so some uncertainty remains concerning their forward projections, particularly on regional scales.
It is expected that the ozone hole recovery combined with a continuing increase in greenhouse gas emissions should continue to strengthen the positive phase of the SAM, though with a trend that is slower than that has been observed over the last 2 decades. Therefore it can be expected that there will be further increases in surface winds above the Southern Ocean in summer and autumn. This will result in a continuing polewards shift of the storm track of the Southern Ocean.
Significant surface warming over Antarctica is projected by models to 2100, with an increase of 0.34oC/decade over land and grounded ice sheets, within a range of 0.14-0.5oC/decade. The largest increase over land is projected for the high altitude interior of East Antarctica. The surface temperature by 2100 is expected to remain well below freezing, in spite of this change over most of Antarctica and will contribute to melting inland.
The largest atmospheric warming that is projected by the models is the sea ice zone off East Antarctica in winter, 0.51 ± 0.26oC/decade , as the sea ice edge continues to retreat and the consequent exposure of the ocean to heating by insolation.
There is low confidence in the regional detail, though there is confidence in the overall projection of warming, because of the large differences in regional outcomes between models.
In the troposphere at 5 km above sea level the annual mean warming rate is projected to be 0.28oC/decade, which is slightly lower than the forecast warming at the surface.
The magnitude or frequency of changes to extreme conditions above Antarctica cannot yet be forecast, which is something biologists need to assess any potential impacts. The extreme temperature range within a given year between summer and winter is projected to decrease around the coasts and to show little change over the interior.
The warming of 3oC that is expected over the next 100 years is faster than the fastest rate recorded in the Antarctic ice cores (4oC, 1,000 years), though it is slower than or comparable to the rates of rise of temperature typical of Dansgaard-Oeschger events that occurred in Greenland in glacial times, or the Bolling-Allerød warming in Greenland 14,700 years ago, and of the warming in Greenland at the end of the Younger Dryas about 11,700 years ago. Therefore it is feasible in terms of what is known about the natural system, however unlikely it may appear.
The amount of precipitation that occurred in the 20th century is generally underestimated by the current numerical models. These underestimating results from problems associated with parameterising the key processes driving precipitation, such as the poor understanding of the microphysics of polar clouds, and because in a coarse resolution model the smooth coastal escarpment causes less precipitation from cyclones than they do in reality. Most models are expected to give net precipitation increases in the future with warmer air and higher atmospheric moisture. Over the coming century most models simulate an increase in precipitation over Antarctica that is larger in winter than in summer, and it is suggested by the output of models that snowfall over the continent may increase by 20 % compared to current values (Bracegirdle et al., 2008). With the expected move to the south of the mid-latitude storm track it can be expected there will be greater precipitation and accumulation in the coastal regions of Antarctica. As liquid water in available to biota immediately, the form that precipitation takes is of biological significance (Convey, 2006) with the balance between rain and snow to change towards rain, especially along the Antarctic Peninsula.
Springtime atmospheric concentrations of ozone are predicted to have recovered significantly by the middle of the 21st century, though not necessarily to 1980 values, as a result of increasing concentrations of greenhouse gasses that have accumulated and warmed the troposphere, which cools the stratosphere further. It is expected that in a colder stratosphere ozone destruction will continue.
There are various trace gasses, such as dimethyl sulphide (DMS) which is generated by plankton, are released from the oceans around Antarctica. The emissions of these gasses, which have a seasonal cycle that is linked closely to the extent of sea ice and to the Sun, are projected to be increased by any loss of sea ice. Emissions of gasses such as DMS would likely be increased in a warmer world, in which the extent of sea ice was reduced, with a minimum in winter and a maximum in summer. Cloud condensation nuclei (CCN) are sourced from the DMS via its oxidation to sulphate. If the numbers of CCN are increased it may lead in an increase in cloudiness and albedo, which would influence the climate of the Earth.
Growth and reproduction may be promoted by increased temperature, though also causing drought and associated effects. Vegetation and faunal dynamics are affected more by the changes to the availability of water than by temperature. It is not clear what the regional patterns of the availability of water may be in the future, but it is predicted by climate models that there will be an increase in precipitation in coastal regions. The tolerance of many arthropods could readily be exceeded by increasing frequency and intensity of freeze-thaw events. Many species may exhibit faster metabolic rates with increases in temperature, which would result in shorter life cycles and local expansion of populations. The catchment of lakes will probably be altered by even subtle changes in temperature, precipitation and wind speed, and of the time, depth and extent of their ice cover, water volume and chemistry, with related effects on lake ecosystems. Invasion by more competitive alien species carried by currents of water and air, humans and other animals would likely also be increased by warming.
The predictive value of existing ice sheet models is in doubt because they do not properly reproduce the observed behaviour of ice sheets. Mechanical degradation, such as cracks that are caused by water which propagates in summer, which changes the lubrication of the base of the ice by an evolving subglacial hydrological regime, or the influence on the flow of outlet glaciers and ice streams of variable sub-ice-shelf melting, are not taken into account by the models. A combination of inference from past behaviour, extension of current behaviour, and interpretation of proxy data and analogues from the geological record, are the basis for predictions of the state of ice sheets in the future.
The regions that are changing at the present are expected to be those that are most likely to change in the future. It is expected that in the Amundsen Sea warmer waters will continue upwelling onto the continental shelf and continue to erode the underside of ice sheets and glaciers. A 30 % probability has been suggested that ice loss from the West Antarctic ice sheet could lead to a sea level rise of 2 mm/year, and a 5 % probability that rate of rise could reach 1 cm/year. Also, there is a concern that the Amundsen Sea Embayment ice could be entering a collapse phase that could result in deglaciation of parts of the West Antarctic ice sheet. It is suggested that a contribution to sea level rise from this sector alone could ultimately reach 1.5 m, so it cannot be discounted that by 2100 sea level could rise by 10s of centimetres. These estimates are based on the assumption that the ice sheets will respond in a linear manner to warming and that contributions to rising sea levels are confined to the West Antarctica ice sheet. If evidence from all marine-based regions is included, together with evidence from abrupt climate changes in the past, the estimates could increase significantly.
Most of the effects leading to ice loss on the Antarctic Peninsula are confined to the northern part, which would contribute a few centimetres of sea level rise. A southerly progression of ice shelf disintegrations along both coasts will result from increased warming. It is suggested an increase in surface melt-water lakes, and/or progressive retreat of the calving front, may precede these. It is not yet possible to predict the timing of the destruction of ice shelves. As the ice shelves are removed the speed of glaciers to the sea will increase. On the Antarctic Peninsula the total volume of ice is 95,200 km3, which is equivalent to 242 mm of sea level, or about half that of all glaciers and ice caps outside Greenland and Antarctica, therefore increased warming may lead to the Peninsula contributing a substantial amount to the global sea level.
A range of global sea level increases from 18-59 cm between 1980-1999 and 2090-2099 was projected by the IPCC’s Fourth Assessment Report. Something they didn’t include was a contribution from changes that were dynamically driven flow changes in portions of either the Greenland or Antarctic ice sheets. It is suggested by recent modelling that there may be a rise of up to 1.4 m, instead of the 59 cm, that was suggested by the IPCC. The spatial pattern of sea level rise projections show the rise will not be uniform, with a minimum in the Southern Hemisphere and a maximum in the Northern Hemisphere in the Arctic Ocean.
The Southern Ocean is suggested by model projections to be an increased sink for atmospheric CO2. The way the ocean responds to increased ocean warming and stratification, which can drive both increases in CO2 uptake by biological and export changes, and decreases by changes of solubility and density, will determine the magnitude of the uptake.
Ocean circulation and water masses
The ability of the models to simulate ocean behaviour is limited, which is an important constraint, as they have a key role in eddies in the transport of heat from north to south in the Southern Ocean. As a result ocean models that are components of General Circulation Models (GCMs) are deficient in having typical grid spacing of about 100 km in the horizontal, which is larger than the typical ocean eddy. An intensification of the ACC in response to the southwards shift and intensification of the westerly winds over the Southern Ocean is generally predicted by the models. In the Drake Passage the increase in the transport of the ACC that is predicted for 2100 is expected to reach a few Sverdrups (1 Sv = 1 million cubic metres/second). A small displacement for the core of the ACC, which is <1o in latitude is expected to result from the enhanced winds.
In the Southern Ocean the warming that has been observed at mid-depth and the surface layer is projected to continue and eventually reach almost all depths. The warming is expected to be weaker close to the surface than in other regions. There could be enhanced ocean ventilation as a result of enhanced divergence of surface waters that is induced by the increasing wind stress and the associated upwelling. It is suggested by model calculations that by 2100 bottom waters could warm by 0.25oC. The density would decrease and hence the ventilation of the Antarctic Bottom Water.
It is suggested by the models that there will be a decrease in the annual average total area of sea ice of 2.6 x 106 km2, which is 33 %. Winter and spring are the seasons when most of the retreat is expected, which will reduce the amplitude of the annual sea ice area cycle. It is not possible for the current generation of models to provide a precise regional picture of the changes that should be expected.
It is likely the area of permafrost will be reduced, which will be accompanied by a subsidence of the ground surface and associated mass movements. The northern Antarctic Peninsula and the South Shetland and South Orkney Islands, as well as coastal areas in East Antarctica, are the areas in where change is most likely. A risk to infrastructure is implied by the forecast changes.
Experimental evidence comprises the majority of evidence of how benthic organisms may cope with rising temperature. A key trait of Antarctic marine animals is being typically ‘stenothermal’, i.e. being capable of living within a limited temperature range. They would be highly sensitive to significant warming if they are truly so limited. It has been shown by experiments that most species have upper temperature limits above which temperatures are lethal of less than 10oC, some surviving no more than a 5oC change. A rise of this magnitude in the Southern Ocean by 2100 is considered to be extremely unlikely. Though the behaviour of organisms can be affected by rising temperatures long before the lethal temperature is reached; whether the feeding, swimming and reproduction, as well as other critical activities, can be carried out by populations or species may determine if the survive the coming temperature increases.
By 2100 the temperatures of the bottom water on the continental shelf are suggested by model projections to be between 0.5 and 0.75oC warmer, with the exception of the Weddell Sea where temperatures are expected to be lower. As the warming of the surface and bottom waters are projected to be no more than 0.75oC it is suggested that there may be less effect on the marine biota than is found in lab experiments, over this time scale at least. It is near the core of the ACC that warming is projected to greater than 1.5oC.
Several of the Antarctica taxa have been found to have a distribution that extends across a range of sites or depths with a temperature range that is much greater than ‘typical Antarctic conditions’. Many populations of typical Antarctic species have been found at South Georgia, in spite of maximum summer temperatures there having been 3oC warmer than the Antarctic Peninsula. It therefore appears there may be a conflict between experimental and ecological evaluations of vulnerability, which suggest the ecological context may be crucial.
Marine ice algae will begin a continuous decline as their habitat is lost if the cover of sea ice continues to decrease, which may result in a cascade of higher trophic levels in the food web. Extinction might be expected of species depending for their survival on any of the trophic levels that ultimately depend on the presence of sea ice algae at the base of the food web, such as some fish, penguins, seals and whales. It is suggested by climate models that there is not likely to be a complete loss of sea ice within the next 100 years, as seems to have been the case in previous interglacials, with sea ice not being completely lost. Algal blooms are expected to occur more often as sea ice declines which would supply food to benthic organisms on the shelf. It is expected there may be a decline in suspension feeders that have adapted to limited food supplies, and to their associated fauna, as there is expected to be an increase in the phytodetritus on the shelf.
It is expected that among the largest ecosystem changes on Earth will be changes from a unique ice-shelf-covered ecosystem to a typical Antarctic shelf ecosystem, as ice shelves collapse, with primary production being high during the short summer.
It is believed there will likely be some thinning of aragonite skeletons of the pteropods that comprise an important part of the plankton at the base of the food chain if pH levels of the surface waters of the ocean increase in acidity by 0.2-0.3 units by 2100; also there is a potential threat to benthic calcifiers such as corals. Because there are low concentrations of CaCO3 the Southern Ocean is at higher risk from this than other oceans.
Continued warming of the ocean and expanded tourism and scientific activity may lead to the establishment of non-indigenous species by 2100, with a consequent reduction or extinction of some species that are locally endemic, given the slow growth rates and high degree of endemism among Antarctic species. It is likely the invasion of new species will remain restricted to isolated areas where invaders can survive at the physiological limits. To date it is not clear if the finding of a very few ‘non-indigenous’ macroalgae and invertebrate animals are rare occurrences at their natural southern limits of distribution, or are the first stages of a marine biogeographical shift that has been induced by ocean warming.
There are a number of disturbing agents affecting the marine biota:
1. Increased ice loading and coastal concentrations of large icebergs calved from the collapse of ice shelves, which result in more ice scour;
2. Increased coastal sedimentation that is associated with melting ice, which smothers benthos and hinders feeding;
3. Surface waters freshening which results in stratification of the water column; and
4. Thermal events such as those associated with El Niño events.
Climate change chronic impacts include:
1 disintegration of ice shelves which exposes new habitats;
2 long-term iceberg scour decreases leading to decreased local biodiversity but increased regional biodiversity;
3 Physiological effect of direct warming leading to a reduction of performance of critical activities and therefore geographic and bathymetric migration;
4 Benthic responses to changes in the pelagic system, especially in the food web;
5 An increase in acidification which leads to skeletal synthesis and maintenance problems; and
6 A slight degree of deoxygenation of surface waters which ultimately leads in deeper layers to more serious deoxygenation. The advantage for survival is minimised by the absence of wide latitudinal and environmental gradients around the Antarctic continent.
It is likely that fur seals will respond to most changes in extreme climate events, such those caused by the El Niño-Southern Oscillation (ENSO). The sea ice is depended on for completion of the life cycle of emperor penguins and other species. The populations of these animals are likely to be affected by a significant decline in sea ice, leading to Antarctic species being displaced by immigrating subantarctic species.
According to Turner et al. it appears to be unlikely that more than a few species will become extinct by 2100, either as a result of not being able to cope ecologically or physiologically with such an increase, or restricted to an area by an above average temperature increase.
Biodiversity studies, together with sound data handling and dissemination, will allow a better understanding of the evolution of life in the marine environment, and the extent of potential it has to respond to change. A legacy of knowledge for future generations, in the form of a comprehensive information system, can be provided by bridges between different disciplines and international programs.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|