Characteristics of Antarctic sea ice | Increasing Antarctic sea ice | Wind and movement | Changes to sea surface temperatures | Increased precipitation | Difficulty in measurement | Climate model simulations of sea-ice trends | Summary | References | Comments |
Characteristics of Antarctic sea ice
The Antarctic continent is surrounded by seasonal, floating sea ice. This sea ice, which comprises mainly frozen sea water, with occasional icebergs from glaciers and ice shelves, covers a minimum of 3×106 km2 in February to a maximum of 18×106 km2 in September. This effectively doubles the size of Antarctica in the winter. Most of the summer sea ice stays in the Weddell Sea, where it is relatively protected from the ocean currents.
The Arctic regularly reaches ever smaller extents of end-of-summer minimum extents of sea ice. This changing sea ice extent is cited by the IPCC as an indicator of a warming world. However, sea ice extent is growing in Antarctica . In fact, it’s recently broken a record for maximum extent.
However, aside from the fact that many people confuse land ice, sea ice and ice shelves, it’s important to note that there are huge differences between the Arctic and the Antarctic. The Arctic is an ocean surrounded by land. The Antarctic is land, covered by ice, surrounded by ocean. Sea ice in the Arctic is generally thick, multi-year sea ice that survives several seasons, whereas the sea ice in Antarctica largely melts away each summer. Antarctic sea ice is mostly thin (~0.6 m thick ), single-year sea ice. It’s also warmer, more saline and more mobile than Arctic sea ice .
Clearly, these two different regions will have very different responses to climate and oceanic change, and these differences will affect sea ice response. And it’s also important to remember that, while sea ice is increasing in Antarctica, glaciers and ice shelves are all melting rapidly, producing large volumes of fresh water.
Increasing Antarctic sea ice
The figure below (from Ref. ) shows the total variability of Antarctic sea ice extent over the last 34 years. Decadal monthly averages almost overlap, and there is little change in seasonal variability.
The trend maps in the figure below show changes in sea ice extent in winter, spring, summer and autumn. The trends are mostly significant near the ice edge. Positive trends are evident in the Ross Sea, with autumn and summer negative trends mainly confined to the Bellingshausen and Amundsen seas. These trends are showing very small increases in total sea ice area and extent; the trend magnitude is approximately one third as large as the trend in decreases in the Arctic. Further, the increases are strongly controlled by region (some regions are showing ever smaller sea ice extents); the Ross Sea has shown the greatest increase in sea ice extent in Antarctica.
The increase in total Antarctic sea ice extent 1978-2012 is slightly positive at 1.5 ± 0.3% per decade. On a seasonal basis, the trends in ice extent and ice area per decade are:
|Season||Sea ice maximum extent||Sea ice-covered area|
|Winter||1.2 ± 0.5%||1.9 ± 0.7%|
|Spring||1.0 ± 0.5%||1.6 ± 0.5%|
|Summer||2.5 ± 2.0%||3.0 ± 2.1%|
|Autumn||3.0 ± 2.0%||4.4 ± 2.3%|
So, the largest trends in sea ice extent and in ice-covered area are in autumn. The trends are higher for ice area than for ice extent, indicating less open water, which may be related to changes in ice drift and wind patterns.
While changes in Antarctic sea ice extent remains an exciting topic for further research, there are a number of reasons put forward that explain these trends.
Wind and movement
Changes in atmospheric dynamics and winds are an important driver of regional sea-ice trends. Ozone and greenhouse forcings cool the Antarctic stratosphere, which increases the stratospheric vortex and tropospheric zonal winds. This results in an increase in the Southern Annular Mode . Increases in the Southern Annular Mode (SAM) signify increased westerly winds  and a rigorous isolation and cooling of parts of the Antarctic continent .
Because the Arctic is a semi-enclosed ocean, there is little scope for sea ice movement. Ice in the Arctic is thicker as a result of collisions, which means that the ice will last longer. This means that much of the Arctic sea ice lasts for several seasons, leading to permanent ice cover at the pole. However, in the Antarctic, there are far fewer such constraints. The sea ice is able to move around far more freely. It floats northwards to warmer waters, where it melts away almost entirely. Changes in the winds around Antarctica therefore change ice-concentration trends around Antarctica  by influencing sea-ice production and melt rates . The pattern of wind change is complex, but variations in winds can help to explain some of the regional patterns in sea-ice formation . Where the wind blows to the north, the sea ice is blown north where it melts, resulting in increased sea-ice extent. Where the winds blow south, the sea ice is blown towards the continent, resulting in decreased sea-ice concentrations.
Polynyas are areas of persistently open water in regions where sea ice is usual. The water remains unfrozen as a result of processes that either prevent ice from forming or that move ice out of the area. Polynyas are therefore an important part of sea-ice production. An increase in the extent of polynyas in the Ross Sea from 1978 to 2008 contributed to sea ice production . The resulting increased ice export accounts for a large proportion of the increased trend in ice production. Changes in wind circulation alter ice production and export in and from these polynyas.
Changes to sea surface temperatures
As the glaciers and ice shelves melt on the Antarctic continent, freshwater is added to the oceans. This layer of cold, fresh water on the ocean surface freezes easily . When combined with increased ocean stratification due to this enhanced run off , sea-surface temperatures are depressed, encouraging sea-ice formation.
A recent modelling study has shown that increases in fresh meltwater flux from melting glaciers and ice caps on Antarctica under various IPCC standardised global warming scenarios offsets the decline in sea-ice area and to even further encourage the increases in sea-ice extent, especially in winter (in summer, air temperatures are too high to support significant sea-ice growth) .
Increased stratification has further implications. Suppression of ocean circulation overturning decreases the ocean heat flux available to melt ice, leading to an increase in net ice production.
Warmer air holds more moisture, and so precipitation is increasing around Antarctica . Strong warming in the middle latitudes of the Southern Ocean can lead to an enhanced hydrological cycle, with enhanced evaporation and moisture content in the lower troposphere . This additional moisture is transported poleward, where it results in increased precipitation. Increases in snow and rain falling onto the ocean contribute to the freshening of the ocean surface in the high latitudes of the Southern Ocean. Fresher, colder water freezes more easily, so this mechanism may contribute to the growth in area of Antarctic sea ice.
Furthermore, the increased weight of snow on the sea ice may force it deeper into the water, forming thicker sea ice when the snow refreezes. Deeper snow also insulates the ice, protecting it from melting .
Difficulty in measurement
Sea ice is measured by repeated images taken by satellites orbiting the Earth . Passive Microwave Sensors were developed in the late 1960s, with the ability to measure sea-ice edge, surface composition and soil moisture. These measurements have been taken with approximately a daily resolution from the late 1970s onwards.
The microwave emissivity of sea ice is higher than the ocean, which means that ice-covered areas have a higher brightness temperature than the ocean . However, warmer surfaces also have a high brightness temperature, so it is difficult to distinguish between cold sea ice and a warm ice-free ocean. Scientists therefore use simultaneous measurements at multiple frequencies and polarisations (the difference in emissivity between sea ice and ocean varies with frequency and polarisation) . Other factors complicating the measurement of sea ice include weather interference, cloud, thin ice, and so on.
Two algorithms for estimating sea ice extent from these measurements were developed in the 1980s. The ‘Bootstrap’ algorithm is one of the most widely used ice-concentration products, and forms the basis of the observations of sea ice made in the IPCC reports.
A change in the inter-calibration across two different sensors on successive satellites caused a substantial change in the long-term trend in sea-ice extent. There is apparently an error in either the current dataset or the one used prior to the mid-2000s. The authors of this particular study suggest that observations should be re-examined to determine the sensitivity of observations to this change in the dataset.
Climate model simulations of sea-ice trends
Climate models simulate a decline in ice extent, thickness and volume in Antarctica. Equilibrium models cannot currently reproduce trends in Antarctic sea ice variability . Virtually all equilibrium climate models simulate a strong decrease in the area of sea ice . This may be because global climate models do not currently incorporate ice-shelf / -sheet/ -climate interactions. Basal melt from ice shelves is therefore disregarded. These equilbruim models may give an idea of what may eventually happen. Simulations with models that do include these interactions, particularly simulating the effect of extra freshwater from melting glaciers and ice caps, do simulate growths in Antarctic sea ice [10, 12].
Transient climate models are more able to capture the transient response of sea ice to changes in the winds. A modelling study by Marshall et al.  showed that changes in the winds tend to push the ice edge northwards, increasing ice extent. These winds also push the ocean surface northwards too, which effectively brings warmer water to the surface and eventually counters the increasing sea ice trend after a few decades. These transient models show that not enough time has elapsed for the equilibrium response to be achieved. We may well see the trends reversing in a few decades.
Antarctica is a unique environment, and the complex interactions between ice, ocean and atmosphere have led to a unique set of circumstances that have resulted in sea ice growth. It may be explained by many factors, or most probably by a combination of several. Climate change is a complex process governed by multiple feedbacks between different parts of the system; complex interactions between the melting land ice and ice shelves fringing the continent and changes in wind stress are all implicated in controlling Antarctic sea ice extent. Further, more work is required to ascertain the reliability of observations of sea ice increase given the recent discovery of an error in the algorithm used to quantify and map sea ice over the last few decades.
- NSIDC: Sea ice characteristics
- NSIDC: State of the Cryosphere: Sea Ice
- Skeptical Science
- NASA Earth Observatory
- RealClimate: clarity on Antarctic sea ice
- Shrinking ice shelves
- Glacier recession
- Ice shelves, sea ice and icebergs
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