Understanding the interactions between sea ice, the surrounding ocean and the atmosphere is critical for accurate weather predictions and climate projections. Scientists are studying these interactions through measurements of sea ice extent, thickness, concentration, drift and snow thickness above the ice. Changes in these attributes significantly alter the ocean-atmosphere interaction, ocean circulation and the marine food web.
Australia has more than 20 years of ship-based sea ice thickness measurements, which provide a baseline against which to measure future change and data for climate models. These include observations from the Sea Ice Physics and Ecosystem eXperiment (SIPEX) in 2007, and SIPEX-II in 2012, which studied the physics and biology of the sea ice, and the interactions of the sea ice structure, thickness and snow properties and their effects on the under-ice algae and ecosystem of the Southern Ocean.
These in situ sea ice measurements are used to validate local satellite measurements and improve the instruments and techniques used to make these measurements. For example, observations from the Australian sea ice research campaign, Antarctic Remote Ice Sensing Experiment (ARISE) in 2003, are being used by NASA and the European Space Agency to validate and develop satellite algorithms. As satellite measurements become more accurate they will be used to measure sea ice (and any changes) on a large scale.
How is sea ice extent changing?
Satellite measurements show the average annual sea ice extent in the Arctic has declined by 3.5–4.1% per decade since 1979, while summer extent has decreased by 9.4–13.6% per decade.
In Antarctica the changes have been much more subtle and regionally variable. The western Antarctic Peninsula region has shown a decline in sea ice extent, particularly in the Bellingshausen Sea, consistent with the recent change to more northerly winds and surface warming observed there.
In contrast, sea ice in the Ross and Weddell seas is expanding. These changes involve both changes in sea ice extent and in the length of season during which sea ice is present each year.
The Fifth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC AR5, 2013) concluded that there had been a small increasing trend (about 1.49 ± 0.18 % per decade) in Antarctic sea ice extent for the period of reliable satellite records (since 1979).
The cause of the contrasting responses of the Antarctic and Arctic sea ice is the subject of lively debate.
What are the projections for the future?
An ensemble of numerical climate models predicts that Antarctic sea ice area will reduce by a third by 2100. Such reductions will be a result of feedback changes between the sea ice and oceanic and atmospheric circulation. Changes in sea ice seasonality (the timing of annual sea ice advance and retreat) are also expected, which will impact on the ecosystems of the Southern Ocean including its wildlife.
What impacts will changes in sea ice extent and duration have?
Reduced sea ice formation will potentially slow the global ocean overturning circulation and will result in increased absorption of the sun’s heat by the ocean at high latitudes. It will also provide greater access for ships to the higher latitudes and may lead to a significant increase in tourist vessels in the Antarctic. This will have implications for Australia’s search and rescue responsibilities as well as for resource management.
What is driving the changes in sea ice?
In the western Antarctic Peninsula, sea ice decline has largely been driven by an intensification of more northerly winds during autumn-spring, leading to wind-induced ice compaction. The sea ice changes are also coincident with an increase in average winter air temperature of 5.8°C between 1950 and 2005, attributed to climate change.
In the western Ross Sea region the increase in sea ice has been attributed to both a strengthening of westerly winds and a more frequent southerly outflow of winds from the continent, associated with the persistence of a deep low-pressure anomaly in the Amundsen Sea.
Intensive research is continuing using both modelling and observations to better understand changes in the large-scale patterns of atmospheric circulation around Antarctica, their complex impacts on observed changes in sea ice, and possible feedback mechanisms involved, as well as connections with atmospheric processes in other parts of the world.
Does the ozone hole have an effect on sea ice?
Recent research published by scientists from the British Antarctic Survey suggests that the ozone hole is delaying the impact of greenhouse gas increases on the climate of Antarctica and contributing to the increase in Antarctic sea ice (BAS press release). As ozone levels recover towards the end of the century, however, sea ice is expected to decline.
How are we monitoring sea ice change?
Sea ice cover (area, concentration, extent, seasonality) has been monitored from satellites since the late 1970s. These data provide a solid view of the two dimensional (horizontal) sea ice space. However, methods of accurately and routinely measuring and monitoring the third dimension — sea ice thickness and volume — over large-scales, are only now emerging, with satellite radar and laser altimeters showing great potential.
Most of our knowledge on the changes in Arctic sea ice thickness comes from recently de-classified sonar data from military submarines. No such data exists for the Antarctic. However, Australian researchers have developed a technique for measuring sea ice thickness that has resulted in the first circumpolar maps of Antarctic sea ice thickness ever published.
These data provide a valuable baseline for climate studies. However, ongoing monitoring of changes in Antarctic sea ice thickness requires more precise measurement techniques. These are currently being developed and implemented by scientists at the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) and include airborne laser and radar altimetry for surface mapping, and under-ice sonar measurements using an autonomous underwater vehicle for measuring ice thickness. In addition, ACE CRC research validates satellite-derived information on the thickness of snow cover on sea ice.
Data from these programs will be used to improve the interpretation of satellite data and will ultimately contribute to the production of more reliable global ice thickness products.