Understanding how sea ice, the ocean and the atmosphere interact is critical for weather predictions and climate projections.

Sea ice extent, thickness, concentration, drift and snow thickness above the ice are all important elements of global systems. Changes in these attributes can have wide reaching consequences. They can significantly alter the ocean-atmosphere interaction, ocean circulation and the marine food web.

How is sea ice extent changing?

Satellite measurements show the average annual sea ice extent in the Arctic has declined by 3.5% to 4.1% per decade since 1979. The summer extent has decreased by 9.4% to 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. This is consistent with the recent change to more northerly winds and surface warming in that region. 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 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?

Numerical climate models predict that Antarctic sea ice area will reduce by a 3rd by the year 2100. Such reductions will be a result of feedback changes between the sea ice and oceanic and atmospheric circulation. Changes in the timing of annual sea ice advance and retreat (seasonality) are also expected.

What is driving the changes in sea ice?

In the western Antarctic Peninsula, intense northerly winds during autumn-spring have led to wind-induced ice compaction. The average air temperature in winter has also increased by 5.8 °C between 1950 and 2005. This is attributed to climate change. These factors have led to sea ice decline in the region.

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.

Does the ozone hole have an effect on sea ice?

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. As ozone levels recover towards the end of the century, sea ice is expected to decline.

How are we monitoring sea ice change?

We have used satellites to monitor sea ice cover since the late 1970s. The data provide a solid view of the horizontal sea ice space.

Measuring sea ice thickness and volume over large scales is more difficult. 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.