Sea ice extent

Page - September 11, 2009
Every year in mid-September, eyes turn north to find out what the minimum sea ice extent is and find out how it compares with previous years.

An indicator of change

The annual sea ice minimum (and maximum around February) is an effective indicator of how the Arctic sea ice responds to increasing temperatures, currents and atmospheric changes. Sea ice extent - simply put, the area of the Arctic Ocean covered by sea ice varies every year, but trends can be established over time.

The settlement of Tasilaq in Greenland. ©Greenpeace / Nick Cobbing

Measuring sea ice extent

Satellite measurements of sea ice have only been available since 1979 when satellites were first commissioned for this purpose.  This does not mean, however, that no previous data exists. From ship and aircraft observations, it is possible to partially recreate the trends.

Extent or volume?

Ideally, sea ice volume - extent multiplied by thickness - is the best indicator of the health of the Arctic sea ice. However, satellites have only been able to measure this properly for a few years. In order to establish longer term trends, sea ice extent is still the best measurement available, although small patterns in volume change are beginning to appear.

During the first half-century, ice extent in all seasons remained essentially constant. Beginning about mid-century, the summer minimum extent began to shrink while the winter maximum remained unchanged. Starting in about 1975, the maximum, too, began to shrink.

Old ice versus new ice

The age and thickness of sea ice is just as important as extent. First-year ice - ice that has frozen in one winter - is not more than one to two meters thick. Once that ice survives one or more summer melt seasons it becomes multi-year ice. Multi-year ice is commonly three to four meters thick, not only because more ice freezes each winter, but because plates of ice known as "floes" collide due to wind, current and tide.

When ice floes collide it's a lot like plate tectonics: two continents collide and a ridge of mountains is formed. In the case of sea ice, when ice floes collide a ridge of ice is formed. Ridged ice can grow 5 meters high and extend 10-25 meters deep below the water. Thin, first-year ice is more susceptible to melt than thick, multi-year ice. Scientists are finding that less and less of the Arctic sea ice is made up of multi-year ice, meaning the sea ice is not only receding, it's thinning, too. And that makes it more susceptible to even more melt.

NASA/Goddard Space Flight Center Scientific Visualization Studio

Melting ice

What do these measurements tell us? The annual sea ice minimum and maximum extent has been shrinking in the last decades:

The contrast is particularly clear when comparing satellite images several years apart.

While year to year anomalies can be explained by short term weather changes, long term trends - like the one appearing with 30 years of satellite data - cannot. The last two annual minimum sea-ice extents are the lowest since satellite records have begun. They are on par with the trend of continuously shrinking sea ice.


Sea ice melt does not contribute to sea level rise the way the melt of ice on land does - when an ice cube melts in a glass of water, the glass doesn't overflow.

However, the shrinking and receding sea ice still has dire consequences. First, as the white ice that normally reflects sunlight away from Earth melts, more of the dark open water of the Arctic Ocean is is exposed, absorbing heat and causing more ice to melt. This is a positive feedback loop where ice melt causes more ice to melt. Second, distinctive Arctic species such as the polar bear, walrus and ice seals depend on the sea ice; they cannot survive without it, so as the sea ice shrinks and thins, these animals' continued existence is jeopardized, as are the Arctic peoples whose cultures and ways of life have depended on the animals and the ice for millenia.