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Understanding Polar Sea Ice Extent

By Mark Bennett
June 30, 2010

An issue that has consistently been front and centre in climate change talks is the dramatic reduction in the northern polar sea ice extent. With all of the discussion around sea ice extent, many interested parties do not actually know what constitutes the extent and how it is derived.

Currently, the sea ice extent is derived from passive microwave emissions received by the SSMR and SSM/I satellites. NASA launched the Scanning Multichannel Microwave Radiometer (SMMR) in 1978, and the Defense Meteorological Satellite Program (DMSP) launched the first of the Special Sensor Microwave/Imager (SSM/I) sensors in 1987. The raw data from these satellites is transformed by an algorithm whereupon it can be displayed in GIS format. It is now possible to have access to up to date sea ice extent data daily from the National Snow and Ice Data Center (NSIDC) website.

Of great interest is what constitutes the sea ice extent. Most would assume that it is the edge of the solid ice cover; most would be wrong. The transformed data is classed according to ice concentration. The concentration is the ratio of ice and ice fragments to open water. As consensus has determined, the ice extent is where the ice concentration is greater than or equal to 15%. What this means is that all that is required is 15% ice to 85% open water. This is hardly pack ice that one could walk on. This is more akin to an ice cube floating in your drink.

Now, of grave concern (besides the loss of extent) is the conversion of the polar ice from high concentration to low concentration. This means that much of the solid, 100% concentration sea ice now exists as chunks of ice floating in ocean water. This lower concentration ice is very susceptible to the effects of wind and oceanic currents which easily disperse it. Not only does this result in a far smaller ice extent but it also slows the formation of any solid, multi year ice which could protect the icepack from rapid decline as well as providing the habitat necessary for many Arctic wildlife species and the indigenous peoples that depend on them.

The importance of the Arctic sea ice cover is multi-faceted. Its effect on the climate system of the planet has only recently been discussed. Exactly how our climate will respond to such a loss can only be the subject of conjecture but it is safe to say that the effect will be dramatic.