Sea ice area and extent

Definitions

Sea ice area and extent give a 2-dimensional view of how much sea ice there is.  These metrics are expressed in (millions of) square kilometers.

Sea ice area is the total surface area covered by sea ice.  For example if a certain pixel in the satellite images consists of 60% sea ice and 40% open water, then 60% of the pixel's surface area is included in the total sea ice area.

Sea ice extent is a simpler metric that uses a binary classification:  it considers each pixel in the satellite images to be either 100% sea ice or 100% open water.  This is based on a threshold value:  if  the pixel consists of more than 15% sea ice then it is considered to be a sea ice pixel, whereas a pixel that consists of less than 15%  sea ice is considered to be an open water pixel.  The total sea ice extent is the sum of the surface area of all the sea ice pixels.  There are historical reasons for using this metric.

Sea ice area is in principle a more useful metric than sea ice extent.  But in practice, sea ice area is harder to measure, especialy in summer, when the satellites have a hard time distinguishing melt ponds on the ice from open water.  However sea ice area is still a very useful indicator of the state of the sea ice even in summer.  Sea ice extent is usually more robust against measurement errors, although it can also have artifacts, especially near coastlines.  Other factors such as clouds, precipitation and warm air advection can also affect the measurements.

For more information on the differences between sea ice area and extent, see the NSIDC website.