The area of sea ice cover is an important climate feedback. Figure 20 in "Storms" shows the large decrease of Arctic sea ice area that occurred at the end of the warm season in 2007. This sudden loss of sea ice is a cause of concern because sea ice area causes an amplifying climate feedbacks. As the area of ice decreases, increased absorption of sunlight by the darker ocean causes more sea ice melting. The huge sea ice loss of 2007 caused some scientists and other people to speculate that all Arctic warm-season sea ice may be lost within five years.
Updates of Fig. 20 in "Storms", and also for the maximum months. (Also in PDF.)
Sea ice cover is probably not that unstable. The figure below shows Arctic and Antarctic sea ice cover in the summer months of maximum insolation, as well as the ice cover in the months with maximum and minimum ice area. It is the sea ice area in April-August, when the sun is high in the Arctic sky, that determines the degree of sea ice feedback in the Northern Hemisphere. This figure suggests that the September 2007 sea ice minimum did not have a correspondingly large effect on the sea ice area at the time of maximum insolation.
Arctic and Antarctic sea ice extent at their minimums, maximums and seasons of maximum and minimum insolation. (Also in PDF.) The "extent" includes the area near the pole not imaged by the sensor. It is assumed to be entirely ice covered with at least 15% concentration. [This statement and data are of National Snow and Ice Data Center, Boulder, CO; ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/.
It seems likely that all September Arctic sea ice may be gone within a few decades, if human- made greenhouse gases continue to increase. On the other hand, as discussed in "Storms", if Earth's energy balance is restored by decreasing atmospheric carbon dioxide to 350 ppm or less, it may be possible to stabilize or increase the area of Arctic ice.
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