The one used are for: September 1987, September 1994, and September 2001. It is fairly obvious that the ice concentration decreased over both seven year periods. The ice concentrations in the Arctic Ocean during September are when the ice extent there is at its minimum. I believe the concentration are reducing because the ice is thinning. Even if the ice has not thinned, the average concentration appears to have dropped from 90% in 1983 to 75% in 2001. Since the extent of the ice has remained unchanged this represents a decrease in ice volume of at least 16% in 14 years, much greater than the 3% per decade currently thought to occur. For instance see Bob Grumbine's FAQ.
With the ice concentration decreased, there is a possibility of a positive feedback happening during the summer. As the concentration drops, the leads widen, and more water is exposed to the Sun. Water absorbs more heat than the ice which reflects the sunlight. The warmer water will melt more ice. More melting ice means more water exposed which could lead to a runaway effect. One summer we may see the ice suddenly disappear completely.