Again, I'll put our estimates in context of some other estimation methods.
- Climatology 1979-2000: 7.03 million km^2
- Climatology 1979-2008: 6.67 million km^2
- Linear Trend Climatology 1979-2008: 5.15 million km^2
- Wang, Wu, Grumbine model: 4.4 million km^2
- Wu, Grumbine, Wang model: 4.1 million km^2
- Grumbine, Wu, Wang statistical ensemble: 3.9 million km^2
The statistical ensemble reflects two items. One is, last year being a new record, well below the estimate (by 0.8 million km^2), pulled down the best curves that can be fit through the data. The second is, the curve itself shows a zone of steep decline. This is where the feedback of more open water -> warmer ocean -> thinner ice -> easier to melt ice -> more open water in summer is starting to take off rapidly. (If the basis for my using this curve is reasonably correct, that is.)
The linear 'climatology' estimate last year was 5.23 million km^2, versus the observed 3.61. Too high that year, and every year 2007-2012, and very likely many more to come. Merely points to a straight line not being a good representation of how the ice pack is changing, which is no surprise. But it is also a reminder that talk of sea ice 'recovery', which will no doubt occur in some corners when (if) 2013's extent is greater than 2012's, is nonsense.