collapse of the old society

Person who does lots of stats checking in. That's a good question. We usually refer to Sigma (1-sigma, 2-sigma... 6-sigma) as the probability that an observation could occur by random chance.

The probability of 6-sigma occurring by random chance is about 1:1-billion.

So you're definitely right to characterize it as an outlier. In terms of sea-ice this means that based on our observations of ice extent recorded going back to 1989 (based on the image) it is extremely unlikely we would expect to observe a sea ice extent so far below the norm suggesting something else (climate change) explains the deviation.