I teach basically graduate statistics and use the data related to this to teach Time Series Analysis. TSA basically uses filters that are some combination of cyclical, trend, and event. Arctic sea ice has a wonderful 12 month cycle and declining trend. Events are difficult to come by, other than 2012 (and this site offers a wonderful explanation). Here is my question, would the temperature anomaly be best interpreted as an event (eruption or heightened activity)? Constant activity would be swept up in the cycle or trend, but this appears different. I am teaching the class this fall and want to make sure I am offering an explanation that makes sense. BTW, I do treat atmospheric CO2 levels as a lagged covariate to teach that. Anyway, rambling on too long, but want to make sure I represent to the students that I teach what the data seems to indicate.