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I reckon 'combination' is the right word. The (quite intuitive I think) premise is that multi-year ice tends to be thicker on average than single-year ice, therefore less multi-year ice equals thinner ice overall and so a smaller minimum extent is more likely. Questions that would be very useful to have answers for. At maximum extent: What is the average thickness of single year ice? What is the average thickness of two year old ice? three? four? And importantly what is the variance of all these? At the point of maximum extent you might find two-year old ice at the edge of last year's min extent is only 2% thinner than neighboring one-year old ice. Maybe ice thickness has a more significant relationship with "distance from last year's min extent boundary" than with it's age. I dunno, this might be completely wrong. I could spend a lot of time hunting all this down, but experts who have done this already have concluded the single-year, multi-year difference is significant so I will stick with that.

Perhaps it's not forcings at all but internal variation, ie climate noise. ENSO is only one component of that so the ENSO-corrected graph still contains such noise. It's possible that the rise to 2001-2002 temp levels was aided by positive contribution from internal variation. It was rather higher than 90s levels afterall and got there fairly quickly. In which case the drop/flatness since then has been largely due to that dropping out, perhaps even being followed by a bout of negative contribution in the last few years. Id like at least another 10 years of not much changing before concluding warming has ended.

Good post. Considering arctic sea ice is a barrier between the upper ocean and lower atmosphere that largely exists throughout the entire year, what effect does removing that barrier have? I am thinking it's a step change. The change in albedo is one step change that you point out, but others I have thought about are evaporation - how is that affected and what effects will that have in turn? Will the reduction of a sea ice barrier allow the wind to mix the upper ocean more? I am no expert on any of this so for all I know these are stupid questions, but I can think of lots of possibile step changes like these. Only need a couple of them to work to get a significant effect.