...
All that said - we do get consistent and timely measures with equally consistent comparison values for decades of history - we can get wrapped up on daily data variations, but the bigger picture of year over year and decade by decade change is valuable and with all it's limitations seems to be a valid measure of what is happening.
Good discussion. But this particular point, i can't agree with. Decades of history means nothing if you have the system changing its mechanics - not just amounts of this and that. And Arctic does exactly that, especially this melt season, because of earlier and much larger areas of open water during this melt season - if we compare to "decades of history". See, those areas absorb lots of heat which historically was spent to melt ice in those areas - but this season there was no ice to melt then and there, and so that heat warmed up water column instead. And much of it ends up bottom melting more ice - just some place else and quite much later during melt season.
I am convinced amount of this action is both very significant for this melt season, and quite unprecedented anyhow "historically". This year's pacific side was so open so early, etc... We still do not see full extent of all the absorbed heat during last ~50 days, but we certainly see some of it in action now. And bottom melt is exactly the sort least noticeable from above, until things start to fall apart - which then looks very much like that picture jdallen posted, i believe.
Further, warmer water column alters not only ice melting later-on, perhaps even more importantly - it alters weather systems around, thermal gradients near remaining ice, slows or halts any otherwise possible re-freeze processes. You know how land areas around the globe which are near large bodies of water have their local climate "moderated" by such a big water body? Year-round. Well i see no reason for similar effects to not happen in Arctic, and they gotta increase as open water areas appear larger and earlier.
This all exactly invalidates estimates of current situation based on historical measurements. How much it does? How big is the error for every particular date / region / measurement kind? Obviously, hard to tell. But i prefer to err on the safe side of things and limit usage of "historical comparisons", when there are reasons enough to be unsure what are the answers to those questions.
Especially since even largest bodies already made that same mistake, and quite many times. Like IPCC predictions of summer Arctic ice holding up to some 2080s or so, which were official and all-serious not much more than just a decade ago - were much based on "historical comparisons" indeed. I guess you know how different is what was observed and the line IPCC had in that prediction for up to current time? More than humiliating - i'd say, shameful.
So you see, can't quite rely on history anymore. Too fast a change is happening there. The ice desert of Arctic is not quite there anymore...