Dark deeds of definitions?

In comments on the sea ice post, both Rob Dekker and Chris Randles has queried an apparent change to the definition of “ice free” as applied to the Arctic. As any fule kno, the definition of “ice free” usually used is “less than a million square km” (1MSK) in order to account for the misc pockets of stuff that will hang around Greenland. FWIW, I’m not sure how useful this defn is, or whether it will survive closer analysis as we get closer to the event, but we’re decades away now so it hardly matters.

Except, there’s an unclarity. Is it 1MSK for a single year (maybe not; there could be a freak year); averaged over 5 years (generally the way things get done); or “for at least five consecutive years”? The latter appears to be what the AR5 SPM says (figure SPM.7 caption): The dashed line represents nearly ice-free conditions (i.e., when sea ice extent is less than 10^6 km2 for at least five consecutive years). For further technical details see the Technical Summary Supplementary Material {Figures 6.28, 12.5, and 12.28–12.31; Figures TS.15, TS.17, and TS.20}. RD and CR point out that earlier drafts (including the final draft) don’t quite say this, that the “five consecutive years” is new. OTOH, in the “final” draft what becomes SPM.7 is SPM.6, so it wasn’t really final; and the dashed line seems to be absent entirely, rather than being redefined.

Also, SPM.7 shows a 5-year-running-mean of sea ice, so when things cross the dashed line, that shows that a 5-year-mean is below 1MSK. So the text, about “consecutive” years, would appear to be wrong anyway.

Perhaps I should go off and read chapter 12 which ought to be the source for this. Figure 12.30 just has the line at 1MSK, and of course the careful language “nearly ice-free Arctic Ocean”. And some text:

A nearly ice-free Arctic Ocean (sea ice extent less than 1 × 10^6 km2 for at least 5 consecutive years) in September before mid-century is likely under RCP8.5 (medium confidence), based on an assessment of a subset of models that most closely reproduce the climatological mean state and 1979–2012 trend of the Arctic sea ice cover. Some climate projections exhibit 5- to 10-year periods of sharp summer Arctic sea ice decline—even steeper than observed over the last decade—and it is likely that such instances of rapid ice loss will occur in the future. There is little evidence in global climate models of a tipping point (or critical threshold) in the transition from a perennially ice-covered to a seasonally ice-free Arctic Ocean beyond which further sea ice loss is unstoppable and irreversible.

So I’m not sure there is anything terribly nefarious in there. Would anyone more closely involved care to comment?

All of this is just an excuse for more pix. Here’s the refuge des Bans, one of the lower down (2000 m) huts and therefore beloved of day walkers. My apologies for not taking it when it was in the sun.

Nice rocks though:

But its not all rocks and ice: