I've been busy with the day job, which has given me time to ponder something I've hinted at before but am now firmly behind. This year isn't a major factor, due to my increased workload in the day job I've had time away from this subject doing something else (often repetitive and boring admin) that has let my subconscious work on the issue - the issue being the long term outlook for Arctic sea ice. Here is what I am now 90% convinced of, having entertained the scenario as a possibility over the last year or so.
I've previously posted about a suspicion that as the ice nears 2m thick it may rapidly crash to near ice free late summer conditions due to a non linear nature between April thickness and the percentage of open water formed by September. PIOMAS April thickness shows a shift around 2m from low percentage of open water formation to high open water formation.
This is similar to panel B of Lenton 2012 "Arctic Climate Tipping Points", PDF, where translating the above graph to that graph, the control variable would be April thickness and the system feature would be the percentage of open water formed in September. Note that the horizontal axis is also reversed.
This would represent a reversible type tipping point. However I have blogged recently about a factor that may stop thickness from dropping further, the transition of the pack to a mainly first year ice pack. That is the thickening of ice over winter.
Since 2000 April average area for the Arctic ocean has been around 9.65M km^2 - i.e. this is the nominal area of ice covered ocean at the volume peak. Assume a nominal 2m equilibrium thickness for the FYI, also assume that thinner ice at the Atlantic edge is to some degree offset by the thicker ice in the Central Arctic. 2m * 9.65M km^2 => 0.002km * 9650km^2 = 19.3k km^3. So an April peak volume of around 19.3k km^3 is the nominal volume associated with an Arctic Sea ice nominal thickness of around 2m.
Now, keeping in mind 19.3km^3, look at what has happened to April Arctic Ocean volume, volumes in thousand cubic km (km^3). Instead of a graph I've copied the following from an Excel spreadsheet, to stress the numbers at the end of the series, and graphically show the decline.
Since the volume loss of 2010 the volume of the Arctic Ocean has levelled at around 19.3k km^3, the same volume as calculated to be the rough volume to be expected with a mainly first year ice pack. I do not think this is coincidental.
All this means I expect to see an end to the previous volume loss trend because this was mainly due to loss of multi year ice (MYI). MYI is now going to fluctuate at low levels in the years to come. Volume and MYI may be up at the end of this season, but it remains fluctuation at a low level compared to the past. By next April I expect to see a volume figure similar to the last four volumes of the series shown above. The end to the period of rapid loss of volume may already have happened, but the time period involved since 2010 is far to short given the variability in the data for statistical significance.
Therefore with regards summer area, extent and volume, I think the spread from 2007 to 2013 is probably typical of what we will see for at least the rest of this decade, with 2012 type drops being followed by a reversion to that range, not a new regime (unlike 2007). I still suspect 2010 may have a lingering effect, but this year suggests it may not be as strong as the post 2007 period. Critical to all this is that I am becoming convinced that the approximate levelling of PIOMAS volume over the last few winters is telling us that the pack is becoming dominated by FYI, whose thermodynamic equilibrium thickness is largely setting the peak volume in April. Even if one year, with exceptionally good melt weather, were to lead to extent below 1 million kmsq, this will be unlikely to be repeated, and for the record, I do not think this is likely anyway. To get to a state of near ice free conditions in late summer we will need to see significant thinning of the winter peak thickness, which needs far greater winter warming. I don't think this is likely to be a fast process.
So I do not expect to see a virtually sea ice free state until later in the next decade - at the earliest, I suspect that Overland and Wang may be proven right in pinning it on the 2030s. In terms of expectations amongst many in the amateur sea ice community this is a slow transition. However in geological terms it remains abrupt.
Appendix for Dromicosuchus, who asked if the process of April ice thickness loss levelling is happening in any of the peripheral seas.