Crandles,
The thickness measure used does not tell us anything about the thickness loss in ice that has not melted out. We can say fairly confidently that the extent loss up to July 22nd requires that all the extent that was less than 1.7m thick according to the April figures has now melted out. In 2012 the July 22nd figure was only 1.66m.
This doesn't mean that the ice that was thicker than 3 m in the original measure has lost 1.7m as well.
The thicker, the ice at the start of the season, the less the thickness will reduce. So ice that was 4m thick might only lose 50cm over the entire season. Chris's figures show that there was a lot more of this thicker ice at the start of this season than in 2012. As a consequence we could see an extent record low without seeing a volume record low.
Theoretically I do accept that is possible. Generally ratio of area to average thickness at minimums has stayed reasonably constant, and we generally see more volume minimum records than extent area minimum records but it might happen the other way around. For this year I think in declining probability we could have:
a year with neither records, or
a year with both records, or
a volume record but not an extent record, or
an extent record but not a volume record
The last three seem low probabilities compared to the first but even the least likely is not something I would completely rule out even if I think it is very unlikely. Maybe I would give subjective probabilities something like 78% 15% 4.5% 2.5%.
We have now moved into the 1.7-1.8m range in the histogram. In this range as well as the next two ranges there is more than 2 1/2 times as much extent as in 2012. That's 3.356 M km^2 compared to just 1.281 M km^2 in 2012. In the past three years all of this range has melted out by the end of the season. I expect it to happen again this year. That takes us down to 4.350 M km^2.
The question at this stage is how much the weather contributes to losses greater than this..
Thanks for confirming that you do believe the 2.5* faster volume melt that your model predicts. You don't seem to have answered if you would believe it if your model predicted 20*faster volume melt rate. What if I asked if you wanted to bet £100 on a record low extent occurring this year?