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Either way, and from the graphs posted earlier, it is clear that sea ice 'volume' is on a persistent and potentially catastrophic decline. 2017 is especially noteworthy, because of the current record low PIOMAS numbers.

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We'll see what September 2017 brings us, but it seems clear that there is a good chance that we are about to find out if the Arctic summer melts ice 'volume' or if it melts 'extent'.

It's an interesting quandary, isn't it Rob?

I was playing about with the monthly figures for PIOMAS, NSIDC Area and NSIDC extent in order to see if I could tease out any clue as to which might be least susceptible to "noise". To do so, I used a simple linear regression in order to obtain a value for the trend, and then subsequently calculated the residuals.

I then compared the Standard Deviation of the residuals with the trend. It's obviously more than a bit crude and simplistic, but my thinking was the higher the value of this ratio, the less time it takes for the genuine underlying trend to emerge from any noise distortion (i.e. natural variability).

The September numbers were...

PIOMAS: Trend = -324 cubic kms per annum, SD of residuals = 1,434 cubic kms

NSIDC area: Trend = -79k sq kms per annum, SD of residuals = 441k sq kms

NSIDC extent: Trend = -87k sq kms per annum, SD of residuals = 550k sq kms

That produces ratios of...

PIOMAS 0.226

NSIDC area 0.18

NSIDC extent 0.159

A possibly more meaningful way of expressing these values might be in terms how many years worth of each trend equates to 2 times the relevant Standard Deviation (i.e. the old 95% confidence level). That comes out as 9 years, 11 years and 12 and a half years respectively.

Using that simplistic approach would suggest that PIOMAS will be the better indicator, as it makes an earlier emergence from the natural variability.

The PIOMAS Daily Arctic Ice Volume graph from Wipneus that you posted is excellent at showing how perilous the end-of-melt-season has become, but, looking at the March-April-May part, it also serves to show that we are still a long way from a totally ice free Arctic. On the 1979-2001 average, the value for the beginning of April is ~ 29,700 cubic kms. The equivalent 2017 value stands at ~ 20,4000 cubic kms - a drop of just over 9,000 cubic kms over a (notional) period of 27 years.

The attached diagrams below show PIOMAS projections for September and for the March-May average volume. If one projects a 2nd order polynomial trend line, the September figure effectively goes to zero in 5 years, but, with a linear projection, this is delayed until about 2032.

The maximum volume is typically attained in April, but with the March-May average and using a 2nd order polynomial fit, the trend does not go to zero until 2050. In fact, the March-May average would still be around 10,000 cubic kms in 20 years time. (Using a linear fit, this trend does not reach zero until almost the end of the 21st Century.)

Anyway, getting back to the melting season aspects, we both know how poor the correlation is for area/extent when the interval gets more than a couple of months. There are various references in the scientific literature to a decorrelation period of just 2 or 3 months for area/extent.

However, using Excel's CORREL, the correlation coefficient between the PIOMAS mean March-April-May residuals and the September residuals (1979-2016) comes out at an interesting 0.65