Central in previous post were the predictions of an ice free Arctic by Peter Wadhams. While reading his predictions several things stood out. One of these things was that his early predictions seemed to refer to the diminishing volume over time (he referred to the PIOMAS volume data and/or the model of Maslowski that apparently showed a similar trend). However, in 2016 he referred to the diminishing extent to show that the prediction still holds.
That was rather puzzling. First, why change from volume to extent when confirming basically the same prediction? Second, when I first started looking into the global warming issue, an often heard argument from the alarmist side was that, even when the extent looked as it was stabilizing, it was the volume that was most important. This made it seem even more odd to me, that an alarmist prediction would go back to extent figures after relying on volume in the past. That is the reverse of what I expected.
The reason for this shift became clear when I found an explanation of what convinced Wadhams to make the 2015 prediction. It was explained as part of a rebuttal he wrote on March 7, 2012 (the second year that he made the 2015 prediction). Here is how he explained it (my emphasis):
There is currently disagreement about when the summer Arctic will become completely ice-free. It depends on what model is being employed. My own view is based on purely empirical grounds, that is, matching the observations of area from satellites with observations from submarines (combined with some modeling) of thickness to give us ice volume. If we think in volume terms instead of area terms, the downward trend is more than linear, in fact it is exponential, and if extrapolated it gives us an ice-free summer Arctic as early as 2015.
What clarified it for me was this graph showing PIOMAS volume data, which illustrated his 2015 prediction:
Seeing that graph, his prediction of an ice free Arctic in 2015 made perfect sense. The trend was indeed exponential downwards and the trend line would collide with the axis in 2015.
That obviously didn’t happen in 2015, so then what happened after 2012 that made his prediction fail so miserably? Just look at the PIOMAS trend after March 2012:
The trend wasn’t exponential down anymore. Since 2012 the trend was even slightly up again, but at that time he wrote the rebuttal it was not really clear what the trend would do. It seemed that it was preparing to go even more down (confirming the prediction) and initially it did, giving the lowest volume since the start of the measuremts. But then the trend went slightly up in the next years. That “exponential” downward trend was an artifact of the low in 2012, combined with lower values in 1982. If the trend would have last, it would indeed be exponential downwards and lead to an ice free Arctic in 2015, but apparently it didn’t.
The shift from volume to extent then isn’t so surprising anymore. The PIOMAS volume trend didn’t justify an ice free Arctic in 2015 anymore, so couldn’t be used to back up the current prediction. Making the switch to the extent, which was at a historic low at that moment (maybe for the reason of the strong El Niño?), a no-brainer if he wanted to maintain the prediction.