Although I respect the expertise of Wadhams, a puzzling thing was the insistence that the downward trend since the measurements is necessarily “abnormal” and caused by our emissions. I have no problem accepting that he saw drastic changes in the Arctic concerning the height / volume of the polar cap during his long career and that this was the reason why he thought that the Arctic would be ice free soon.
For the record, I have no problem whatsoever believing that the Arctic underwent changes in the last 40-50 years. But logically, we only start measuring the ice of the poles since the end of the 1970s. If we only started to measure AFTER the supposed cause started, then logically what is the “normal” extent and volume of the Arctic?
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):
The “Arctic could become ice-free for the first time in more than 100,000 years, claims leading scientist”, that was the surprising title of an article from the Independent. That leading scientist is Peter Wadhams from the university of Cambridge and it was explained that satellite data showed there was just over 11.1 million km2 of sea ice on June 1st this year, compared to the average for the last 30 years of nearly 12.7 km2. Therefor on track to be ice free this year (or next year) for the first time in more than 100,000 years.
That seemed odd to me. I didn’t know that June 1st was such an important date for the Arctic sea ice. My take was that the most important moment was in September, when the extent of ice is at its smallest. When we look at the NSIDC data I indeed noticed that it is lower than usual. But prizes are awarded at the finish, not in the middle of the race and going from 11.1 to 1 million km2 in roughly three months would require quite a steep drop. Not entirely impossible though, in 2012 ice extent dropped from 12.3 on June 1st to 3.4 at its minimum, but then it has to deviate from its current course soon to get from 11.1 to 1 in the same time frame.