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?
The measurements of the pole ice and the observations made by Wadhams all started at the end of what was then considered a cold period, in which it was reported that temperature of the Northern Hemisphere dropped by a hefty 0.6 °C since the 1940s and the sentiment was that we were heading for an imminent ice age. We could for example as well be in the downwards slope of a cycle. How could one then differentiate between an abnormal melt and a multidecadal cycle?
When I first looked at the PIOMAS data (which originally was used to predict the ice free Arctic), then I had the impression that the trend could be interpreted in more than one way. This is the graph as it shows on the website of the Polar Science Center:
I found the already added trendline rather distracting. To better compare the different views, I removed all trend information from the image and also added extra color to the line to make it stand out more. Without any trend information, this is how this graph now looks:
On the Polar Science Center website it is presented with a linear trendline. Adding that back to the previous graph is rather simple knowing the start point and the end point:
The fit is not that bad, but it doesn’t really fit in the beginning, in the middle and the end. Wadhams also didn’t see it that way. He saw a exponential trend downwards. When I add this to the image this is the result (for illustrative purposes only):
This seems a better fit in the beginning and in the middle, but at the end it gave the problem that the ice volume didn’t follow the supposed trajectory after 2012. If that would be the case, the ice would have been gone by 2015, which obviously didn’t happen. Taking the last part into account gives something that resembles a cycle (for illustrative purposes only):
Of course, the future will tell what the exact trend is. Maybe it continues to go up, confirming a cycle. Or go back down in a linear way, confirming the PSC trendline. Or stay stable for a while. One thing is for sure, it is not going down exponential.
The puzzling thing to me however is why this trend is presented as if it is really clear that the decrease is abnormal and caused by our emissions. There is nothing in the data that shows this. If we want to prove that anthropogenic emissions are to blame, then we need to know what the natural base line is, which is not the case because the measurements started at a time the supposed effect already was having an influence. Therefor I have no problem believing that the Arctic changed a lot during the last 40 years, but am wary about how it is being explained without it following from the data.