Renowden’s Threnody Reprised
Two years ago, Gareth Renowden posted a piece titled FIVE YEARS (THRENODY FOR ARCTIC SEA ICE). He modeled changes in arctic sea ice extent as a function of linear trends in sea ice volume and “thickness.” The result was an accelerating decline in sea ice extent; it might have been the first such ‘accelerating decline’ chart I had seen. I bookmarked it for later review. Now here we are, two years later…
Gareth took PIOMAS volume and divided it by NSIDC sea ice extent to calculate a notional “thickness.” He then determined the linear trends for both the volume and the thickness over a 21 year period (1990-2010) and a 10 year period (2001-2010). He then used the trend projectons for volume and thickness to project extent.
I’ve reproduced his methods in the following two charts, the first for the 21 year trends and the second for the 10 year trend. The sea ice volume and extent for 2010 and 2011 have been added; note the four “cross-hair” data points.
There were three things about Gareth’s work that seemed odd to me. The first is that he used sea ice extent rather than area. The second is that he extended his projections from the last data point (2010), rather than from the trend value for 2010. And the last is that thickness remains positive even as volume goes negative.
In the following two charts, I have “corrected” Gareth’s method by using sea ice area and by projecting the trend from the 2010 trend value and not the 2010 data value. The 10 year “zero” date is moved from 2016 to 2017.
It’s pretty obvious that projections of the 21 year trends do a poor job of predicting volume and area values for 2011 and 2012. The ten year trends do better.
But as a “model”, this is still a piss poor model. It is a model in which the volume goes zero (and beyond!) but for which the thickness remains greater than zero. Recall that thickness is a calculated, notional value based on extent or area. The fact that the notional thickness trend isn’t falling in proportion with the volume trend simply indicates that the real sea ice extent and area aren’t falling at the same proportion as the PIOMAS volume. It appears that the arctic sea ice is getting thinner proportionally faster than it is retreating.
And the same chart as the last but extended over the entire time series…
I quote Gareth’s original post from 2010
Gareth (2010): Bottom line: if the relationship between ice volume and extent evident in the NSIDC and PIOMAS data over the last 21 years continues in the near future, then the Arctic will be effectively ice-free in late summer sometime between 2015 and 2020. One interesting observation: the 10 year trend chart above suggests that 2007′s record minimum extent could remain unbroken next year, and the ice would still be on course to disappear within five years. If there’s any upside at all to this message (and I’m struggling to find one) it is perhaps that such a rapid and visible loss of sea ice might finally persuade the international community to take urgent action to reduce the atmospheric carbon load. What a seasonally ice-free Arctic might mean for global climate is something I shall look into a future post.
Source here: threnody.R