Earlier this month the US National Snow & Ice Data Center issued its analysis of this year’s Arctic sea ice minimum — at 4.60 million km2 on September 19, the third lowest extent in the satellite record. However extent (defined here) doesn’t tell you everything about the state of the ice — according to the University of Washington’s PIOMAS ice model 2010 managed to set a new record low for sea ice volume.
In terms of the future of the Arctic sea ice, the volume of ice remaining at minimum is a crucial metric because it represents the size of the heat budget buffer between an ocean with a perennial floating ice cap and one that’s seasonally ice-free. For the Arctic to be ice free in summer, that buffer has to disappear, or become a lot smaller. I’ve been writing about sea ice volume for some time, and considered the overall Arctic heat budget in this post a couple of years ago, so news of the new low volume prompted me to think about what it might mean for the extent metric over the next few years. To do that, I downloaded the NSIDC’s September monthly average extent for the last 21 years, and plotted that against the PIOMAS model’s September average monthly volume (kindly supplied by Jinlun Zhang). Here’s what the data looks like when you plot it on the same chart.
The red line at the bottom, labelled “thickness”, is what you get when you divide volume by extent, and that too has been in decline, reflecting the fact that the loss of volume has been happening faster than the reductions in extent.
“Thickness” is not a real representation of the actual thickness of the ice at minimum — that varies considerably with the age of the ice, older ice being thicker. It’s just a number that depends on the relationship between extent and volume, and that makes it interesting. Here’s what it looks like if you plot it at a finer scale:
In 1990, the sea ice at minimum was 2.2 metres “thick”. In 2010 that had declined to 0.94 metres — reflecting the marked loss in old thick ice over the period. Over the 21 year period, the ice has been thinning at about 4 cm per year, and over the last decade — reflecting the steep recent drop — at 8 cm per year.
For sea ice volume, the PIOMAS numbers show an average drop of 410 km3 per year since 1990, but over the last ten years that loss increases to 740 km3 per year, mainly because of the steep falls in 2007 and 2010. For extent the 21 year trend is a loss of 110,000 km2 per annum, rising to 200,000 km2 per year over the last decade.
So what happens when we project these trends forward? To do this I plotted two graphs, the first assuming that the 21 year trends for volume and “thickness” would continue over the next ten years. From those numbers I calculated the equivalent extent:
By 2020 September average sea ice extent is reduced to 850,000 km2. The following year (not plotted) it reaches a minimum of 90,000 km2. In other words, if the rate of reduction of ice volume continues at the average rate of the last 21 years, the Arctic will be ice free in summer by 2021.
What happens if you plot the same graph, using the current (last decade) trends for volume and “thickness” to project extent? It’s a striking picture, I think you’ll agree:
If these trends continue, then September ice extent dips below 2 million km2 in 2015, before all but disappearing in 2016. Hence the title of this post…
I also looked at what happens if you use the trends in extent and “thickness” to project volume going forward. This results in ice volume reduction at half the rate observed over the last two decades (a quarter of the current rate), and so looks unlikely — although it does postpone an ice-free Arctic summer until 2030/31.
Of course, these projections are very “smooth”, and make no allowance for the year to year variations that are obvious in the records for volume and extent. That “noise” could work to our advantage, and delay the loss of ice. For instance, if volume reductions occur at the rate apparent in 2002-6, then 2020-21 looks possible. On the other hand, just one more year with a drop in volume as large as in 2007 or 2010 could bring the end of the ice before 2015.
Some caveats: the PIOMAS data is model generated, and while regarded by the sea ice community as the best available at the moment, it is subject to uncertainty. The absolute numbers could be wrong, as could the exact trends, but the big picture is all I’m really relying on for these projections. Improved volume numbers should become available as the new Cryosat 2 mission begins to produce data.
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.
My thanks to Jinlun Zhang at the University of Washington for supplying the PIOMAS data. My amateur prognostications should not be taken as endorsed by him or the PIOMAS team, the NSIDC or any respectable sea ice scientist for that matter.