Another month has passed and so here is the updated Arctic sea ice volume graph as calculated by the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) at the Polar Science Center:
There haven't been any major changes compared to last month, and that's good, because it means things haven't gotten worse (according to PIOMAS). February 2017 saw an increase that was almost identical to the average for the last 10 years. Nevertheless, the difference with number 2, has increased from 1776 to 1851 km3. At the same time the difference with last year (now third in the ranking) has been reduced considerably, from 2374 to 1975 km3.
Here's how the differences with previous years have evolved from last month:
And here's Wipneus' version, showing the gap is still intact, but hasn't become larger:
Of course, the trend line on the PIOMAS sea ice volume anomaly graph is still in two standard deviation territory:
Likewise on the Polar Science Center thickness plot:
And now for the interesting bit. If you follow the PIOMAS website closely, you'll have noticed a new graph that was posted this month. The graph shows a time series of both PIOMAS and CryoSat-2 sea ice volume for January:
As you can see, both modeled data and observations have agreed quite well over the years, but this year something has changed. Whereas according to PIOMAS, sea ice volume is currently lowest on record, CryoSat-2 data shows that volume has actually gone up when compared to last year. I had noticed they were diverging last month already, after a commenter posted comparison maps on the Arctic Sea Ice Forum. An interesting discussion ensued, revolving around the questions: Who has it more right and whence the divergence?
Now, the divergence more or less started in October, but really came to the fore in December. It seems very likely that the Atlantic storms we saw crashing into the Arctic (see this blog post) had something to do with it, via two possible mechanisms. On the one hand through strong winds, pushing the ice away from the Siberian coast, towards Greenland and the Canadian Arctic Archipelago, causing ridging of ice floes and new ice to form in their wake. This would suggest that CryoSat-2 data is a closer representation of reality.
But the other mechanism would be that the storms brought a lot of snow with them, and due to the insulative snow properties of snow, the heat flux through the ice slows down, in turn reducing the rate of ice accretion on the underside of floes. This would explain why CryoSat-2 has sea ice volume going up compared to last year, because the satellite's radio altimeter has problems determining freeboard when there's snow on top of ice floes. To quote from Alfred Wegener Institute scientist Robert Ricker's PHD thesis paper: Recent studies show that the influence of the snow cover is not negligible and can highly affect the CryoSat-2 range measurements.I posted more quotes from that paper on the ASIF, in the PIOMAS vs CryoSat thread.
The thing that makes me tend to think that PIOMAS may have it more right than CryoSat-2 besides the probability of increased snowfall, is the fact that those storms (and other factors) brought in a lot of anomalous heat this winter, as evidenced by the CryosphereComputing Freezing Degree Days graph (also check out Zack Labe's Arctic Temperatures page):
Of course, another source of this heat is probably the Arctic Ocean itself, given that its surface was running extremely hot at the end of last year's spectacular melting season. Unfortunately it's very difficult to quantify all these different factors - snowfall, ridging, ocean heat flux (not just at the surface) - because we simply don't have the means to observe them.
If snowfall is the dominant factor that has caused PIOMAS and CryoSat-2 to diverge so much, it spells bad news for the Arctic sea ice pack. Not just because the ice has thickened less due to the snow, but also because snow has a lower albedo than ice, which means melt onset could start earlier under certain weather conditions (mostly cloudy, as the Mortin et al. paper showed last year).
But perhaps CryoSat has it more right, after all, or both have it somewhat 'wrong' and the truth lies somewhere in the middle. That still wouldn't be good, but better than how things currently look according to PIOMAS. Hope springs eternal.