Be ye fairly warned, all; the following is a wall of text of dubious interest to the general populace and of even more dubious scientific merit. Im very hesitant to post this, because its based on that most pernicious of oxymorons, blog science. The analysis seems sound, though, and (more critically) Im not able to find anything in the literature addressing this exact point (there are plenty of papers on the negative growth feedback, but papers dealing with specific volume thresholds seem rarer). I also think it might be rather important, if it turns out to be true. I half suspect sea ice researchers have been aware of this for decades and that its just been communicated poorly outside of the field, but...well, either way, up it goes. Anyway. To my point. Over the past ten to fifteen years, theres been something of a divide in the sea ice community about the approximate date that the Arctic will become ice free in summer. Studies based on modeling of Arctic sea ice have suggested that the Arctic will become ice free in summer in 2080-2100 or (more recently, with the updated CMIP5 ensemble of models) 2050-2080 (cesm.ucar.edu/working_groups/Polar/presentations/2012/stroeve.pdf). Observations, however, paint a very different story, with volume declines in particular pointing towards possible ice free conditions by 2016-2020 (https://14adebb0-a-62cb3a1a-s-sites.googlegroups/site/arctischepinguin/home/piomas/grf/piomas-trnd1.png?attachauth=ANoY7cotYnwjXCeWZvFqE_uYQWRQiO0VCq2-d4ZenyuyoIJdS__-pcf16mBEkEpQ-0NMvvrvgeaFbaQs0Pwz6RIg0f6T8CEqJuskVbLLgM1U28cTt7z3ufxpbgq8FFsQ0MWSa74ij5DFBj3hM7Pav2TzSY6bB1dZcbKUyCezRVKfDZ1L3fIZEMO6Tr7N8ppLvUjW2LGkOk04yh-YbnvvmNhDxMuj9-N_eJ_VDSWei-3hQmulqb-b61a39nE4XwxktTHNLHR181DU&attredirects=0 Graph courtesy of Wipneus, on Neven’s Arctic Sea Ice Blog). This volume decline has been driven both by increasing melt and (somewhat surprisingly, at least to me) even more rapidly decreasing maximum volumes, as seen in the following graph (iwantsomeproof/extimg/siv_annual_max_loss_and_ice_remaining.png Courtesy of Jim Pettit, also of Neven’s Arctic Sea Ice Blog. This was made in 2012, and thus does not include 2013 and 2014, both of which had/will almost certainly have somewhat higher volumes preserved than of late. The general picture’s the same, though). Total meltout will occur when the two curves on that graph intersect--that is, when seasonal melt is equal to volume at the start of the melt season. Naively, looking at that graph one would suppose that ice free conditions must inevitably come in the course of just a few more years, at most, and that’s certainly what I supposed. It was hard for me to see what could stop the decline--and certainly difficult to see what could not only stop it, but maintain a plateau so close to the edge for decades on end while the world continued to warm It may be, though, that we’re going to get a reprieve--although I still doubt that it’ll be a multidecadal one, as the models posit. Chris Reynolds of the Dosbat blog recently put up a very thought-provoking post arguing that the current decline in maximum thickness following winter is about to hit a plateau, and after thinking on the matter for a while, I find myself agreeing with him. His argument is based on the fact that first year ice--that is, new ice that was produced over the course of a single winter, and hasn’t survived multiple summers--always grows to ~2 m thick following the winter freeze. This value has declined slightly in recent years as winters have grown milder in the Arctic, but not by very much; most of the decline in Arctic volume at the maximum has been driven by the destruction of thicker multiyear ice. Of course, in a given melt season, both first year ice and multiyear ice will be destroyed--but the multiyear ice takes, well, multiple years to be replenished, while the first year ice grows back in a single season. Over time, the multiyear ice dwindles as it’s destroyed faster than it can be replaced, while the thinner first year ice grows more plentiful. However, we’re reaching a point where the Arctic sea ice is composed of almost nothing BUT first year ice; the multiyear ice has been dwindling so much, and the melts lately have been so aggressive, that it exists only as a small fraction of the total ice volume. What Chris Reynolds observed was that this puts a fundamental restriction on any further volume loss--because no matter how much first year ice is lost in a given melt season, it will ALL be replenished during the following winter, unlike multiyear ice. We may, in short, be about to hit bottom, only to find that, very fortunately, “bottom” is still just a little bit higher than the amount of melt that can take place in a given year. ...Of course, the melt each year is growing more and more aggressive, and the thickness of first year ice IS steadily decreasing as Arctic winters grow warmer, just at a slow pace. This isn’t a multiple decade reprieve; even assuming the thinning of the ice stopped dead now, at the current rate of melt increase we’d still be facing ice free conditions in summer in the late 2020s or the early 2030s. And the thinning won’t stop dead; as the globe continues to warm, it will continue apace. Chris Reynolds estimates a likely point of intersection of volume after winter and volume melted during summer of ~2025, which is only about five to ten years after an estimate based on a naive extrapolation of the volume trends--but still, every little bit added to what you’ve got is a little bit more. And, of course, I have to bear in mind the possibility that I--and the various researchers arguing for an early meltout, like Peter Wadhams and Wieslaw Maslowski--am wrong about the whole thing, and that the modelers are right and these last few years were just a bizarre fluke. There’s so precious little ice left that it’s a little hard for me to believe that, but nonetheless it’s important for me to keep that in mind. ...So, why bring all this up in the first place? It’s very interesting to me, really, and I thought I’d provide an update on my current thinking on the subject, for anyone who’s interested. My anticipation of the possibility of meltout within just a few years was motivating some of my planning for the future, and I haven’t discounted that possibility just yet, by any means--but this does seem to hint that I may have a little more breathing space than I had been afraid of.
Posted on: Mon, 28 Jul 2014 00:22:42 +0000
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