The Arctic has seen rapid sea-ice decline in the past three decades, whilst warming at about twice the global average rate. Yet the relationship between Arctic warming and sea-ice loss is not well understood. Here, we present evidence that trends in summertime atmospheric circulation may have contributed as much as 60% to the September sea-ice extent decline since 1979. A tendency towards a stronger anticyclonic circulation over Greenland and the Arctic Ocean with a barotropic structure blah blah you’re not reading this bit are you in the troposphere increased the downwelling longwave radiation above the ice by warming and moistening the lower troposphere. Model experiments, with reanalysis data constraining atmospheric circulation, replicate the observed thermodynamic response and indicate that the near-surface changes are dominated by circulation changes rather than feedbacks from the changing sea-ice cover. Internal variability dominates the Arctic summer circulation trend and may be responsible for about 30–50% of the overall decline in September sea ice since 1979.
In case you’re completely asleep, I’ve bolded the bit of general interest. Can it really be true that our beloved Arctic sea ice decline is no more than natural variability? Well of course they aren’t saying that; but they are saying that a substantial fraction of it might be. Are we really going to sit still for that? Of course not, I’m going to quibble, but don’t get too excited; this isn’t a Curry paper, it isn’t fatally flawed. Should you believe it? If you’re a septic no you certainly shouldn’t, because this paper is totally reliant on models for it’s conclusions, and we all know that a good septic doesn’t trust models, only hard data. Carbon Brief has a go at spinning it as Humans causing up to two-thirds of Arctic summer sea ice loss, study confirms but I’m not sure that’s entirely satisfactory.
But enough of that. What does it say? Weeell an awful lot of it is about the connection between September sea-ice extent and the preceding summer (June–July–August, JJA) atmospheric circulation. We choose thisp receding 3-month window because sea-ice extent anomalies have a ∼3-month decorrelation timescale and the reference for that is Blanchard-Wrigglesworth et al., one of the authors. Again, without being able to pick any obvious holes I feel somewhat uncomfortable with that; the idea that September ice depends just on JJA circulation doesn’t feel at all right. Having decided that, though, they then run a variety of model experiments, for example “nudging” the circulation back to re-analysis, with and without an ocean-ice model underneath. And the result seems to be that it is mostly the circulation forcing the sea ice, rather than the sea ice changes forcing the atmosphere. This kinda-fits the “information flow” meme from way back so I should be prepared to accept that mostly. having done that they then convince themselves that most of the circulation changes that matter to the ice are not GW forced, and so must be natural variability; and hence the conclusion. If you took all of this at face value then they’d have solved one of the puzzles, that on the whole models show much less ice decline that reality. But of course if the decline is substantially a freak of variation, not forced, that would fit.
The flaw in this overall, without looking at the details, is that it’s hard to see a near-40-year trend and being so much natural variability. That seems to be asking for an awful lot of one-way variation.