Lately Alaska has been feeling the heat, particularly in the part of it you’d least expect: the northern part. As many have noted, places like Deadhorse have had heat-wave conditions despite being within 50 mi. of the Arctic ocean, and old records haven’t just been broken they’ve been smashed. It’s not just last week, the whole year (so far) has been unusually warm in our northernmost state.
A big contributor to the rapidity of changes in the Arctic has been the disappearance of Arctic sea ice. Ice is highly reflective, sending incoming sunlight back to space, but when sea ice melts it uncovers the ocean beneath, which absorbs that sunlight, increasing our planet’s energy imbalance. This also affects the flow of air, bringing about changes in the very nature of how air masses move about — changes which are implicated in the shifting patterns and persistence of heat waves, storms, and particularly rainfall.
Yes, we really are changing the fundamentals of weather, in ways that the victims of recent floods and heat waves can tell you, we already don’t like. Even greater changes are coming, and we’re gonna hate those even more.
The latest figures on Arctic sea ice are not encouraging. After a record low extent in May …
… we’ve had another record low in June …
It’s all part of the trend, which has been decline — not just of the annual minimum (usually in September) but year-round.
If we estimate the seasonal cycle in the usual way, and subtract it from the extent to compute sea ice anomaly, we see interesting behavior recently:
Since about 2007 it seems to be showing wild fluctuations. But that’s a false impression proceeding from the heat-oppressed Arctic; it’s really due to the fact that the seasonal cycle itself changed (it got bigger), so subtracting the old seasonal cycle still leaves part of the new seasonal cycle in place.
One way to ameliorate that effect, in order to focus on the trend in mean rather than the trend in seasonal cycle, is to compute an “adaptive anomaly” — one for which the seasonal cycle is allowed to exhibit changes. I’ve done so, and came up with this “adaptive” seasonal cycle:
Removing that from the extent data gives us a better (or at least, more trend-focused) anomaly calculation:
Not only is the decline visible, the continued decline is plain. But those in denial about man-made climate change and its dangers — let’s call ’em “deniers” — will still crow about how Arctic sea ice is in “recovery.” It’s a clear case of “those who will not see.”
If we want to find departures from simple linear decline, a good way to get clues is to smooth the anomalies. Here are two ways, one complex (a modified lowess smooth) and one not so complex (a Gaussian smooth), together with the error ranges which both return (for which I’ve included the effect of autocorrelation):
They’re in broad agreement, giving a visual impression of steady decline until about 2002, followed by faster decline until about 2008, followed by a return to the previous rate of decline.
To know whether or not those indicated changes are real or just the impression given by random noise, a good way is to apply changepoint analysis. That confirms that these changes are indeed real, and gives the following “continuous piecewise linear” model of the anomalies:
We can also compare the smoothed values (in red and blue) to the piecewise linear model (from changepoint analysis) (in brown):
This particular graph shows just about all we can be confident in. The data are noisy enough that those are the only changes we can assert with genuine statistical justification. Yes there was a speedup of ice loss from about 2002 through 2006, after which the rate of decline is indistinguishable from its previous rate.
But that won’t stop deniers from denying. The “Arctic sea ice recovery” idea has been pushed many many times — it just hasn’t yet been true. Perhaps the most hilarious aspect of the “recovery” idiocy is the fact that if we take the data prior to the change in slope (which changepoint analysis puts at late 2001) and use it to forecast what followed by its linear trend, we get this:
Arctic sea ice was already in dramatic decline, but since its most recent trend change it has been consistently lower that would have been expected, even accounting for the already-serious declining trend. Some recovery!
For those unfamiliar with the way deniers deny, let me tell you what’s behind their “recovery” nonsense. By finding a small enough time span, ingoring all context, and seeing only what they want to see, they can make a graph which they think shows “recovery.” It’s totally misleading, so to them it’s salvation, because truth is against them, so they must deny it.
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