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Are observations of Arctic sea ice in accord with what the IPCC forecasts says they should be? And as such is the ice reducing in an exponential or linear way?

For instance, from https://www.scientificamerican.com/article/how-the-ipcc-underestimated-climate-change/:

Projection: The IPCC has always confidently projected that the Arctic ice sheet was safe at least until 2050 or well beyond 2100. Reality: Summer ice is thinning faster than every climate projection, and today scientists predict an ice-free Arctic in years, not decades.

And then is this prediction based on the curved line on the left (found here)
or the straight one on the right (found here)
(both are from NSIDC data):

icecomparison

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    $\begingroup$ Just for clarity's sake, "ice sheet" is not the same as "sea ice," which both of those graphs track. Sea ice is usually a meter or two thick and is highly seasonal, growing in local winter and shrinking in local summer. Ice sheets like those on Greenland and Antarctica are up to several kilometers in thickness and, while much harder to measure precisely, are subject to much less variability. $\endgroup$
    – jeffronicus
    Jan 9, 2018 at 5:00
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    $\begingroup$ "And as such is the ice reducing in an exponential or linear way?". The left figure actually shows a logistic decrease. People tend to just put a straight line through data because nearly every data analysis software supports this feature. Doing more than a straight line is to complicated for most users (personal opinion). Logistic decrease sounds realistic. However, the fit does only connect time and ice extent. If we change other conditions (stronger warming) the predicted logistic curve might be as wrong as any other curve. $\endgroup$
    – daniel.heydebreck
    Jan 9, 2018 at 8:11
  • $\begingroup$ OK but these examples aren't layman's guesses they're by scientists. A well constructed model should tell you what kind of a line should be drawn and I believe the problem here is that, according to some, the observations aren't fitting the models, most problematically the models used by the IPCC. So if it's a curve when everyone says it should be a line then there's something wrong with the way physics is being applied in the models, yes ? And that means that things are going awry up north sooner than anticipated ? $\endgroup$
    – Jimmy Widdle
    Jan 9, 2018 at 12:29

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The IPCC said "Although the Hadley Centre climate model underestimates sea-ice extent and thickness, the trends of the two models are similar. Both models predict continued decreases in sea-ice thickness and extent (Vinnikov et al., 1999), so that by 2050, sea-ice extent is reduced to about 80% of area it covered at the mid-20th century." The graph on this IPCC page shows a predicted decline in annual average extent of about half a million km$^2$ from 2000 to now. Your monthly average graph shows a somewhat faster decline over this period.

The SA article talks about the much more drastic decline in minimum (September) extent. enter image description here I can't find any IPCC prediction of minimums, so you could say the IPCC failed to predict it or you could say it's in accord with what the IPCC did predict about annual averages.

Ice volume is also declining rapidly. enter image description here

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  • $\begingroup$ Those quotes from the TAR (AR3) are quite old now, it's probably better to refer to the AR5 (e.g., Tech Summary 5.5.5 or Chapter 12.4.6), although I don't think the bottom line has changed much tbh. It does mention that the models don't simulate the observed decline though. $\endgroup$
    – Deditos
    Jan 10, 2018 at 13:15
  • $\begingroup$ So, if models don't predict the actual decline is the science wrong ? $\endgroup$
    – Jimmy Widdle
    Feb 19, 2018 at 0:23

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