Arctic surprise

In 2007, IPCC AR4 produced this figure showing projections of changes in Arctic sea ice extent in the summer (July-September). The different colours represent a wide range of different scenarios for future emissions. Observations (1979-2020, added purple line) have decreased far more rapidly than projected in the CMIP3 models used at the time, when plotted on the same scale with the same reference period.

This demonstrates the concept of a climate-related ‘surprise’, or what might have been considered a low-likelihood event at the time.

The original figure is here. Also see Stroeve et al. (2012) for a CMIP5 comparison (Fig. 2a), and Notz et al. (2020) for a CMIP6 comparison (Fig. 2f).

3 thoughts on “Arctic surprise

  1. That the physics of the extension of the Artic ice cup is not fully mastered and modeled, I am not very surprised. That there are still people to vehemently shout “freedom” when recommended to save on fuel, there I feel totally shocked.
    Well done again, Ed. Keep going … and keep it a smile, it won’t harm !

  2. Hi Ed,
    Nice figure. Rather than a “surprise” could it be due to the models under-representing the amplitude of the low frequency (decadal-like) variability ? We know they struggle in this complex region.

    1. Thanks Eric – yes, I agree some of the trend could be variability related, but you might have hoped that the models at the time would have considered the subsequent observed trend at least as plausible rather than way outside the ensemble spread. But, there were no large ensembles in CMIP3 to sample variability and sea-ice models were far less sophisticated. So, given the model projections available the observations were perhaps a surprise even if part of the cause is variability. Maybe expert judgement would have suggested that the ensemble spread was too narrow?

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