Uncertainty in the sensitivity of Arctic sea ice to global warming in a perturbed parameter climate model ensemble

Abstract: The retreat of Arctic sea ice is a very likely consequence of climate change and part of a key feedback process, which can accelerate global warming. The uncertainty in predictions in the rate of sea ice retreat requires quantification and ultimately reduction via observational constraints. Here we analyse a climate model ensemble with perturbations to parameters in the atmosphere model. We find a large range of the sensitivity of Arctic sea-ice retreat to global temperature change, from 11 to 18% per °C. This is placed in the context of the uncertainty obtained by alternative model ensembles. Reasons for the different sensitivities are explored and we find that differences in the amount of ocean and atmospheric heat transported from low to high latitudes dominates over local radiative contributions to the heat budget. Furthermore, we find no significant relationship between the uncertainty in sea ice response to climate change and climate sensitivity.

An ensemble of AOGCMs (coupled atmospheric ocean global circulation models) were run assuming 1% annually increase levels of CO2 until there is four times pre-industrial levels. From these, the fractional change in sea ice were computed relative to the pre-industrial baseline period. The sea ice changes for just the HadCM3 model are shown above. This reproduces previously work that has shown the Arctic sea ice decreases linearly as global-average temperature rises [Gregory et al., 2002].

A quick calculation shows that the fractional sea ice anomaly will become zero if the global-average temperature rises by 6.9C above pre-industrial levels. Numerous posts around the Internet, such as here, show how the arctic sea ice has declined in recent years. They also show that the sea ice decline is not linear; the ice seems to be melting faster.

I’m too lazy to reproduce a globally-averaged temperature plot, but luckily for you, I already did one (with something else on it too) in a previous post.

Ignore the blue line. The black line is the global average temperature. I forget which temperature I used, probably GISTemp. It doesn’t really matter, they all are extremely similar. But from that plot, we can see that the temperature increase has not been linear. Some may say it even looks like a hockey stick. [Note: the time scales of the sea ice plot at Tamino and the temperature plot above have vastly different time scales. Keep that in mind before trying to directly compare them.]

This figure is reproduced only for the third plot, which is the ocean to ice heat flux. Notice that over small global temperature changes this is approximately linear. As a 3C warming is approached, the relationship suddenly becomes non-linear. I know that there has been much discussion about “tipping points” in the climate system, and I think this is an important one.

They write that “[t]he AR4 ensemble exhibits a SITS [sea ice temperature sensitivity] range from 4% per deg. C to 15% per deg. C whilst the ocean ensemble SITS cover a range from 15 to 22% per deg. C. Thus, while there is a significant degree of overlap, no ensemble alone can capture the full spread (Figure 2) [not reproduced here]. The atmosphere and ocean ensembles both have members that are significantly more sensitive, measured in terms of SITS, than the AR4 ensemble. We do not establish here if one ensemble is more realistic than the others.”

If the sea ice anomaly plots currently features at Tamino are correct, and there is no reason to suspect that they aren’t, then it seems clear that the AR4 models have been underestimating the loss of sea ice. Since it appears that there are now models that do a better job of handling the physics, it would be interesting to see them compared to observations instead of just being compared to other ensembles.

Ridley concludes that “[a]n analysis of the processes causing ice melt in the atmospheric ensemble reveals that the increase in local ocean temperature is the dominate component, with local longwave forcing being the second most important term. A study of the large-scale regional energy budget suggests that much of the extra heat associated with ice melting comes from heat transported into the region from mid-latitudes, rather than local radiative forcing.”

References:
Gregory, J. M., P. A. Stott, D. J. Cresswell, N. A. Rayner, C. Gordon, and D. M. H. Sexton (2002), Recent and future changes in Arctic sea ice simulated by the HadCM3 AOGCM, Geophys. Res. Lett., 29(24), 2175, doi:10.1029/2001GL014575.

Ridley, J., J. Lowe, C. Brierley, and G. Harris (2007), Uncertainty in the sensitivity of Arctic sea ice to global warming in a perturbed parameter climate model ensemble, Geophys. Res. Lett., 34, L19704, doi:10.1029/2007GL031209.

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