Arctic Sea Ice decline in the 21st CenturyDéclin de la banquise de l’Arctique au 21ème siècle

3) Is sea ice in our model retreating faster than in other models?

Figure 4 shows September ice retreat in 16 models that were archived for the IPCC AR4. The most extreme predictions are from models that have too much or too little sea ice extent compared to observations, so it is important for a model to produce the correct sea ice coverage in the past. Some of the spread is expected from natural variability, but much depends on differing model sensitivity relating to the representation of sea ice, heat transport by the ocean, and cloud cover. It is not possible to identify the most accurate model prediction, although I think it is safe to rule out some of the outliers owing to their poor match to the observations.

About half of the models become ice-free in September during the 21st century. I included one ensemble member from our model, CCSM3, which is in the middle of the pack until about 2020. Our model run retreats faster than most after about 2020, but it isn’t radically different.

There is considerable uncertainty in future model projections, and Figs 2 and 4 illustrate why it would be better not to focus too much on the year 2040, which to our dismay was highly publicized. The more important message from models is that all but a few outliers predict enourmous sea ice retreat this century. At least a few respectable models predict a nearly ice-free Arctic by midcentury, with a retreat that may be punctuated by rapid events.

Figure 4: Northern Hemisphere sea ice extent in September from model integrations submitted to the IPCC AR4 with observations from satellite era shown in black.

4) Is it too late to save the sea ice?

The future emissions scenario discussed here is one that assumes modest increases in emissions. If humans can reduce the rise in emissions compared to this, then sea ice retreat would be slower and rapid events would be rarer, according to the IPCC AR4 models.

5) Have we crossed a tipping point?

I don’t think we have yet. If we fix the greenhouse gas and aerosol levels at year 2000 values and run the model into the 21st century, the sea ice retreats for only another decade or two and then levels off (some of the ensemble members even recover a little bit). So according to our model, the sea ice does not appear to have passed a threshold yet. We have not done an exhaustive study of any years beyond today, so unfortunately we cannot say with certainty that no tipping points exist. The bottom-line: The retreat can be surprisingly rapid even without clear evidence of a tipping point.

I thank Dr. Holland for valuable suggestions to improve this post and providing Fig 1. I thank Ian Eisenman for computing ice extent from the IPCC AR4 models shown in Fig 4. I look forward to reading your comments and questions.

Dans notre publication (avec notre co-auteur Bruno Tremblay), nous avons examiné la couverture de glace de mer de l’Arctique au mois de septembre dans des simulations climatiques des 20ème et 21ème siècles, et observé certaines décennies correspondant à des retraits très rapides. Le cas le plus extrême correspondait à une diminution de 6 à 2 millions de kilomètres carrés en dix ans (voir la Figure 1). Ceci est près de 4 fois plus rapide que le déclin observé au cours de la dernière décennie.

Figure 1 : Extension de la glace de mer dans l’hémisphère nord produite dans une simulation du CCSM3 (Modèle de Système Climatique Communautaire, version 3) (noir) et observée par satellite (rouge). La courbe bleu clair correspond à une moyenne glissante sur 5 ans. Les 3 cartes du bas montrent la concentration de glace de mer en Septembre au cours de trois décennies spécifiques.

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