Arctic Sea Ice decline in the 21st Century

As illustrated in Fig 1, the sea ice retreat accelerates during the 21st century as the ice decays and more sunlight is absorbed by the ocean (the positive ice-albedo feedback). Increasing ocean heat transport under the sea ice adds to the melt back. The retreat appears abrupt when natural variability in the ocean heat transport into the Arctic Ocean is anomalously high. We did not find clear evidence of a threshold, which can be difficult to identify given the variability and complexity of the climate system. Therefore we can neither verify or rule-out the existence of a tipping point. Regardless, the rapid declines seen in our runs are a serious concern.

Figure 2: Northern Hemisphere sea ice extent in September for all seven integration of the CCSM3 with observations from satellite era shown in black.

Most common questions asked by journalists

1) How does our model compare with the trend in the observed record?

The trends in the seven ensemble members for 1979-2006 span the trend in the observations: Some members retreat a little faster and some a little slower, as expected from the random natural variability in the runs (see Fig 2). The model also reproduces the mean and variance of the observations with good fidelity.

2) Other scientists are predicting an ice-free Arctic in September by the year 2060-2080, why is this model predicting it 20-40 years sooner?

First consider estimates based on extrapolation from the observational record. I’ve heard these numbers quoted in the media, but I have not seen a reference to a scientific paper that discusses the analysis in any detail. Figures 2 and 3 illustrate the danger of making an estimate of the future from the observational period. The future trend is not linear, the observational record is too short and the ice-free time is too far in the future to trust extrapolation. If one carries out such an exercise anyway, extrapolation from a linear fit to 1979–2006 gives a zero intersect (indicating the first ice-free year in the future) at about 2110 (see Fig 3). If instead one uses just the last decade, the extrapolation gives 2060. Both estimates are questionable, and so instead we turn to climate models.

Figure 3: Extrapolating into the future from the observational record.

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.

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