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Computer models and uncertainty in predictions of sea ice loss

Computer models predict that by 2035, the Arctic could be nearly sea ice free in summer
  Figure 1. Model projections of sea ice thickness when the Arctic is nearly ice free in September, within 30 years. Units for sea ice thickness are meters. Figure from Wang and Overland1

About computer models: Computer models are combined with observations of sea ice coverage to yield predictions of the number of years for Arctic sea ice extent to drop from current values to nearly ice free in summer (Figure 1). Although there is considerable variability in the model predictions, many computer models show an accelerating decline in the summer minimum sea ice extent during the 21st century. Confidence that climate models provide credible quantitative projections of future climate is build upon their demonstrated ability to reproduce observed features of recent climate.

About the computer model predictions: Six of the 23 IPCC Arctic models, which most faithfully reproduced observed features of the recent climate, were run using the 2007-2008 September ice extents as a starting point for projections into the future. Based on the predictions from these six models selected for their ability to simulate current conditions of ice extent, an Arctic free of sea ice in September may occur within 30 years (as early as the late 2020's).1

Uncertainty in model predictions: The six computer models selected for their ability to simulate current ice extent were run with two different medium to high future emission scenarios. There is also variability between different runs of the same model due to natural variability (think about variation in the weather from week to week). Another source of uncertainty of model predictions is due to the difference between the models.The uncertainty in computer model predictions in the timing of a nearly summer sea ice free Arctic is largely due to within-model contributions from natural variability and between-model differences. The projected number of years computer models project for sea ice extent to decline from present conditions to a summer ice free Arctic has an average of 30 years with a first quartile distribution of 9 years (Figure 2).1

Number of years estimated for sea ice extent to decline from current value to nearly summer ice free Arctic.  Estimated by six IPCC models which most faithfully reproduced observed features of the recent climate.
Figure 2. Vertical bars indicate estimated number of years for sea ice extent to decline from the current value to a summer ice free Arctic. Runs by each of the six IPCC models are shown in the six colored boxes. Figure from Wang and Overland1

Each vertical bar indicates the estimated number of years for sea ice extent to decline from the current value to a summer ice free Arctic. For example the first gray bar on the left in the red box is a computer run with the CCSM3 IPCC model that predicted about 27 years until a summer ice-free Arctic.

Each colored box encloses runs made with one of the six IPCC computer models selected for faithful reproduction of recent climate. The height of the vertical bars inside a colored box indicates the variability in the prediction of the computer model due to natural variability and different IPCC anthropogenic emission scenarios.

References

1 Wang, M., and J.E. Overland (2009): A sea ice free summer Arctic within 30 years? Geophys. Res. Lett., 36, L07502, doi: 10.1029/2009GL037820.