model is a good strategy for reducing uncertainty about how fast sea ice might be lost during a climate change, even if it does not reduce uncertainty about the accompanying global-mean warming. The committee’s strategy for climate modeling in the United States is intended to facilitate these advances and improve the understanding of the uncertainties in climate model projections (Chapter 14).

Although improvements in uncertainty characterization and quantification will proceed, particularly in the context of various kinds of climate model ensembles, it is less clear that convincing means of combining known qualitative (i.e., structural) uncertainties with these quantitative methods will be developed. Moreover, while much attention has been paid recently to developing means of differentially weighting different ensemble methods, we are not yet at a point where a consensus on how to proceed has been reached. Obviously limits to predictability constrain reduction in uncertainty, a possible issue for decadal forecasting. A probabilistic framework, rather than methods used in deterministic prediction, better characterizes uncertainty. Work on better characterizing uncertainty will need to be done on an ongoing basis. The committee suggests that a working group in the proposed annual climate modeling forum would be an appropriate venue to explore these issues (see Chapter 13).

Recommendation 6.1: Uncertainty is a significant aspect of climate modeling and should be properly addressed by the climate modeling community. To facilitate this, the United States should more vigorously support research on uncertainty, including

•  understanding and quantifying uncertainty in the projection of future climate change, including how best to use the current observational record across all time scales;

•  incorporating uncertainty characterization and quantification more fully in the climate modeling process;

•  communicating uncertainty to both users of climate model output and decision makers; and

•  developing deeper understanding on the relationship between uncertainty and decision making so that climate modeling efforts and characterization of uncertainty are better brought in line with the true needs for decision making.

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