Quantifying the Uncertainties in an Ensemble of Decadal Climate Predictions

Meaningful climate predictions must be accompanied by their corresponding range of uncertainty. Quantifying the uncertainties is nontrivial, and different methods have been suggested and used in the past. We propose a method that does not rely on any assumptions regarding the distribution of the ensemble member predictions. The method is tested using the Coupled Model Intercomparison Project Phase 5 1981-2010 decadal predictions and is shown to perform better than two other methods considered.

In the figure we show reliability diagrams for the different estimation methods of the surface temperature uncertainty range. The observed frequency represents the spatial average over all the grid cells. (left column) The reliability for the equally weighted ensemble. (right column) The reliability for the EGA forecaster. (top row) The reliability of the Gaussian method. (middle row) The reliability of the RMSE-corrected method. (bottom row) The reliability of the asymmetric method.

The improved estimate of the uncertainties is of great importance both for practical use and for better assessing the significance of the effects seen in theoretical studies.