Thank you for your feedback!  Yes, predictions are difficult (especially about the future), but when we talk about predictions in future decades, we need to keep in mind the difference in our "initial value" predictions (like predicting ENSO based on the current conditions and the physical equations that use those conditions, as discussed in this post) and projections based on changes in radiative forcing like greenhouse gases.  Both factors can be important for decadal and multi-decadal predictions, but radiative forcing changes (like increases in greenhouse gases) become more important as we move farther ahead in time.  That means that we cannot predict details of the climate variations many decades ahead (the initial value part of the prediction) but we still have the potential to predict average changes in climate due to the changes in radiative forcing. 

Please feel free to cite the URL of this blog.  Thanks again!