Climate prediction is based on two basically different methods, both of which claim to be "empirical."So the IPCC model allows lambda, term for climate sensitivity, to remain fixed with respect to all 'feedback' over the course of the doubling of atmospheric CO2 (dT_surface) They choose this metering method by deriving the feedback component from one set of data points? What is the justification for the approximation? The prediction that variability is minuscule and will have no meaningful effect on the measurement results? Do you feel the 'unlikely less than' 1.5C prediction for the IPCC model is a result of the prediction based on one set of data points as compared to the 'unlikely <' 2.6C prediction of other models mentioned in the wiki climate sensitivity page? I could have been interested in this science if the present science wasn't so depressing. Thanks for trying to exercise intellectual honesty in the face of being the bearer of ill tidings. For me it's not interesting to have disagreements on data when it's all bad news. My greatest fear is the one rpenner wrote about. We will wait until it's to late and in the meantime let the oligarchs make all the calls.
One has very detailed, highly specialized, investigations of the many known aspects (for example, rate of water vapor condensation on aerosols in the troposphere vs the stratospheric as function of relative humidity), by many experts followed by some model that integrates all this data it can to project what (with different probability given, of course) will happen. IPCC's reports issued at half decade intervals are most important of this group but are unable to predict correctly (as linerarized)* more than 5 years into the future. So the whole process is repeated about every 5 or 6 years to correct the prior errors. AFAIK not one of their predictions remained valid a decade after it was first made.
The other does not pretend to understand or even to know all the important aspects but looks at the ice core and other records relating to ancient climatic changes, especially the more rapid ones, and then fits the data with relatively simple math terms of its model and keeps adjusting the model's parameters until they give accurate representation of a sub set of the ancient records. Then they test their model on some other sub set (different period) of the ancient data. If it survives that test, they use it to predict what will happen in the future. The simple (two terms) model of graph at end claims to have only 12% error in its "prediction test" and did predict the turn down in temperature rise that seems to be starting now some years ago, in disagreement with the IPCC's prediction of continued rise.
I strongly recommend you take 5 minutes, to watch this "Climate Cheetah" video - it is informative and entertaining:
http://www.youtube.com/watch?v=RbY9Oyi2VW0&list=PLIbt-C7z11l1ZZqsQYTxCtbrdawW9wGdQ&index=2
It answers your questions better than I can.
* And nations must sign off (and some even reword sections before doing so) all IPCC reports. IPCC reports are not scientific, but a political compromise agreeable to the oil industry and 195 nations who MUST approve release of the report.
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