Boris said,
The GCMs include all known feedbacks. Not sure what you mean by “unprovem.” The WV feedback, as Richard notes, is well founded in theory and the models predicted the correct drying of the stratosphere after the Mt. Pinatubo eruption. so we have strong confidence that the models’ predictions for CO2 warming are good as well.
Unproven as it applies to WV feedback is incorrect. Perhaps it should have been stated that the sensitivity assumptions were unproven. Is there any empirical evidence that CO2 sensitivity in the 21st century will be 3,4,5 times greater than previously observed in the 20th century? What about other feedbacks, such as, PDO, AMO, La Nina, solar winds and solar cycles. These all exist and correlate with, though not prove causation, changes in climate can be affected by many natural factors. These are just as plausible as CO2 causing GW. Are any of these natural variation used in the GCM’S and if so, what do the models say their affect is? How well do GCM’s factor cloud cover into their predictions? Do we know whether cloud cover causes warming or warming causes cloud cover? So many unanswered questions referring to GCM’s
