FallibleFiend - Don't you think it's interesting that the Scientific Basis (which was written by the actual researchers and scientists) did not have the term "exact" in that sentence, but yet the synthesis report (which was a summary of the full technical report, and was pulled together by bureaucrats) did include the word "exact".
It is a very significant word, and one may ask the question why was it included in the synthesis report, but not the original technical report. Perhaps the bureaucrats knew something the researchers didn't.
What the text surrounding that quote makes clear, is that the climate system is too complex, too non-linear, too bloody chaotic to accurately model it, and predict into the future. The non-linear nature of most(all?) of the climate processes, mean that relationships, which have been parameterized based on current climate conditions, will likely not hold true in the future. Without knowing how these relationships will interact in a changed climate, you can't predict forward.
A perfect example is convection. Convection is the primary method of heat transport from the lower troposphere to the stratosphere. In a troposphere with more energy (as is the case in a warmer climate), there will be increased convection (this shouldn't be a debatable point). How much more though? Is it a linear increase? logarithmic? exponential? Anybody who says they know the answer is lying. So if you don't know how the main heat transport engine will react to a warmer surface, how can you actually predict how much warmer it will be?
The tripe about developing model ensembles to develop probability distributions is laughable at best. This is analogous to taking 20 people, giving them a gun and tell them to shoot at an invisible target - that's over there.....somewhere. Do you expect any of them to hit this invisible target? Of course not.......taking the IPCC approach, you'd take the "average" of all 20 shots, and say "here's our probability distribution of where that blinkered target is - and look, it's in that general area that we thought it was! I told you!!!!!".
As long as models are developed based on incomplete knowledge of our current (and future) climate processes, all our model simulations are nothing more than shots in the dark. But hey, we can always add some "flux adjustments" to make the models give us the answers we want - right?
I'll be eagerly awaiting your response as to why the Synthesis Report included the word "exact" whereas the technical report did not. I'm sure there must be a good reason.
btw - yes, I do agree that humans have impacted the global carbon cycle. The 500,000,000,000,000 dollar question is "does it matter?".
