I agree that we're dealing with symptoms,without a 'total knowledge' of how the  "patient's" system operates, which clouds predictive ability. This lack of 'total knowledge' has never stopped medical science( or any other branch of science for that matter) from advancing. 

When the whole of mankind's future may hang in the balance, it behooves a society ( if rational and sane ) to err on the side of caution( as in let's not make the patient's problem worse) based on the best science we have. Waiting for a 'perfect plan' is a luxury that mankind really does not have. We have no way of knowing if it's "too late" or not.  If it is(too late), then what have we lost by reducing ( rather than increasing ) GHGs? If it isn't then the gain should be obvious.

 I guess the choice is ring up 30 knots of speed, don't spare the coal and damn the icebergs ahead to a likely certain end, or give future generations a fighting chance. Our new skipper doesn't think 30 knots is fast enough. Sorry kids.