Steven Mosher: Sensitivity has nothing to do with C02.
The “sensitivity” that matters in the policy debate is the sensitivity to change in atmospheric CO2 concentration. The TCS that was estimated by Isaac Held was a sensitivity to a doubling of CO2 over a period of 70 years, which would be the result of a continued 1% per year increase over that time.
One assumption that is dubious is that a change in CO2 can be related to a change in forcing (in Watts/m^2) throughout all future climate change. Another is that the sensitivity defined as you defined it will be constant throughout future climate change. Dr. Curry put it this way: Yes, that equation is used like some sort of iron law. What the equation actually means in terms of expectations for future climate change from greenhouse forcing is not what its proponents think it means, wherein lies the problem.
Neither “sensitivity” (wrt CO2 or radiative “forcing”) can be estimated on present data without the assumption that it is constant throughout climate change, and there is no good reason to assume that either is constant.