Once allowance has been made not only for the IPCC’s reduction of the feedback sum f from 2.05 to 1.5 W m–2 K–1 and the application of Eq. (1) to the relationship between TS and FS but also for the probability that f is not strongly positive, for the possibility that a 50% increase in CO2 concentration is all that can occur before fossil-fuel exhaustion, for the IPCC’s estimate that only half of equilibrium sensitivity will occur within the century after the CO2 increase, and for the fact that the CO2 increase will not be complete until the end of this century, it is difficult, and arguably impossible, to maintain that Man can cause a dangerous warming of the planet by 2100.
Indeed, even one ignores all of the considerations in the above paragraph except the first, the IPCC’s implicit central estimate of global warming this century would amount to only 1.1 K, just within the arbitrary 2-K-since-1750 limit, and any remaining warming would come through so slowly as to be harmless. It is no longer legitimate – if ever it was – to maintain that there is any need to fear runaway warming.
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