The climate sensitivity value of 3°C ± 1.5°C that was calculated in 1979 by Jule Charney.
Why is this value politically important?
It is important to the climate debate because it became one of the key metrics used to justify policy recommendations made by the first three IPCC reports - and this is problematic because it exemplifies one of the problems that make AGW such a hard sell.
Why does it bother skeptics?
When Charney created his range, the state of climate modeling was such that just two authoritative values for climate sensitivity were available:
(1) Syukuro Manabe's which showed 2°C and
(2) James Hansen's which showed 4°C.
According to Richard Kerr in Science Magazine: "Charney chose 0.5°C as a not-unreasonable margin of error, subtracted it from Manabe's number, and added it to Hansen's. Thus was born the 1.5°C-to-4.5°C range of likely climate sensitivity that has appeared in every greenhouse assessment since..."
Now this is a legitimate example of how science moves towards the truth. You approximate and then increase the precision of your approximation as research accumulates, your information gets better, and the models become more finely tuned.
However, it is difficult to convince people to voluntarily shut down, say, the coal industry and give up their livelihoods based on this method of reasoning.
1. The source for the Manabe anecdote is an article by Kerr CLIMATE CHANGE: Three Degrees of Consensus. Click here to read it.
2. Charney sensitivity assumes that land surface, ice sheets, and atmospheric compositions (chemistry and aerosols) stay the same, i.e., that there are no feedbacks. This is in contrast to James Hansen's long term sensitivity which includes these feedbacks.