Zeke Hausfather weighs into the climate sensitivity argument at the Yale Climate Forum (crossposted at the AGU). Like Skeptical Science before him, he chooses to base his analysis on a 2008 review by Knutti and Hegerl.
So what about climate sensitivity? We are left going back to the IPCC synthesis, that it is “likely” between 2 C and 4.5 C per doubling of CO2 concentrations, and “very likely” more than 1.5 C. While different researchers have different best estimates (James Annan, for example, says his best estimate is 2.5 C), uncertainties still mean that estimates cannot be narrowed down to a far narrower and more precise range.
The problem with this of course is that the science has moved on since Knutti and Hegerl and we now have all the papers like Aldrin et al and Ring et al that find much lower sensitivities than the IPCC range. Add in the warm bias from use of uniform priors and the old IPCC range of 2-4.5 is untenable.
When Hausfather moves on to policy he is equally shaky ground:
Ultimately, from the perspective of policy makers and the general public, the impacts of climate change and the required mitigation and adaptation efforts are largely the same in a world of 2 or 4 C per doubling of CO2 concentrations where carbon dioxide emissions are rising quickly.
The problem is that climate sensitivity of two gives you a cost of carbon that is much too low to justify most of the mitigation measures currently being deployed. Chris Hope put the cost of carbon for 1.6-degrees climate sensitivity at around $18/ton. The cost of mitigating emissions through wind power is more like $100. Even if you can justify wind with a climate sensitivity of 4, you will struggle to do so for 2.