IPCC Reductions in Carbon optimistic at best and unachievable at worst

This discussion doesn't even involved any cost-benefit calculations, just whether these scientists in Nature think that the reductions in Carbon emissions are technically feasible.

Here we show that two-thirds or more of all the energy efficiency improvements and decarbonization of energy supply required to stabilize greenhouse gases is already built into the IPCC reference scenarios. This is because the scenarios assume a certain amount of spontaneous technological change and related decarbonization. Thus, the IPCC implicitly assumes that the bulk of the challenge of reducing future emissions will occur in the absence of climate policies. We believe that these assumptions are optimistic at best and unachievable at worst, potentially seriously underestimating the scale of the technological challenge associated with stabilizing greenhouse-gas concentrations. . . .

The lack of notions of costs and benefits or incentives is troubling, but within their own model I suppose that this is still useful.