How much Warming does Peak at 450 ppm Produce?

John Quiggin at Crooked Timber:

“the world is likely to achieve stabilization of greenhouse gas concentrations around 450 parts per million by 2050, and reduce that to 350 ppm by 2100. …

"Should this have been a surprise? It has been for me. As an economist, I'd have thought an outcome like this would have required a global commitment to an emissions trading scheme with a carbon price on a rising trajectory to $US100/tonne or so. In fact, we've seen nothing of the kind. There has been no real global co-ordination, and where carbon prices have been imposed, they have been low and limited in scope."

“Instead, we've had a series of favorable technological surprises of which the most striking have been the plummeting cost of solar photovoltaics, and advances in battery technology allowing both low-cost electricity storage and affordable electric vehicles. … But maybe I shouldn't have been surprised. There were a lot of potential technological options out there, and we only needed a couple to work. … On the other hand, there's no obvious technical reason why we couldn't have developed most of these technologies decades ago. ... 

"A question I haven't yet been able to find a good answer on is: how much warming would a trajectory peaking at 450 ppm and declining to 350 ppm ultimately produce?"