I haven't done any specific analysis on what the right price ought to be for carbon. I do believe it's appropriate that in line with things like forward-looking regulations, it would make sense to increase prices over time, to tell people in advance what they're likely to be. I don't think it's appropriate to set a price for carbon until we understand what damage is likely to be done by a tonne of carbon.
That gets me back to the notion that I'm not sure we understand what these long-term damages are going to be. If my presumptions about the release of methane from the permafrost are real, I'm going to suggest to you that the damages are going to be intense. If IPCC estimates up to 4.5 degrees centigrade in changes--and there's an article in the New Scientist from two weeks ago that indicates that IPCC work did not include some of the commonly discussed material among scientists, including this feedback loop--then we could be looking at global temperature increases substantially above 4.5 degrees by the end of the century. This would have a dramatic impact on things like food production, fish production, rising sea levels, and so forth within the lifetime of my grandchildren.
Until we know the answer to that, I don't know how we price the value of getting rid of a tonne of carbon today.