The alleged 'social cost' of carbon

"[A] recent article in 'Nature' magazine ... claims to calculate how much worse off humans will be for each additional ton of CO2 released. The costs are summed over a period of almost three hundred years, from now to 2300.... [A]bout two thirds of their social cost of carbon is incurred after 2100.
    "This raises serious problems. To begin with, CO2 output as a function of GNP depends on the technology for producing power. An order of magnitude reduction in the cost of either nuclear power or storage would almost entirely eliminate the use of fossil fuels, as would the development of cheap fusion power, either of which could happen in the next fifty years. That makes any estimate of CO2 output over the next three centuries a guess about unknowable technological change.
    "Almost all of the article’s estimated cost of carbon is from either increased mortality or reduced agricultural output. Mortality from increased temperature depends on medical technology, home insulation and cooling technology, and probably other technologies. Agricultural yields depend on agricultural technologies. We have no way of predicting those effects. 

    "How does the article deal with technological change? As best I could tell, it ignores it...."

~ David Friedman, from his article 'Inflating the Cost of Carbon' [hat tip David P. Henderson]