Not much, if at all, argues Jim Manzi at The American Scene:

The total net expected benefits of the best-imaginable program to combat global warming are about $3.4 trillion. This is about 0.17% of the expected present value of total global income.

Compare the current Waxman-Markey bill to “an optimally-designed and optimally-implemented” carbon tax. Consider what it would be like, in the real world, to cut a global deal. Now consider what it would be like to enforce this for more than a century, not only in Sweden, Japan and Australia, but in China, India and Brazil. Compare this to “a globally-harmonized, optimally-designed and optimally-executed emissions mitigation program.” Do you think the economic drag the real-world deal would create might cause the planet to lose more than 0.17% (or for that matter, 1.7%) of the present value of future income as compared to the case without such a deal? It is extremely likely, in my view.