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BT,
Your question is a complex one and the subject of an entire IPCC report involving years of research from some of the world's best think tanks.
At the end of the reckoning the EV equation is very solidly in the realm of there being a significant net damage to economies, and mitigation being +EV overall. See here for example (I strongly encourage anyone to read the actual reports if they're interested in the topic):
http://en.wikipedia.org/wiki/Economi...global_warming
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Then it should be easy for you to put out a number with a margin of error. So what is the number?