Positive externalities is a bit of an odd way to phrase it—if it’s just counting up the economic value (i.e. price) of the fossil fuels, doesn’t it also disregard the consumer surplus? In other words, they’ve demonstrated that the negative externalities of pollution outweigh the value added on the margin, but if we were to radically decrease our usage of fossil fuels then the cost of energy (especially for certain uses with no good substitute, as you discussed above) would go way up, and the tradeoff on the margin would look very different.
Yes, the statement that switching off coal-fired power plants etc. is only true at the margin. However, for the OP’s question, it seems that the sign of “marginal social benefit—marginal social cost” seems crucial.
Positive externalities is a bit of an odd way to phrase it—if it’s just counting up the economic value (i.e. price) of the fossil fuels, doesn’t it also disregard the consumer surplus? In other words, they’ve demonstrated that the negative externalities of pollution outweigh the value added on the margin, but if we were to radically decrease our usage of fossil fuels then the cost of energy (especially for certain uses with no good substitute, as you discussed above) would go way up, and the tradeoff on the margin would look very different.
Yes, the statement that switching off coal-fired power plants etc. is only true at the margin. However, for the OP’s question, it seems that the sign of “marginal social benefit—marginal social cost” seems crucial.