No, if you had read the source I linked you'd find out that there are decent models that put firm prices on CO2 based on assumptions of how quickly you want to decarbonize the economy. The assumed externality is the need to decarbonize and thus the cost of achieving this of course scales on the time horizon you want to achieve this in and your assumptions on technical developments in the meantime. Good models usually come with some complexity of this sort.

So what is the positive externality of emitting CO2?

sorry, wrong thread.