Starting in the 1970s, Nordhaus constructed increasingly comprehensive models of the interaction between the economy and additions of carbon dioxide to the atmosphere, along with its effects on global warming. Economists use these models, along with assumptions about various magnitudes, to compute the “social cost of carbon” (SCC). The idea is that past a certain point, additions of carbon dioxide to the atmosphere heat the earth and thus create a global negative externality. The SCC is the net cost that using that additional carbon imposes on society. While the warmth has some benefits in, for example, causing longer growing seasons and improving recreational alternatives, it also has costs such as raising ocean levels, making some land uses obsolete. The SCC is the net of these social costs and is measured at the current margin. (The “current margin” language is important because otherwise one can get the wrong impression that any use of carbon is harmful.) Nordhaus and others then use the SCC to recommend taxes on carbon. In 2017, Nordhaus computed the optimal tax to be $31 per ton of carbon dioxide. To put that into perspective, a $31 carbon tax would increase the price of gasoline by about 28 cents per gallon.

Nordhaus noted, though, that there is a large amount of uncertainty about the optimal tax. For the $31 tax above, the actual optimal tax could be as little as $6 per ton or as much as $93.

Interestingly, according to Nordhaus’s model, setting too high a carbon tax can be worse than setting no carbon tax at all. According to the calibration of Nordhaus’s model in 2007, with no carbon tax and no other government controls, the present value of damages from environment damage and abatement costs would be $22.59 trillion (in 2004 dollars). Nordhaus’s optimal carbon tax would have reduced damage but increased abatement costs, for a total of $19.52 trillion, an improvement of only $3.07 trillion. But the cost of a policy to limit the temperature increase to only 1.5° C would have been $37.03 trillion, which is $16.4 trillion more than the cost of the “do nothing” option. Those numbers will be different today, but what is not different is that the cost of doing nothing is substantially below the cost of limiting the temperature increase to only 1.5° C.

This is from my biography of William Nordhaus in The Concise Encyclopedia of Economics. It is now on line.