### The Social Cost of Carbon

I participated in a NPR interview on Marketplace on the topic of the “Social Cost of Carbon”.   A different way to say the same thing is; “What is the benefit of not producing another ton of carbon?” While President Obama will ask a “Dream Team” of economists and climate scientists to answer this question (and I hope their answer is somewhere in the range of \$20 < X < \$55), I have no idea how they will write down a credible methodology for justifying their favorite number.  But, let me sketch the challenge through an algebra example.

To simplify this very hard problem, suppose that the only consequence of climate change is to raise average temperature levels.  Let’s simplify further and assume a linear relationship so that every ton of carbon released always increases atmospheric CO2 concentrations by d units.  Let’s make another linearity assumption and assume that a one unit increase in CO2 concentrations increases average average temperature by m degrees.  Let’s make another linearity assumption and assume that world GDP declines by \$f for every increase of 1 degree in average temperature.  Now that we have made all of these crazy assumptions, the world GDP cost (measured in dollars) of an extra ton of CO2 always equals =  d*m*f.    You wanted an equation and I have supplied one.  But, isn’t it silly? As an economist, I propose that we focus on the “f”.

Our economy is not physics.  There are no universal constants for how environmental conditions impact us!  That’s the whole point of why I wrote Climatopolis.  As we grow richer and smarter and as we experiment, we become more resilient in figuring out new strategies so that we suffer less from events such as temperature change. We are urbanizing and fewer of us work in agriculture.  Now, I agree that  the impact of CO2 concentrations on average temperature is also likely to be a convex rather than a linear equation. In this case, as China and India continue to consume fossil fuels, the social cost of carbon could rise over time even as we get better at adapting.

My point is that a number is needed as the “Social Cost of Carbon” in order to help justify activist carbon policy but the intellectual justification for the number that is picked is going to being almost funny.  Perhaps it is important for economists to be more honest about the assumptions we do make when creating an economic model that generates a single “answer” to a very hard question.

## 3 Replies to “The Social Cost of Carbon”

1. Matt,

I heard your piece on NPR and, while thoughtful, it left out the costs of climate change to non-human species. We – at least the rich nations – may be able to adapt just fine. We may even be willing and able to absorb the adaption costs of billions of poor people who can neither afford the adaptations on their own nor had anything to do with causing the problem in the first place. But what are we to do about mass extinction of flora and fauna? What is the social cost of extinction?

2. And cue the scientists: “Published online in the journal Ecology Letters, the study concludes that most land-based vertebrate species evolve too slowly to adjust to the dramatically warmer climate expected by 2100. If they can’t make high-speed adaptations or move to a new ecosystem, many terrestrial animal species will cease to exist, the researchers report.”

http://www.huffingtonpost.com/2013/07/16/climate-change-evolution_n_3604280.html

Matthew E. Kahn is a Professor at the UCLA Institute of the Environment, the Department of Economics, and the Department of Public Policy. He is a research associate at t…

Matthew E. Kahn is a Professor at the UCLA Institute of the Environment, the Department of Economics, and the Department of Public Policy. He is a research associate at t…