How likely is it that the world will warm not just by 2 degrees Fahrenheit by the end of this century, but by 11 degrees? What would happen to the planet? And, to avoid a much hotter world, what should we do and when should we do it? In Climate Shock, Gernot Wagner, the lead senior economist at the Environmental Defense Fund, and Martin L. Weitzman, a professor of economics at Harvard University, address those questions.
They claim that if the concentration of carbon dioxide in the air rises from its current level of 400 parts per million (ppm) to 700 ppm, the probability that the world will warm by more than 11 degrees is 11 percent. If that were to happen, they claim, we would experience serious rises in ocean levels and more and more-violent storms, to name two major consequences. To avoid that, they propose a Pigovian tax of at least $40 per ton of carbon dioxide emitted and, they say, "We must act now." If we don't substantially reduce our carbon usage soon, at some point we will find ourselves using "geoengineering" to reduce the earth's temperature by a few degrees. And they fear that geoengineering could get out of control and have unintended consequences. Better, they say, to impose a stiff carbon tax now.
How convincing is their case? Not very. They could be right, but they don't tell us nearly enough to justify their most important claim: an 11 percent probability of a much warmer climate. And, while they often profess relative certainty in the body of the book, they tend to relegate some of the most important doubts and controversies to the footnotes. That's a problem because few people read footnotes. Also, the authors judge competing policy responses to global warming asymmetrically. Specifically, they advocate a stiff carbon tax throughout, always claiming that it's the obviously right thing to do, without ever considering whether such a tax might have unintended consequences. But when they consider geoengineering solutions--technological methods to alter the climate that they admit would cost a small fraction of the cost of a carbon tax--they raise the specter of unspecified unintended consequences and even construct a scenario in which a mysterious foreign government could engage in unchecked geoengineering.
This is from "Egad, Geoengineering!", my review of Climate Shock in Regulation, Winter 2015-16, p. 74.