Let this be our national goal: At the end of this decade, in the year 1980, the United States will not be dependent on any other country for the energy we need.

—President Richard Nixon, January, 1974

“If all economists were laid end to end, they would not reach a conclusion.” So said George Bernard Shaw. The propensity of economists to waffle makes N. Gregory Mankiw’s claim all the more startling: “Among policy wonks like me [economists], there is a broad consensus.” Economists can’t reach a conclusion, never mind a consensus. But he’s right—economists have reached a consensus in favor of his conclusion that “we need a global carbon tax.”

Because economists favor market-based approaches, their tilt toward a tax may seem paradoxical...