Mr. Sanders would spend more than $2 trillion to build new wind, solar and geothermal electricity-production infrastructure through government-run utilities. He would spend another $2 trillion buying people electric cars. Though he proposes totally electrifying car and truck transportation, he also wants to spend $607 billion linking U.S. cities with high-speed rail, which, under his plan, would represent a major cost for meager carbon benefits.

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Mr. Sanders insists that his plan would be paid for through new taxes levied against fossil-fuel companies, cuts in military spending and new income tax revenue from the jobs he claims his plan would create. The senator promises 20 million “good-paying, unionized jobs.” Only about 6.1 million people are unemployed in the United States. Though some currently employed Americans could try to trade up to the cushy gigs envisioned by Mr. Sanders, many of them would not have the skills required to weatherize homes or install solar panels.

Mr. Sanders also promises to make his plan unnecessarily expensive by ruling out a long-established source of carbon-free electricity: nuclear power. Not only would he halt the building of new plants, but he also would deny re-licensing to the existing ones that now provide about 20 percent of the nation’s electricity.

As with practically every grandiose program Mr. Sanders proposes, we are left wondering what the democratic socialist would actually do as president. Nothing resembling his climate plan could pass Congress, even with a strong Democratic majority. Mr. Sanders typically retorts that he will lead a political revolution. But he will not change the fact that the nation is ideologically pluralistic.

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On climate policy, the key is to get the most bang for the nation’s buck. The task is so large that direct government spending on projects such as power plants is a recipe for unconscionable waste. Mr. Sanders’s promise to divert national wealth into proven boondoggles such as high-speed rail is another red flag.

No central planner can know exactly how and where to invest for an efficient and effective energy transition. That is why economists continue to recommend that the government take a simple, two-pronged approach: invest in scientific research and prime the market to accept new, clean technologies with a substantial and steadily rising carbon tax. People and businesses would find the most effective ways to avoid the increasingly high, tax-inflated costs of using dirty fuels. Maybe that would mean building huge new solar farms throughout the country. Maybe it would mean massive energy efficiency gains driven by home retrofits or new appliances. Maybe it would mean continuing to accept some role for nuclear power.

We do not know, precisely, what the most efficient path looks like. We are also certain that Mr. Sanders does not.