Climate change is the single biggest challenge facing humankind. Yet the next president of the United States — the world's second-largest greenhouse-gas emitter and a critical actor in climate policy — does not believe it is happening, or at least that humans have a role in driving it. If Donald Trump actually wants to "Make America Great Again," as his campaign slogan declared, he will need to change his attitude and embrace the climate agenda.

So far, the situation does not look promising. Despite a mountain of scientific data, Trump claims that there is no evidence that humans contribute to global warming. He once even called climate change a "hoax," invented by the Chinese to make U.S. manufacturing less competitive (though he later walked back that accusation). He has not, however, rethought his broader skepticism about human-driven climate change.

Reflecting this line of thinking, Trump has announced his intention to reverse carbon-emission limits for coal-fired power plants, step up fossil-fuel production, and roll back support for wind and solar power. He has also pledged to pull the U.S. out of the global climate-change agreement concluded last December in Paris. Such a reversal would be catastrophic for global efforts to tackle climate change.