As United Nations climate talks kick off in Turkey on Monday, Secretary-General António Guterres has an urgent message for world leaders: “This is a moment of truth.”

He's delivered variations of this ominous warning about climate change, caused by the burning of fossil fuels like oil and gas, in the past decade, often with increasingly hyperbolic language. Three years ago, he even said, “the era of global boiling has arrived, " referring to rising temperatures and increasing extreme weather, both results of climate change.

But this time, it will be his last global warming sermon as head of the United Nations as his second and last term expires at the end of the year. And the sense of urgency is palpable in a world with worsening killer heat waves, storms, floods and droughts amid ever higher temperatures and, on top of all of it, El Niño, a naturally occurring climate pattern that scrambles weather systems around the globe.

“This is a moment of truths.…