Donald Trump speaks during a campaign stop at the Signature Flight Hangar at Port Columbus International Airport in Columbus, Ohio, on March 1. (John Minchillo/Associated Press)

Todd Stern was the U.S. special envoy for climate change from 2009 until April 2016.

Donald Trump vows that once in office, he’s “going to cancel the Paris climate agreement,” which, he asserts, “gives foreign bureaucrats control over how much energy we use right here in America.”

That’s not especially surprising coming from Trump, who has said he is “not a great believer in man-made climate change.” But this particular promise caught my attention, since I led the U.S. negotiating team in Paris and in the seven years leading up to that agreement.

1 of 45 Full Screen Autoplay Close Skip Ad × Trump captures the nation’s attention as he campaigns View Photos The presidential candidate and billionaire businessman leads the field in the Republican race. Caption Businessman Donald Trump officially became the Republican nominee at the party’s convention in Cleveland. Donald Trump speaks at a campaign event at Trump Doral golf course in Miami. Carlo Allegri/Reuters Buy Photo Wait 1 second to continue.

Let’s take a look at Trump’s position on the Paris Agreement, the first genuinely global, durable diplomatic response to the threat — and yes, it’s a real one — posed by climate change.

The bit about “foreign bureaucrats” controlling our energy use is ludicrous. Under the Paris Agreement, no foreigner, from bureaucrat to king, gains an iota of control over U.S. decisions about how much energy we use or, indeed, what our overall energy or climate policy is.

Rather, every country develops its own plan for reducing its greenhouse gas emissions. No country can tell another what it must do. This “nationally determined” structure was exactly what the United States advocated. And some 190 countries, including all the big ones, have submitted plans.

All countries are covered by a strong transparency system for regularly reporting and being reviewed on their emissions inventories and the progress they are making toward targets. But again, these targets are a matter for the countries themselves to determine.

Indeed, the Paris Agreement ought to be embraced not only by those who seek strong action on climate change but also by all those who traditionally opposed climate agreements because China and other emerging economies seemed to get off scot-free. Rather than excuse these less developed countries, the Paris Agreement adopts a flexible means of differentiating among countries, keyed to their capabilities.

But suppose Trump disagrees and wants to keep his promise to “cancel” the Paris Agreement. For starters, he couldn’t do that even if he were foolish enough to try. Leaders of more than 190 nations endorsed the agreement. The United States has no power to cancel it. This isn’t reality TV. You can’t tell sovereign leaders around the world “you’re fired,” and you can’t tell them a multilateral agreement they just entered is canceled.

Of course Trump could, in theory, pull the United States out of the Paris regime, but that would be stunningly misguided. During the course of this century, climate change, with the impacts it produces — such as severe droughts and floods, extreme heat, massive wildfires, rising sea levels and super storms — has more capacity to disrupt life as we know it and to threaten both human welfare and national security than any other issue, save nuclear conflict.

Climate change is happening now, intensively, all over the world. It’s getting worse. We can’t hope to contain it without joint global action. The Paris Agreement is our vehicle for doing that. Trump would have us walk away? Really?

Further, if a President Trump were to stick to his wrongheaded notion that climate change is a hoax, and unilaterally withdraw from the Paris Agreement, he would also inflict severe diplomatic damage on the United States. U.S. standing in the world would plummet amid almost universal condemnation. Trump could pound his chest to his heart’s content, but he’d come out of this rash exercise a loser, with U.S. credibility and leverage in tatters.

It’s time to take Trump seriously. He is effectively the Republican candidate for president. Don’t make excuses for his words or assume he doesn’t mean them. We’re playing for keeps now.