And now, in this millennium, we’ve watched the warning start to play out. We’ve seen 2014 set a new global temperature record, which was smashed in 2015 and smashed again in 2016. We’ve watched Arctic sea ice vanish at a record pace and measured the early disintegration of Antarctica’s great ice sheets. We’ve been able to record alarming increases in drought and flood and wildfire, and we’ve been able to link them directly to the greenhouse gases we’ve poured into the atmosphere. This is the largest-scale example in the planet’s history of the scientific method in operation, the continuing dialectic between hypothesis and skepticism that arrived eventually at a strong consensus about the most critical aspects of our planet’s maintenance. Rational people the world around understand. As Bloomberg Businessweek blazoned across its cover the week after Hurricane Sandy smashed into Wall Street, “It’s Global Warming, Stupid.”

But now President Trump (and 22 Republican senators who wrote a letter asking him to take the step) is betting that all of that is wrong. Mr. Trump famously called global warming a hoax during the campaign, and with this decision he’s wagering that he was actually right — he’s calling his own bluff. No line of argument in the physical world supports his claim, and no credible authority backs him, not here and not abroad. It’s telling that he simultaneously wants to cut the funding for the satellites and ocean buoys that monitor our degrading climate. Every piece of data they collect makes clear his foolishness. He’s simply insisting that physics isn’t real.

But it’s not just science that he’s blowing up. The Paris accord was a high achievement of the diplomatic art, a process much messier than science, and inevitably involving compromise and unseemly concession. Still, after decades of work, the world’s negotiators managed to bring along virtually every nation: the Saudis and the low-lying Marshall Islanders, the Chinese and the Indians. One hundred and ninety-five nations negotiated the Paris accord, including the United States.

The dysfunctional American political process had already warped the process, of course. The reason Paris is a series of voluntary agreements and not a real treaty is because the world had long since understood that no binding document would ever get two-thirds of the vote in our oil-soaked Senate. And that’s despite the fact that the agreement asks very little of us: President Barack Obama’s mild shift away from coal-fired power and toward higher-mileage cars would have satisfied our obligations.

Those changes, and similar ones agreed to by other nations, would not have ended global warming. They were too small. But the hope of Paris was that the treaty would send such a strong signal to the world’s governments, and its capital markets, that the targets would become a floor and not a ceiling; that shaken into action by the accord, we would start moving much faster toward renewable energy, maybe even fast enough to begin catching up with the physics of global warming. There are signs that this has been happening: The plummeting price of solar energy just this spring persuaded India to forgo a huge planned expansion of coal plants in favor of more solar panel arrays to catch the sun. China is shutting coal mines as fast as it can build wind turbines.