Bill McKibben is the Schumann distinguished scholar in environmental studies at Middlebury College and founder of the global climate group 350.org.

President-elect Donald Trump has already begun to back off some of his promises: Maybe not all of Obamacare has to go. Maybe parts of his wall will actually be a fence. Maybe it’s okay to have some lobbyists running the government after all.

But I fear he won’t shrink from the actions he has promised on climate change: withdrawing the United States from the Paris accord, ending President Obama’s Clean Power Plan and okaying every new fossil-fuel plan from the Keystone XL pipeline on down. He won’t back down because those are hard-to-hedge choices and because he’s surrounded by climate-change deniers and fossil-fuel insiders who will try to ensure that he keeps his word.

So let’s be entirely clear about what those actions would represent: the biggest, most against-the-odds and most irrevocable bet any president has ever made about anything.

It’s the biggest because of the stakes. This year has been the hottest year recorded in modern history, smashing the record set in 2015, which smashed the record set in 2014. The extra heat has begun to steadily raise sea levels, to the point where some coastal U.S. cities already flood at high tide even in calm weather. Global sea ice levels are at record lows, and the oceans are 30 percent more acidic. And that’s just so far. Virtually every scientific forecast says that without swift action in the next few years to cut carbon emissions, this crisis will grow to be catastrophic, with implications for everything from agriculture to national security that dwarf our other problems.

Donald Trump will enter the White House with an environmental policy agenda opposed to that of the Obama administration and many other nations that have pledged support to the Paris climate agreement. The Washington Post's Chris Mooney breaks down what a Donald Trump presidency will mean when it comes to climate change. (Daron Taylor/The Washington Post)

It’s the most against-the-odds bet because at this point there’s so little scientific dispute about climate change. Researchers have spent three decades narrowing the error bars and establishing an ever-clearer picture of the future. There’s always the chance that scientists have overlooked something, but it’s by now so narrow a chance it hardly deserves that description.

And it’s the most irrevocable bet because the next few years are crucial. This makes global warming unique: If you take away Obamacare, poor people will suffer until something replaces it — which would be bad, but that suffering would not make it harder to fix the problem later. Climate change, however, comes with a time limit, which is why senior scientists last week were saying that if Trump carries through with his wager, it might well be “game over.” If he loses his bet, he will have cost us the last years in which we might have made a real difference.

Against all this, Trump has merely the conviction that climate change is a hoax. It’s a conviction more or less shared by the man he has put in charge of his energy and environmental transition team, Myron Ebell of the Competitive Enterprise Institute, and a handful of other climate-change deniers at websites such as WattsUpWithThat.com . Some, like Ebell, are funded by the fossil-fuel industry, and others are quite sincere freelancers who have involved theories about how some of the thermometers measuring the planet’s climate have been placed too near to airport runways or believe that sunspots or cosmic rays or “natural cycles” will soon cool the Earth. They are contemptuous of the consensus science (the product of “a lot of third-, fourth- and fifth-rate” researchers, says Ebell) and of anyone who takes it seriously. (Pope Francis, in his encyclical on climate change, was “scientifically ill informed, economically illiterate, intellectually incoherent and morally obtuse,” says Ebell.)

It’s easy to see why these kinds of pronouncements might appeal to Trump. It’s not just that they’re spoken in the brash language he likes to use, but they made it easy for him to justify, say, his promises to restore the nation’s coal mines to their glory days. It would indeed be much easier for all concerned if global warming were hogwash.

But as far as anyone knows, he has never tested his beliefs by sitting down with scientists for even a cursory examination of the data. So someone who has his ear needs to tell him that the opinions on which he’s relying are marginal at best.

And that friend might remind him, too, of the difference between issues governed by opinion and those governed by fact. If you don’t think poor people should get subsidized medical care, that’s ugly, but it’s an opinion you’re entitled to hold. Science isn’t like that: The heat-trapping properties of the carbon dioxide molecule simply a re. Which is why, even if we fail in our efforts to stop Trump from making his bet, it’s important for history to note what’s going on. One man is preparing to bet the future of the planet in a long-shot wager against physics.