It probably won’t surprise you to hear that when it comes to climate change, as with so many issues, Mr. Trump has gone deep down the rabbit hole, asserting not just that global warming is a hoax, but that it’s a hoax concocted by the Chinese to make America less competitive.

The thing is, he’s not alone in going down that rabbit hole. On other issues Republicans may try to claim that their presidential nominee doesn’t speak for the party that nominated him. We’re already hearing claims that Mr. Trump isn’t a true conservative, indeed that he’s really a liberal, or anyway that liberals are somehow responsible for his rise. (My favorite theory here, one that has quite a few advocates, is that I personally caused Trumpism by being nasty to Mitt Romney.)

But when it comes to denial of climate change and the deployment of bizarre conspiracy theories to explain away the evidence, Mr. Trump is squarely in the Republican mainstream. He may be talking nonsense, but anyone his party was likely to nominate would have been talking pretty much the same nonsense.

It’s interesting to ask why climate denial has become not just acceptable but essentially required within the G.O.P. Yes, the fossil-fuel sector is a big donor to the party. But the vehemence of the hostility to climate science seems disproportionate even so; bear in mind that, for example, at this point there are fewer than 60,000 coal miners, that is, less than 0.05 percent of the work force. What’s happening, I suspect, is that climate denial has become a sort of badge of right-wing identity, above and beyond the still-operative motive of rewarding donors.

In any case, this election is likely to be decisive for the climate, one way or another. President Obama has made some serious moves to address global warming, and there’s every reason to believe that Hillary Clinton would continue this push — using executive action if she faced a hostile Congress. Given the technological breakthroughs of the last few years, this push might just be enough to avert disaster. Donald Trump, on the other hand, would do everything in his power to trash the planet, with the enthusiastic support of his party. So which will it be? Stay tuned.