We now have a pretty good idea who will be on the ballot in November: Hillary Clinton, almost surely (after the South Carolina blowout, prediction markets give her a 96 percent probability of securing her party’s nomination), and Donald Trump, with high likelihood (currently 80 percent probability on the markets). But even if there’s a stunning upset in what’s left of the primaries, we already know very well what will be at stake — namely, the fate of the planet.

Why do I say this?

Obviously, the partisan divide on environmental policy has been growing ever wider. Just eight years ago the G.O.P. nominated John McCain, whose platform included a call for a “cap and trade” system — that is, a system that restricts emissions, but allows pollution permits to be bought and sold — to limit greenhouse gases. Since then, however, denial of climate science and opposition to anything that might avert catastrophe have become essential pillars of Republican identity. So the choice in 2016 is starker than ever before.

Yet that partisan divide would not, in itself, be enough to make this a truly crucial year. After all, electing a pro-environment president wouldn’t make much difference if he or (much more likely) she weren’t in a position to steer us away from the precipice. And the truth is that given Republican retrogression and the G.O.P.’s near-lock on the House of Representatives, even a blowout Democratic victory this year probably wouldn’t create a political environment in which anything like Mr. McCain’s 2008 proposal could pass Congress.

But here’s the thing: the next president won’t need to pass comprehensive legislation, or indeed any legislation, to take a big step toward saving the planet. Dramatic progress in energy technology has put us in a position where executive action — action that relies on existing law — can achieve great things. All we need is an executive willing to take that action, and a Supreme Court that won’t stand in its way.