If anyone needed another reminder of what’s at stake in this election, we saw it last weekend with the announcement of a vitally important agreement to reduce worldwide use of hydrofluorocarbons. Some 197 nations embraced an international pact to phase out use of those chemical compounds, which are highly damaging because of their high heat-trapping capacity.

It’s the kind of progress we simply won’t see if Donald Trump is elected. He, after all, believes man-made climate change is a hoax, a view in keeping with his conspiratorialist views on any number of matters. Among experts, however, there’s a broad consensus that human activity is the major cause of climate change — and that prompt and determined action is necessary to stave off the worst effects of global warming.

Hydrofluorocarbons (HFCs) are widely used in air conditioning and refrigeration. Although they are less of an overall threat than carbon emissions, they are far more potent a climate-change accelerator on a ton-for-ton basis. Developing nations, however, have been reluctant to transition from HFCs to more expensive substitutes, fearing that doing so would put air conditioning beyond the reach of some of their citizens.

But with a differentiated schedule that allows some nations a later date to start tapering down use of those compounds, forward-thinking leaders found a way to get to yes. Nor is it the only recent progress. With action by the European Parliament, India, and China, enough countries have now ratified the Paris climate change agreement for it to take effect.

All this is significant. But it wouldn’t have happened without the persistent leadership the United States has shown under President Obama. Indeed, it’s very possible that the Paris agreement on climate change will lose momentum, or even be abandoned, if Trump becomes president. Under him, there would likely be no anti-global-warming progress beyond what market forces — that is, cheap natural gas, which is supplanting coal — would bring about.

Nor would this new pact likely have been possible if it had required Congressional action; like Trump — but unlike conservative parties in other countries — congressional Republicans are in a state of denial about global warming. That lamentable political reality has left President Obama to proceed with executive regulations to implement a plan to curb carbon emissions by about a third, against a 2005 baseline, by 2030. The Supreme Court, however, has blocked implementation of the president’s plan while a legal challenge wends its way through the lower courts.

Unlike Trump, Hillary Clinton believes in climate science. If she is elected, progress on climate change would continue with the United States as a leader. Further, if Democrats take control of the Senate, the majority leader would no longer be Mitch McConnell, who hails from a coal state and is determined to block any measures that hurt coal mining.

But even if Democrats don’t take control of the Senate, if Clinton wins, she will then fill the vacancy on the Supreme Court, and presumably with a justice who has a more expansive view of the Clean Air Act and executive authority than the court’s conservatives have.

All of which is another way of saying that there’s a lot riding on this election. And not just for the United States, but for the world.