Climate activists and government negotiators are heading home as the United Nations climate summit in Paris comes to a close.

In the final days, U.S. Secretary of State John Kerry gave a speech to delegates and the media. He acknowledged that there are still many Americans—even some running for president—who cast doubt on the science of climate change.

“My friends—these people are so out of touch with science that they believe rising sea levels don’t matter, because in their view, the extra water is just going to spill out over the sides of a flat earth,” Kerry said.

LISTEN: “Kerry calls out climate change deniers”

But Kerry said that can’t stop the world from acting to reduce greenhouse gas emissions and attempting to stem the worst outcomes.

“Unless the global community takes bold steps now to transition away from a high-carbon economy, we are facing unthinkable harm to our habitat, our infrastructure, our food production, our water supplies—and potentially to life itself.”

Kerry told delegates that the U.S. and industrialized nations can’t stop climate change alone because 65 percent of carbon pollution comes from the developing world. He said different countries at the conference will have different goals for how to move into a lower-carbon future. And he called for a strong, legally binding enforcement system for those goals.

“That means that we need to require regular reporting from all countries on what they’re doing and how much progress they’re making. And that’s the only way we’re going to know we stand as a global community.”

Kerry doesn’t expect fossil fuel development to stop. But he does want the climate summit to lead the world toward more renewable power. He says that’s important for governments—and also for business.

“What we’re doing is sending the marketplace an extraordinary signal that those 186 countries are really committed. And that helps the private sector to move capital into that knowing that there’s a future that’s committed to this sustainable path.”

Kerry called on global leaders not to let dysfunction get in the way of saving the planet for future generations.