Three wildly different world leaders with divergent personalities, agendas, and backgrounds will be in the same country at the same time, fighting for the same thing—solutions for climate change.

Ahead of the UN General Assembly meeting in New York City, an unprecedented triumvirate will be on US soil: President Xi Jinping, the Communist leader of China's 1.3 billion people and the world's biggest carbon polluter; Pope Francis, the leader of the world's 1.2 billion Catholics, and a self-modeled reformer; and US President Barack Obama, the leader of the world's largest economy (and a man who doesn't need to be reelected). In one way or another, they each have made fighting global warming a core part of their leadership at home and abroad.

The trio isn't publicly scheduled to meet each other in America this week—though they ...