Want climate news in your inbox? Sign up here for Climate Fwd:, our email newsletter.

They have the largest carbon footprints. Also the largest economies. Now, as diplomats meet in Poland for high-stakes climate negotiations, a pitched standoff between the United States and China threatens to slow global action on climate change precisely at a time when the risks of catastrophe are accelerating.

The tensions between Washington and Beijing range from trade to cybersecurity to military rivalry in the Pacific. And while some of those issues have simmered for years, cooperation in the fight against climate change had once been a bright spot, so much so that it propelled the creation of the landmark global agreement in Paris in 2015 to curb greenhouse gas emissions.

But then the Trump administration announced its intention to pull out of the Paris pact altogether, rejecting the scientific consensus that greenhouse gas emissions are warming the pl anet. That represented perhaps the most consequential diplomatic reversal of the Trump era.

“The biggest threats to the planet are the lack of U.S. climate leadership at home and the unwillingness of the U.S. to engage with China,” said Joanna Lewis, a China specialist at Georgetown University. “The rest of the world looks to the U.S. and China for leadership, and it has become clear that, as the alliance has waned, global momentum to address climate change has slowed.”