The possibility that a climate science denier could soon become America’s president is serving as an urgent motivator for diplomats gathering in New York this week for United Nations meetings.

Donald Trump, the Republican presidential nominee, has called climate change a “hoax” and said that as president he would “cancel” the UN’s new climate pact, the Paris agreement, which was finalized last December.

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“The president has plenary authority to manage foreign affairs,” said Michael Wara, an environmental law expert at Stanford. “Since the Constitution is silent as to treaty withdrawal, a president can do that without consulting with Congress.”

Wara pointed out that in 1992, President George W. Bush withdrew the U.S. from an anti-ballistic missile treaty that the Senate had ratified in 1972 without seeking the Senate’s approval. After President Carter withdrew from a defense treaty with Taiwan, the Supreme Court refused to hear a challenge filed by members of Congress.

“The UNFCCC is a forum for negotiating, and withdrawal from that would essentially be saying, ‘We’re not even going to talk about this any more,” Wara said.

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“What are the other countries going to do to retaliate? Emit more greenhouse gas emissions? Of course not,” Wara said. “But it would mean that when we need their help to achieve some other diplomatic objective, they would trust our promises much less.”