US Secretary of State John Kerry had hoped his presence at a Marrakesh conference to decide on the finer points of a historic agreement to stave off climate change would be a victory lap.
Instead, he found himself having to address the uncertainty created by the election of Donald Trump, and what his presidency might mean for the US commitment to the 2015 Paris agreement to cut global greenhouse gas emissions.
In a speech on Wednesday, Kerry urged countries to treat the earth's changing climate as an urgent threat, citing melting glaciers, stronger storms, and record-breaking droughts.
"While I can't stand here and speculate about what policies our president-elect will pursue, I will tell you this: In the time that I have spent in public life, one of the things I've learned is that some issues look a little bit different when you're actually in office compared to when you're on the campaign trail," he said.
Trump has called climate change a hoax, and said he would rip up the Paris deal, halt any U.S. taxpayer funds for UN global warming programs, and revive the US coal sector.
If he follows through on his promises, he would undo the legacy of President Barack Obama, who has made climate change a policy priority and called the rising temperatures and other fallout from climate change "terrifying".
Kerry spoke of his trip last week to Antarctica, where he met scientists alarmed at the trends.