During a bilateral session in his hometown Chicago, Stern took Xie to a Chicago Cubs baseball game and also a meeting with Rahm Emanuel, then mayor of the city.

"We were really quite friendly, and we joked a lot and all of that," he said.

Guardian journalist Fiona Harvey, who attended many of Xie's news conferences at UN climate summits, described him as "a colossus in climate negotiations".

He was hugely respected and always worth listening to, she said.

"It was always useful to attend his press conferences, and I interviewed him many times, from which I always came away more enlightened," Harvey said.

At his COP28 news conference, Xie expressed his gratitude to Harvey and another foreign journalist for attending "every one of my news conferences".

But he especially singled out his long-standing friendship with US counterpart John Kerry.

"Kerry and I have known each other for over 20 years. We are good friends. Why? Because we share a common idea," Xie said. "That is being committed to protecting the environment and addressing climate change.

"We think this is a great cause because it benefits future generations and also can make our planet more beautiful."

Kerry also recently announced that he will step down from his role.

But Xie emphasized that he and Kerry will not leave the climate community.

"Both of us will continue to make a contribution, and make efforts to bring this process forward," he said.