As the special climate envoys of China and the United States, Xie Zhenhua and John Kerry have spent years on opposite sides of negotiating tables.

They are rivals, but also bosom friends.

During the COP28 climate change conference late last year, Xie, 74, attended a private gathering for Kerry's 80th birthday.

"We are good friends. Why? Because we share a common idea," Xie told a news conference during the annual United Nations event, which was held in Dubai, United Arab Emirates.

"That is being committed to protecting the environment and addressing climate change. We think this is a great cause because it benefits future generations."

The remarkable friendship that Xie and Kerry forged not only helped bridge rifts between their governments, but also defined the global climate agenda, experts said.

However, the two veterans are retiring from climate diplomacy. Shortly after China announced on Jan 12 that Xie, who began steering China's climate diplomacy in 2007, had retired from his role, Kerry told media that he was also to leave climate diplomacy.

Liu Zhenmin, a former vice-minister of foreign affairs, has been appointed Xie's successor. US President Joe Biden will tap John Podesta, a Democratic political strategist and energy expert, to replace Kerry, The Washington Post reported late last month.

Experts said China and the US may need a "transitional period" to adapt to bilateral climate diplomacy without Xie and Kerry, but that would not cause big trouble for long-term climate cooperation.