An aerial photo of Sanjiangyuan National Park in Northwest China's Qinghai province. [Photo provided to China Daily]

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.

Wang Yi, vice-chair of China's National Expert Panel on Climate Change, said the competitive and cooperative engagement between Xie and Kerry occurred against the backdrop of the Paris Agreement, which changed the paradigm of global climate diplomacy.

The accord adopted a bottom-up structure to set emission targets for different parties. The change made strong leadership especially important in global climate negotiations, which involve almost 200 parties, he said.

China, as a representative of developing nations, and the US, which represents developed countries, had, without doubt, forged the most important bilateral relationship for multilateral action on tackling global climate change, Wang said.

"Xie and Kerry were pushed to the foreground in this special era," he said.

Li Zheng, president of Tsinghua University's Institute of Climate Change and Sustainable Development, said Xie and Kerry made historic contributions to the world by ensuring that China and the US exercised climate leadership.

President Xi Jinping and then US president Barack Obama laid the foundations of international support for the Paris Agreement, which was adopted in 2015, by making a joint public announcement on climate change in Beijing in late 2014.

Xie, then deputy head of the National Development and Reform Commission, and Kerry, then US secretary of state, played an important role in bringing about the announcement by their heads of state, Li said, adding that they were also behind five other China-US joint announcements and one joint declaration on climate change.

He said they also contributed a lot to safeguarding multilateralism under the Paris treaty and the accord's authority.

"When parties were at their wits' end trying to reach a consensus (at COP26 in 2021) in Glasgow, for example, China and the US released a joint declaration … thanks to the efforts of Xie and Kerry," Li said, adding that the joint declaration turned the situation around and helped ensure the success of COP26.

Most recently, they came to an ambitious agreement in California in November for both sides to triple renewable energy capacity by 2030.That had, in turn, been adopted as a global target at COP28, he said.

De Boer said their close personal relationship had allowed the US and China to continue an informal climate dialogue despite geopolitical tensions.

"Their relationship has not only helped to bring the formal US-China climate dialogue back on track, I believe it has even contributed to some progress in broader US-China relations," he said.

Rajendra Shende, who in 2007 was coordinating lead author for a report by the UN Intergovernmental Panel on Climate Change when it shared the Nobel Peace Prize with former US vice-president Al Gore, said the legacies left by Xie and Kerry were in the form of lessons learned.

One of the lessons was that bilateral rapport should be able to show tangible results that benefit not only two countries but the whole world.

"Climate change is a global issue, not bilateral," he said. "But bilateral ties can trigger global benefits."

Wang of the National Expert Panel on Climate Change said Xie and Kerry were competent climate diplomats with strong, comprehensive leadership skills and political wisdom.

Their retirements will certainly have an impact, and China and the US may have to go through a transitional period in their climate diplomacy, Wang said.

But that won't matter much in the long run, because the two countries generally still hope to have better communication with each other, and that would provide a solid foundation for climate cooperation.

Li said that Xie and Kerry had formed a very good working relationship that followed a rule that Xie defined as the "comfort principle", which was to take care of the core interests of the other side and not to breach its bottom line.

Li said their successors will need some time for mutual adaptation, but he is optimistic about the future of China-US climate cooperation.