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

They are rivals, but also bosom friends.

During the COP28 climate change conference late last year, Xie, 74, took his 8-year-old grandson to 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 UN 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 stage, 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 Party 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.

Significant contributions

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 the Institute of Climate Change and Sustainable Development at Tsinghua University, said Xie and Kerry made historic contributions to the world by ensuring China and the US exercised climate leadership.

President Xi Jinping and former 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 the joint declaration turned the situation around and helped ensure the success of COP26.

Dimitri de Boer, regional director for Asia at the environmental law organization ClientEarth, said that together with the other parties in the UN Framework Convention on Climate Change, Xie and Kerry had "laid the groundwork for strong global climate action, which is now extremely urgent".

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.

Broader impact

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.

Most important, they had shown that joint climate action could be a force for improved trust, 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.

Both of them wanted to "move even further forward", he said, with Kerry pushing China to set even more ambitious climate targets, and Xie hoping that China could follow its own path in tackling climate change while also striving to ensure developing nations got more support.

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 of the Institute of Climate Change and Sustainable Development at Tsinghua University said 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.

"Such rapport in a relationship means that they can have more in-depth, thorough and frank exchanges," he said. "And it's easier for them to strike a balance and reach goals."

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

Both China and the US need to promote green innovation and forge ahead with a green, low-carbon transition to cope with the challenges brought by climate change," he said. There is demand for cooperation from the national level to the local level in both countries and also among their enterprises, Li said.

"Tackling climate change concerns the global public good," he said. "The two sides also have the demand. I think these provide a good foundation for China-US climate cooperation."

Both Wang and Li said they believed Xie and Kerry would continue to play roles in meeting the global climate challenge.

Xie said at COP28 that he and Kerry would not leave the climate community and would "continue to make a contribution".

While Xie was absent from the World Economic Forum annual meeting in January, he entrusted Wang to discuss some issues with Kerry in Davos.

"Xie asked me to communicate with Kerry on the issue of illegal deforestation, a problem they are both concerned about, and discuss with him how they will conduct research on long-term strategic cooperation in the field of tackling climate change," Wang said.