As the final fraught hours of the Cop28 UN climate summit approached last December, a small but vital meeting was missing from the official agenda. While thousands of grumpy delegates trooped the vast and glitzy Dubai venue, a private gathering in a quiet corner marked a joyful milestone. John Kerry, the US presidential envoy for climate, celebrated his 80th birthday, and guest of honour was his 74-year-old Chinese counterpart, Xie Zhenhua. It was a warm and familial occasion: even Xie’s eight-year-old grandson was present. “Why are you still working so hard, at your age?” he asked of Xie and his “good old friend” Kerry. “Because we are all passionate about this common cause of addressing climate change,” was Xie’s answer, recounted at an unusual joint US-China press conference after the summit ended. For both men, who have been pivotal to climate negotiations since before the signing of the landmark 2015 Paris agreement, that hard work is now over. First Xie and then, last week, Kerry announced his retirement, leaving a global climate stage that their close personal relationship has helped to define. Despite rising tensions between their countries, they forged a US-China collaboration on climate action that has been essential to reshaping the global economy – but also a frustrating barrier to climate progress. The world’s two biggest emitters – China, with about 30% of global carbon dioxide emissions, and the US with about 14%, plus the world’s biggest oil and gas production – hold the planet’s future in their hands. Relations between the countries were traditionally strained, with the US seeing in China a dangerous growing rival and China building its own alliances and power bases, fuelled by its increasing economic might. “The relationship on the climate between the US and China is super-important,” says Todd Stern, former US climate envoy, who worked first with Hillary Clinton as secretary of state, and then with Kerry for the Paris agreement. “It has been for a long time the most important bilateral relationship in the world, on the climate.” Both countries have been alternately praised and criticised for their climate actions. China has been the engine of global green growth, with its massive investments in wind, solar and electric vehicles driving down costs dramatically, and has been a hero to developing countries, offering aid and investment. But the world’s second biggest economy remains highly dependent on coal, and has sometimes obstructed progress – at the Durban UN Cop in 2011, holding out until the final moments on accepting the roadmap that eventually led to the Paris agreement, and at Cop26 in Glasgow in 2021, forcing a last-minute weakening of the commitment to phase out coal into a phase down. The US has also played a dual role: pushing for the 2015 Paris agreement and unveiling $369bn in green investment under the Inflation Reduction Act but simultaneously ramping up its oil and gas production to become the world’s biggest producer. It has also signally failed to provide the vital climate finance that developing countries have long been promised, and stalled on the “loss and damage” rescue funds that are life and death issues for the world’s poorest. It was good to have a “grownup” back in the role after Trump, said Mohamed Adow, director of the Power Shift Africa thinktank, but the US is still a major disappointment on climate action. “Kerry and the US fought hard against the creation of the loss and damage fund, and too often it acts as a major blocker when it comes to progressive climate agreements. Hopefully the new envoy can have a better legacy than Kerry,” he said. And Harjeet Singh, global engagement director for the Fossil Fuel Non-Proliferation Treaty Initiative, argued that both countries need to do more: “Their claims of climate leadership ring hollow with continued investment in fossil fuels.” Nevertheless, the Xie-Kerry relationship, which goes back more than 25 years, has brought what many argue are rich dividends for the climate talks and for much of the past decade – with a large gap for Donald Trump’s presidency – Kerry and Xie, both keenly and seriously interested in climate change, spearheaded their countries’ climate engagement and helped to turn it into an arena for cooperation. “Xie built a set of lasting partnerships with US officials that underpinned the relationship of the world’s largest economies, and saw them through good times and bad,” said Rachel Kyte, former World Bank climate chief and UN special representative. “Kerry is probably one of the few people in the US who could bring the Biden administration and the wider body politic together to do a deal with China, when so many others in Washington just saw danger everywhere.” “The Paris agreement would not have happened without those two,” economist Lord Stern told the Guardian. “They built up a relationship of trust between their two countries that was essential. That has in turn given stability and a sense of commitment.” Both men have shown a deep understanding of the climate crisis and its urgency, and political nous in navigating climate action through the shoals of economic rivalry and diplomatic tensions. “They are also very nice guys, real human beings, with a deep, deep sense of decency, both of them,” added Lord Stern. Their warm personal rapport was frequently in evidence. At Cop28, the US and Chinese delegations were housed strategically near to each other, with the back entrance of the US offices opening on to the Chinese pavilion, facilitating an apparently impromptu meeting between the two men as the conference began, to their mutual and clear delight. Both men, while driven and energetic, have been showing signs of fatigue at recent climate meetings, however. Xie has spent the last few wearing trainers for his bad back and suffered a serious but undisclosed illness last year. Kerry contracted Covid in the late stages of Cop27, in Sharm El-Sheikh, and had to conduct the final hours of negotiations from his hotel room, though his punishing work schedule resumed soon after. They leave the stage at a crucial juncture. With Donald Trump again threatening to upend US action on the climate, as well as a host of other issues, the looming US presidential election is of primary importance. But China is just as critical: Bernice Lee, of Chatham House, pointed out that the country is deploying vast amounts of low-carbon technology, adding: “The Xi administration wants to be regarded well on the world stage – climate is the best platform for them to assert leadership.” Xie’s apparent successor is Liu Zhenmin, a top diplomat who views climate as a vital brief, while the Biden administration has yet to indicate who will replace Kerry. Paul Bledsoe, a former Clinton White House climate adviser now with American University’s Center for Environmental Policy, said the Kerry/Xie relationship should be the model for their governments, to act before it is too late. “With luck, their successors will realise that Kerry and Xie together just barely began to put the world’s two largest economies on paths toward far lower emissions and greater climate cooperation, in a shared determination to prevent global destabilisation and climate calamity,” he said.