U.S. climate envoy John F. Kerry implored China on Thursday to move more aggressively to slow global warming, as the world’s two largest greenhouse gas polluters continue a delicate and fraught diplomatic dance ahead of a crucial international climate summit this fall.

“We’re behind the eight ball sufficiently that we need to reach for the highest ambition,” Kerry told reporters as he concluded a two-day visit to the northern city of Tianjin — his second trip to China this year. “We think that China can do more. We’ve said that very clearly. I said that to every leader I’ve spoken to in the last few days.”

Kerry acknowledged that China, whose greenhouse gas emissions now eclipse those of developed countries combined, has made important strides in areas such as renewable energy, and he said officials briefed him on far-reaching climate policies currently in the works. But if the sprawling nation of 1.4 billion people continues to construct coal plants and remains reliant on fossil fuels well into the future, he added, it threatens to unravel the efforts of countries around the world.

“The stakes are very high,” Kerry said. “It’s not a matter of ideology or politics or geostrategic think. It’s a matter of mathematics and physics.”

Kerry described conversations with his Chinese counterparts this week as “constructive” and “polite.” And while he remained optimistic Thursday that the United States and China could set aside deepening tensions in other areas and work together to slow the consequences of a warming planet, diplomatic strains between the two nations were on display again in recent days.

During a video meeting with Chinese Foreign Minister Wang Yi on Wednesday, Kerry was warned that worsening U.S.-China ties could derail cooperation on climate change, one of few areas of potential collaboration between the two rivals.

Wang called on the United States to stop treating China as “a threat and an adversary,” adding that climate change “cannot be separated” from the broader geopolitical environment and linking climate with other diplomatic obstacles in the increasingly tense relationship.

“The U.S. side hopes climate change cooperation can be an oasis in China-U.S. relations,” Wang said, according to a statement from the Foreign Ministry. “But if the oasis is surrounded by desert, sooner or later the oasis will also become desert.”

Kerry’s visit also put into sharp contrast how much has changed since world leaders forged the Paris climate accord in late 2015.

That unprecedented agreement became a reality, in part, because President Barack Obama and Chinese leader Xi Jinping came together the previous year and announced that their nations would embrace the global pact. Every other nation on the planet subsequently joined the Paris agreement, which seeks to limit warming to “well below” 2 degrees Celsius (3.6 Fahrenheit) over preindustrial levels.

Six years later, much has changed.

The United States walked away from the Paris agreement under President Donald Trump, and the federal government largely abandoned climate action until President Biden made the issue a priority again this year. Meanwhile, China is now responsible for roughly 27 percent of the world’s emissions — more than double that of the United States.

Two months out from COP26, a United Nations climate summit billed as a make-or-break moment in which the world must finally muster the political will to rapidly cut carbon emissions, there are few signs of the kind of high-profile partnership that helped usher the Paris agreement into being in 2015.

Rather, everything from trade restrictions to the covid-19 pandemic to China’s treatment of Uyghurs has strained the U.S.-China relationship. Meanwhile, the Biden administration has worked in tandem with European Union leaders to lock in ambitious new climate promises and pressure other major economies to follow suit — none more than China.

There have been flickers of optimism.

In April, the United States and China vowed in a joint statement to work together to combat climate change “with the seriousness and urgency that it demands.” That rare bit of common cause, which included few specifics, came as Kerry traveled to China to meet with his counterparts ahead of a White House climate summit.

Kerry in July called on China to reach peak emissions earlier than its current target of 2030. Without dramatic reductions by China, Kerry and others have said, the most ambitious aim of the Paris accord — limiting warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) — would be essentially impossible. The world has already warmed roughly 1.2 Celsius, with few signs of slowing as greenhouse gas pollution continue to rise.

China, which has said it plans to achieve net zero emissions by 2060, has bristled at the notion that it isn’t pursuing meaningful climate action. The nation has noted that it is still considered a developing country and should not necessarily be held to the same standards as developed nations.

“The reality is that without China successfully managing its net-zero commitment domestically and supporting others to do so overseas, the 1.5 target is unachievable,” Rachel Kyte, dean of the Fletcher School at Tufts University and a former U.N. climate adviser, said in an email Thursday.

“Finding a way to cooperate, without the Chinese feeling that the U.S. is dictating terms, or without the U.S. feeling as if it is hostage to Chinese ambitions, is at the heart of climate cooperation,” she added. “While many aspects of the U.S.-Chinese relationship may be framed as zero sum, cooperating on climate change — to be able to safely prosper on this planet — is not one of them.”

Kerry said Thursday he expects China to detail “very soon” how it plans to peak emissions and meet the aims of the Paris accord. Such an announcement, he said, would likely give “some indication” of whether the looming U.N. climate summit can deliver the kind of momentum that scientists say is crucial.

While he was briefed on parts of China’s plan, Kerry said, he did not know all the details and declined to elaborate before China unveils its intentions on its own timeline. “Is it enough? I don’t know the answer to that yet. But I do know they talk of it in terms that it will affect every aspect of society,” he said.

Observers say Wang Yi’s warning this week, in line with increasingly muscular posturing by Chinese diplomats over the past few years, may be an effort to ensure that China is not seen as bowing to the United States.

“It’s a very different geopolitical context between China and the U.S.” compared to the lead-up to the Paris agreement, said Nat Keohane, president of the Center for Climate and Energy Solutions. “What has changed, I think, is that China is in a more self-confident posture relative to the rest of the world and to the U.S. on a variety of things — more assertive, more willing to push its own interests.”

What remains true, he added, is that whatever path China chooses in the months and years ahead will shape the futures of people around the planet. “Whether China goes in a high-carbon or low-carbon direction is one of the most important forks in the road on climate,” Keohane said.

Kerry’s trip to Tianjin marks the third visit to China by a senior Biden administration official. As part of a campaign to isolate climate change from other points of diplomatic tension, Kerry has held more than a dozen talks with Xie Zhenhua, the top envoy on climate, even as ties between the two countries have deteriorated over human rights, technology and trade, the South China Sea, Hong Kong and Taiwan.

Kerry, who is also visiting Japan this week, noted Thursday that the United States and the E.U. have committed to cut emissions by more than half by the end of this decade. Other countries have also announced stronger targets, and still more could lock in bolder promises in coming weeks. But China remains the world’s biggest and most important question mark.

“It really depends on the choices at this point that China makes,” he said. “We’ve made our choice.”