After several years of dismissing global action to fight climate change, US leadership was formally challenged this week by China announcing bold new climate pledges.
Former Vice President Joe Biden has pledged to reinvigorate US climate leadership if he wins the November 3 election against incumbent President Donald Trump.
Re-establishing that leadership role, however, may not be so easy, according to US and Chinese diplomats involved in past climate negotiations.
The 2015 Paris Agreement hinged on a pact between China and the United States, the world's two biggest emitters, to cooperate on climate action. Now, the United States under Trump is poised to exit the treaty on November 4, the day after the election.
And the once-careful negotiations between Washington and Beijing have unraveled to what experts say is the worst level in years. Under Trump, the United States has launched a trade war against China and blamed Beijing for the Covid-19 pandemic, while China has cracked down on pro-democracy protests in Hong Kong, imprisoned Uighurs in Xinjiang and escalated tensions in the South China Sea.
This week, the situation got even trickier as China's President Xi Jinping announced plans to be carbon neutral by 2060 and urged the world to step up to the challenge.
Making global climate progress without reviving the US-China relationship would be impossible, according to former US climate envoy Todd Stern and other key figures behind the Paris agreement.
China produces 29% of global emissions - more than the EU and United States combined. Taken together, the three regions account for just over half of global carbon dioxide emissions.
Biden's team would need to balance the forces of competition and cooperation with China, or else renewed climate cooperation won't get off the ground, Stern said.
"We will have to learn to manage a relationship marked by both competition and collaboration, working with allies to stand up against unacceptable Chinese behavior where necessary, while seeking to collaborate where we can and must," he wrote in an essay for the Brookings Institution this month.
In his speech at the UN General Assembly on Tuesday, Xi said China's CO2 emissions would peak before 2030. His pledge for China to achieve carbon neutrality before 2060 also marked the country's first commitment to a long-term target.
The announcement amounted to a "framing of competition between the US and China," said Andrew Light, who served on the US strategy team in UN climate negotiations under Obama.
In effect, Xi set the agenda on future climate negotiations, getting ahead of pressure from a potential Biden presidency to rein its coal use and plans to build coal plants worldwide, Light said.
Biden already has pledged that the United States will produce carbon-free electricity by 2035 and achieve net zero emissions across the economy by 2050. But his plans will require either executive action that can be challenged in court or legislation that would need to pass through Congress.
Biden would also find the EU much more assertive on climate today, compared with during the Obama era, as the bloc has placed climate action at the center of its policy framework, pledging to impose a carbon border tax and to invest in clean technologies.
If Trump wins the 2020 presidential election, China would take "advantage of the fact that the US has been absent on this front" and "enhance its global positioning" around climate change, said Peter Kiernan, lead energy analyst at The Economist Intelligence Unit.
Officially, China insists its position on climate negotiations will remain the same, regardless of who wins the election in the United States, and claims that re-engagement with the United States is not necessarily a priority.
China's top climate official, Li Gao, said in a September 7 speech that while China would "proactively" and "unswervingly" fulfill its national commitments on climate change, global political complications were making things harder.
"Under an accumulation of factors such as unilateralism, protectionism and the spread of the novel coronavirus, the handling of global climate change is facing more difficulties," he said.
The Biden campaign has said the United States under his leadership would seek to work with China again on climate change, but would push Beijing to curb exports of coal technology and reducing the carbon footprint of its Belt and Road Initiative – a massive infrastructure project that would stretch from East Asia to Europe.
China has under construction hundreds of new coal plants and could build even more in the next five years.
The country is also expected to rely on energy-intensive infrastructure projects to try to accelerate its post-Covid-19 economic recovery.
Biden adviser John Kerry, former secretary of state and key player behind the Paris agreement, said that China's buildout of coal domestically and abroad would negate any past progress on climate change.
"That's going to kill the efforts to deal with climate," Kerry said earlier this month in a live-streamed discussion. This is why the United States needs to rebuild its climate partnership with China regardless of other disagreements, he argued.
"We are going to have to reach out, build up, but also be absolutely firm about the things that we disagree with."
Cementing the US-China bilateral agreement in 2014 took over four years of work and included the personal outreach of Stern, Kerry and Obama chief of staff John Podesta with their Chinese counterparts.
That same level of outreach would need to happen quickly now, including "confidence building measures" to help ease the "strong forces of nationalism in both countries," said David Sandalow, former under Secretary of Energy under Obama and China expert at Columbia's Global Energy Policy.
Those measures could include reopening diplomacy in areas such as green finance, or partnerships on carbon capture technology, he said.
That effort, even if difficult, is still possible if not essential, said Paul Bodnar, a State Department climate negotiator under Obama.
In the first year of Obama's administration, the US-China relationship "wasn't particularly rosy," Bodnar noted. Other nations were also wary of US climate leadership, after President George W Bush withdrew the US from an earlier global climate pact, the Kyoto Protocol.
"It took us three years to dig ourselves out of the hole of distrust we found ourselves in," Bodnar said.
Still, he said, regardless of what else is going on in the US-China relationship, they will have to find a way to work together.
"The fate of the planet depends on it. There is no other option," he said.