(CNN) One question looms above all others in United States climate envoy John Kerry's climate diplomacy agenda: What can the US do about China?
The two countries -- the world's biggest greenhouse gas emitters -- have seen bilateral relations plunge to their lowest point in decades over human rights, security and trade. While Washington has sought to keep climate talks separate from thornier subjects, Beijing has had other ideas.
As Kerry pressed Chinese officials to be more ambitious in their plans to move away from coal faster and slash greenhouse gas emissions, they pushed back to ask what could be done about US sanctions, according to Kerry's own account of the talks. CNN has reached out to the Chinese foreign ministry for comment, and officials have previously said that US political pressure in areas like rights and trade was hindering cooperation on climate.
All eyes will be on the two countries during the United Nations climate summit in Glasgow this November, also known as COP26. Just about every COP in recent history has essentially come down to the US and China rivalry, and Glasgow looks to be no different.
Two big things have happened since the Paris Climate Accord in 2015 that could make US and Chinese cooperation on climate more difficult: broader US and China relations haven't improved much since the end of the Trump administration, and China has complained that the goalposts have moved from keeping global warming below 2 degrees Celsius to unequivocally at or below 1.5 degrees.
Worsening climate change is evident in the catastrophic rains and flooding that has hit both the US and China this summer, and Kerry has hammered home the point to Chinese officials that climate is not a partisan or ideological issue -- it's one that is very tangible and impacting the entire world.
"Climate change is one of the only areas where there's real dialogue in working together," said former White House and State Department official David Sandalow, who now is a fellow at Columbia University.
As President Joe Biden's top international climate official, Kerry's mission is to convince China to abandon coal as a power source, and speed up its timelines to both peak its emissions (currently sometime before 2030) and go carbon neutral (by 2060). Kerry told reporters on Thursday morning that if China brings another 200 gigawatts of coal power online, as it has planned to do, it would "undo the ability of the rest of the world to achieve a limit of 1.5 degrees."
"We have an opportunity to have positive impact in Glasgow, but it really depends on choices at this point that China makes because we've made our choice," Kerry said. "President Biden has put America on a course to have a 50-52% reduction" of carbon emissions relative to 2005 levels by 2030.
Still, Biden's own decarbonization goal has not yet been met, and current and former US officials and lawmakers told CNN it largely depends on whether Congress passes a $3.5 trillion budget reconciliation bill containing most of Biden's climate agenda, which includes a massive investment in clean energy tax credits as well as a new program to push the country's electricity to 100% renewable by 2035. It's uncertain whether that can pass Congress, as it faces some opposition even from within Biden's own party, and whether it indeed will give Kerry and Biden the climate credibility they so badly need at COP26.
Former US climate officials and lawmakers told CNN the US needs to demonstrate action, not promising rhetoric, at Glasgow this fall.
"Kerry needs the domestic action to have any chance to organize the diplomatic posture," said Center for American Progress founder and Obama climate adviser John Podesta.
China will be eager to show it isn't bending to the US, meaning Kerry will need to ensure he doesn't push so hard that it backfires. He could give Beijing more political ammunition to not cooperate, or to make demands, such as dropping sanctions for rights abuses.
Negotiating with China in the run-up to COP26 will be a complex balancing act. Kerry, a former secretary of state, has a good working relationship with Chinese Climate Envoy Xie Zhenhua, who negotiated the Paris Accord with the Obama administration. The two just met in Tianjin, spending about 20 hours with each other over two days. One former US official told CNN that while Xie cares deeply about the issue of climate change, he also is not going to break ranks from China's party line.
China's greenhouse emissions largely come from coal -- which generated over half of the country's energy in 2019. The country's emissions far outpace other developed nations. The nonpartisan Rhodium Group found China emitted 27% of the world's total emissions in 2019, compared to 11% from the US, 6.6% from India and 6.4% from the European Union. China is also financing new coal projects overseas, including in African countries, which could be a big focus at COP. Some lawmakers and US officials want to see America spending more to help developing nations modernize and green their electricity grids to help move them away from coal.
At the same time, China is also making strides in renewable energy, producing much of the world's solar panels.
"China's climate progress is filled with irony and has been two steps forward, one step back," Li Shuo, a climate analyst with Greenpeace in China, told CNN in an email interview. "It is the biggest manufacturer, investor, and developer of renewable energy on one hand, but single-handedly burns half of the world's coal."
Still, warm relations between Kerry and Xie won't erase four years of lost climate diplomacy and worsening tensions under the Trump administration. Kerry also met via videolink with China's Foreign Minister Wang Yi, who told Kerry that the US should stop seeing China as a "threat and opponent," according to a Chinese foreign ministry statement Wednesday.
On climate and other issues, China is adamant that it doesn't want to be seen as caving to pressure from other countries.
"President Xi Jinping has repeatedly emphasized that [we] address issues on climate change not because other countries ask us to, but because we want to do it ourselves," China's foreign ministry told CNN in a statement. "No matter what other countries do, China will unswervingly and actively respond to climate change and successfully implement its own climate goal into place."
To prompt action from China, the US will need to galvanize its own allies to push Beijing harder. But to get its allies on a united front, the US needs to work on its own climate credibility.
"I don't think it's the US alone because we don't have enough carrots or sticks to get them to change," said Podesta, who oversaw the Obama administration's climate agenda. "But the US can organize diplomatically as well as have credibility to make something happen here."
To do that, the Biden administration needs to get moving and bring some climate wins to Glasgow.
Former California Gov. Jerry Brown, chair of the California-China Climate Institute at UC Berkeley, told CNN that the Biden administration's narrative that the US is the world's climate leader holding China to account was overblown. It's not even clear if there is enough political will in the US to move away from fossil fuels, he said.
"All this posturing, this moral self-righteousness is counterproductive," he said, "and it's also based on a false premise that America is following a better path than it currently is."