Global momentum is building on the climate crisis but action will be impossible without two nations, China and the United States, which together account for more than half of emissions -- and whose governments don't get along.
Ahead of the COP26 summit in Glasgow, experts believe that breakthrough US-China cooperation could be the catalyst for a historic agreement on climate change -- but also that frosty ties between Washington and Beijing are not, so to speak, the end of the world.
Both nations have stepped up efforts to curb emissions, although analysts say that actions are far too modest to meet a UN-backed goal of keeping the planet's temperature rise to 1.5 degrees Celsius (2.6 degrees Fahrenheit) and avoiding the worst effects of climate change.
"If the national governments of China and the US are not able to agree on anything of substance, I think there may well be room for serious action anyway, because both countries are able and willing to do a lot on their own," said Mary Nichols, who led major climate initiatives as chair of the California Air Resources Board.
"But that doesn't mean that it's irrelevant," she said. "Without an explicit agreement, other countries will be reluctant to act."
US President Joe Biden's administration has described Beijing as his country's top long-term challenge and raised pressure on concerns from human rights to Taiwan to trade but has sought engagement on climate.
"It is not a mystery that China and the US have many differences. But on climate, cooperation -- it is the only way to break free from the world's current mutual suicide pact," John Kerry, the US climate envoy, said in a recent speech.
- 'Race to top'? -
Kerry has traveled twice to China despite a chill in relations. But on his latest visit, Foreign Minister Wang Yi issued a warning.
"It is impossible for China-US climate cooperation to be elevated above the overall environment of China-US relations," Wang said.
The remarks raised concern in Washington that the Biden-Kerry approach could backfire, allowing China to use climate as leverage.
But Chinese President Xi Jinping soon afterward took a major step by telling the United Nations that Beijing would stop funding coal in its overseas infrastructure-building blitz, although it is still investing at home in the dirty but politically sensitive form of energy.
Alex Wang, faculty co-director of the Emmett Institute on Climate Change and the Environment at the University of California, Los Angeles, said that China and the United States could engage in a "race to the top" on who does more.
"It improves China's global reputation to appear as a positive actor on climate," Wang said.
"If the leaders in China feel like they are becoming laggards, I think it would lead to some pressure to act further, and it would be a reason to disregard the voices from fossil fuel industries or coal industry within the country," he said.
"But without the pressure then the balance shifts in favor of slower action."
He drew a contrast with Biden's predecessor Donald Trump, whose climate skepticism meant little pressure on Beijing to tackle coal.
- Potentially powerful steps -
Nichols, who helped design California's cap-and-trade program that creates a market with incentives for reducing emissions, said one major step would be if China agreed to link efforts to set a common price on carbon.
"That would, I think, send an extraordinarily strong signal to investors and businesses around the world," said Nichols, now a fellow at Columbia University's Center on Global Energy Policy.
California already has linked its market to Quebec, taking a lead in action even before Biden's election.
With so many areas of tension between the United States and China, a multilateral process such as COP26 may be more effective in any case than bilateral talks, said Jacob Stokes, a fellow at the Center for a New American Security.
"Neither side wants to be seen as doing this as some sort of favor to the other side," he said.
And with China already the world's second-largest economy, Stokes said that US policymakers may want to focus climate diplomacy on poorer nations.
"Is it more important to expend effort to get concessions from Beijing or to try to finance expansion of clean power in the rest of the developing world that still has a lot more energy-intensive development to do?"