On a November day back in 2014, then US president Barack Obama and China's leader Xi Jinping stood side by side in Beijing’s Great Hall of the People to deliver a historic commitment to curb carbon emissions.
It was a huge moment, because these two superpowers were then - as they are now - the biggest polluters on the planet.
"The United States of America and the People's Republic of China have a critical role to play in combating global climate change, one of the greatest threats facing humanity," their joint statement said.
"The seriousness of the challenge calls upon the two sides to work constructively together for the common good," it said.
That landmark deal paved the way for the Paris Agreement the following year at COP21, legally binding nations to monitor and report their efforts to reduce greenhouse gas emissions.
Now here we are ten years later, and things are, well, a bit different.
One of the first things President Donald Trump did, after being sworn into office for the second time, was pull the United States out of that Paris Agreement.
Taking a sharp turn away from his predecessor's drive towards green energy with the Inflation Reduction Act - which poured billions into renewable development - Mr Trump promised instead to "drill, baby, drill".
Wind turbines ("pathetic") and solar panels ("big ugly patches of plastic") were out.
Oil and coal were in.
It was a message that Mr Trump was keen to export.
Talking to world leaders at the UN General Assembly in September, he said climate change was "the greatest con job ever perpetrated on the world, in my opinion".
"I'm telling you if you don’t get away from the green energy scam, your country is going to fail," he said.
It soon became clear that it wasn’t just empty rhetoric.
The following month, at a UN-backed conference in London to finalise a shipping emissions agreement, the US, backed by Saudi Arabia and Russia, aggressively lobbied other countries to postpone it.
In a communication issued ahead of the International Maritime Organisation (IMO) event, the US threatened sanctions on countries and individual officials "sponsoring activist-driven climate policies that would burden American consumers".
People were "summoned to the US Embassy in London - intimidation, threats of cessation of business, threats of family members losing visas," according to one European official quoted by Politico.
Organisers were left reeling.
"That was not the normal, regular IMO meeting it is not the way we normally conduct our business," said IMO Secretary General Arsenio Dominguez.
Ralph Regenvanu, the climate minister of the small Pacific island nation of Vanuatu, said it was "unacceptable given the urgency we face in light of accelerating climate change".
But the US Secretary of State Marco Rubio praised the delay as "another huge win" for President Trump.
Now, as all eyes turn towards Brazil for COP30, the US assault on climate policies is hard to ignore.
But there won't be any aggressive lobbying this time, not least because the US isn't even sending a delegation.
The Brazilian hosts brushed off the US boycott.
"All the others are here - we have confirmations for almost the totality of parties within the climate regime," Ambassador Liliam Chagas, Brazil's lead climate negotiator, told RTÉ News.
"Would be better if they could be on board," she added, "but if the US cannot, we will have to keep working with the rest of us who are very concerned with the speed of the impacts of climate change, on our societies".
Speaking via video link from Belém where the final preparations for the two-week conference were under way, she said the heat and humidity there would help to remind delegates of the task at hand.
"You step outside, it's like walking in a sauna," she said.
As the US steps back from its climate leadership, China is stepping up.
President Xi put in a surprise video appearance at the UN's Climate Week in September, to double down on China’s commitment to the Paris Agreement.
In a pointed opener, he said "green and low-carbon transition is the trend of our time, while some country is acting against it, the international community should stay focused on the right direction," he told delegates.
He went on to announce China's 2035 targets to reduce emissions by 7%, increase the share of renewable energy consumption to more than 30% and expand the installed capacity of wind and solar power to over six times 2020 levels.
Still not enough according to climate campaigners, given the 14 billion tonnes of carbon dioxide that pumps into the atmosphere every year.
But the appearance put China in pole position on climate.
After all, it came just a day after his American counterpart had told the General Assembly: "Windmills are so pathetic and so bad".
There's also widespread recognition that China is leading the charge on the green energy transition.
"China is doing an incredible amount of progress in terms of developing clean energy alternatives in China and also providing equipment and what we need to have more clean energy worldwide," Ms Chagas said.
"It's really a great revolution going on," she added.
"China is definitely stepping into the space and vacuum that's been left by the US," said Kaveh Guilanpour, a former EU and UK climate negotiator, now with the Center for Climate and Energy Solutions, Washington DC.
"This is why it's sad to see from the perspective of the US that international discussions about the future shape of the global energy system and the acceleration towards renewable energy deployment is something that's going to be handled at COP30 and the US won't be there."
As the paths of the world's most polluting nations diverge, it has the potential to leave other countries with a dilemma on what some observers have called the "petrostate versus electrostate".
For decades, global geopolitics were shaped by fossil fuels, Rachel Kyte, the UK's climate envoy recently told the Cleaning Up podcast.
"We're now in a world where the geopolitics is still partially shaped by the sort of fat tail of oil and gas, and they're now being shaped by access to the minerals and metals and the kit that you need for renewable energy," she said.
This informs bilateral discussions between India and the US as well as relations between the EU and China, she said.
"And then what you see is Africa, Latin America and Southeast Asia sitting there saying, we don't want to have to be completely, sort of, indebted to one pole or the other," she added.
"We need to find our own way."
