UN talks to implement the landmark Paris climate pact opened in Marrakesh on Monday, buoyed by gathering momentum but threatened by the spectre of climate change denier Donald Trump in the White House.
Diplomats from 196 nations are meeting in Morocco to flesh out the planet-saving plan inked in the French capital last December.
"We have made possible what everyone said was impossible," said French environment minister Segolene Royal at the opening ceremony, in which she handed over stewardship of the climate forum to Moroccan foreign minister Salaheddine Mezouar.
Royal announced that 100 countries have ratified the Paris Agreement, which entered into force last Friday, a record time for an international treaty.
She appealed to other nations to do so by the end of the year.
Amid growing alarm at the gathering pace of climate change and its impacts -- rising seas, deadly storms, drought and wildfires -- the world's nations have moved quickly over the last year to tackle the still-growing problem.
But as 15,000 negotiators, CEOs and activists settle in for the 12-day talks in Marrakesh, all eyes are on the United States, where voting Tuesday could thrust Trump into the White House.
When it comes to global warming, the stakes could hardly be higher, US President Barack Obama has warned.
"All the progress we've made on climate change" -- including the Paris pact, decades in the making -- "is going to be on the ballot," he told TV talk show host Bill Maher on Friday.
Democratic candidate Hillary Clinton has vowed to uphold Obama's domestic energy policies and international climate commitments.
Experts in Marrakesh say the Republican candidate cannot carry out his threat to "cancel" the still-fragile accord, but a Trump victory might cripple it.
"It would be a shock, and I hope we don't see it," Laurence Tubiana, France's top climate negotiator told AFP.
"But if Trump is elected, I am sure that Wednesday morning you will hear everyone at the COP say, 'We'll stick to the Paris Agreement'", she added, using the acronym for the Conference of the Parties meeting.
In Marrakesh, front-line diplomats must roll up their sleeves and work through scores of procedural issues that will make the difference between success and failure.
They have informally set 2018 as the deadline for laying that groundwork, Royal told journalists Sunday.
Concretely, these include how to measure and track each nation's CO2 emissions; disbursing hundreds of billions of dollars in financing in a way that reassures both rich and recipient nations; and setting criteria for compensating poor countries devastated by climate-fuelled storms, droughts or floods.
2018 is also the next high-stakes rendezvous when nations can narrow the so-called "emissions gap" between their carbon-cutting pledges and the level of reductions needed to stave off dangerous levels of warming.
That gap is still huge, and getting bigger every year.
On current trends, the Earth will heat up about three degrees Celsius (5.4 degrees Fahrenheit) above the pre-industrial era benchmark, a sure recipe for climate catastrophe, according to scientists.
The Paris Agreement calls for capping global warming at under 2.0 C (3.6 F), and even 1.5 C (2.7 F) if possible -- a hugely daunting challenge.
"We have to plug that gap," Tubiana said. "The big battle of the next two years is how to get countries to increase their ambition."
The UN talks opened amid encouraging signs. New figures show that renewable energy attracted record investment of 300 billion dollars (270 billion euros) in 2015, outstripping fossil fuels.
Installed capacity of solar, wind and hydro also, for the first time, overtook carbon-intensive coal, which is in sharp decline.
A separate international agreement inked last month ensures the phase out of potent, manmade greenhouse gases known as HFCs, potentially shaving 0.5C (0.9F) off global temperatures by the end of the century.
Businesses -- ever-more present at the UN climate forum -- have also become crucial drivers of change, motivated by the need to anticipate the rapid shift to a low- or zero-carbon global economy.
At the same time, however, climate scientists are sending up red flags.
After two successive record-breaking years, 2016 is shaping up to be the hottest ever registered.
"If we don't start taking additional action now," warned UN Environment Programme head Erik Solheim, "we will grieve over the avoidable human tragedy."