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 urged other nations to follow suit by year's end.

Faced with a crescendo of climate impacts -- rising seas, deadly storms, drought and wildfires -- the world's nations have moved quickly since last year to tackle the still-growing menace.

This year is shaping up to be the hottest ever recorded, topping previous highs in 2014 and 2015.

"We have embarked on an effort to change the course of two centuries of carbon-intense development," said UN climate chief Patricia Espinosa, underscoring the scope of the challenge.

"The peaking of global emissions is urgent," she told the opening plenary.

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.

Royal said this was to be an "African COP", because the continent stands to suffer first and most from climate ravages, but also in recognition of ambitious plans to leapfrog to renewable energy.

- The battle to come -

Morocco, she noted, has said it will generate 54 percent of its electricity from renewables by 2030.

In Marrakesh, front-line diplomats are rolling 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.

The next high-stakes rendezvous, set for 2018, 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.

The Paris deal calls for capping global warming at under two degrees Celsius (3.6 degrees Fahrenheit), but Earth is currently on track to heat up, before 2100, by 3.0C (5.4F) -- a sure recipe for catastrophe, say scientists.

"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.

Renewable energy investment and installed capacity surged in 2015, outstripping fossil fuels in the first instance and overtaking carbon-intensive coal, in the second.

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 -- anticipating the rapid shift to a low- or zero-carbon global economy -- have also become crucial drivers of change.