When the world triumphantly celebrated the signing of the landmark Paris climate pact last December, it was hard to imagine that only a year later it might face an existential threat.

Then again, who could have predicted at the time that a self-promoting reality TV impressario -- and avowed climate sceptic -- was months away from capturing the White House?

"The Paris Agreement was bound to be tested sooner or later," said Myles Allen, head of the climate research programme at the University of Oxford's Environmental Change Institute.

"It has just come sooner than most expected."

Campaign promises to "cancel" the 196-nation deal notwithstanding, there are reasons to think that US President-elect Donald Trump will not seek to derail it, or that he would fail if he tried.

For one thing, the first universal action plan for curbing global warming -- in force since last month -- has already been ratified by the US and 116 other countries.

That makes pulling out a highly visible and lengthy process, lasting at least four years.

"Overtly withdrawing has a cost," both political and economic, said Princeton international affairs professor Michael Oppenheimer.

Countries deeply invested in the agreement -- including China, the European Union and almost all the world's developing nations -- would likely register displeasure in other arenas.

The idea of a carbon tax on US goods, for example, has been mooted.

- Lost opportunity -

But should the US turn away from the global transition to clean energy, the highest cost would be lost opportunity.

In 2015, renewables outstripped fossil fuels globally for the first time in attracting investment, and overtook carbon-rich coal as a source of electricity.

Trump may find that his options within the US are also limited.

Domestically, he has threatened to scrap Barack Obama's Clean Power Plan, defang the Environmental Protection Agency, and shelve incoming regulations designed to push down US greenhouse gas emissions.

Under the Paris Agreement, Washington has pledged to cut US carbon pollution 26-28 percent by 2025 compared to 2005 levels.

US emissions have declined in recent years, albeit slowly.

But the main drivers have come not from the federal government but the market and individuals states, and these forces are likely to dominate no matter what Trump does, analysts say.

"Trump will have little effect on trends in the US power industry, where coal is being rapidly replaced by natural gas and renewables," said William Sweet, an energy expert at NYU Tandon School of Engineering.

Pouring money into new coal-fired power plants -- a sector Trump has vowed to revitalise -- no long makes economic sense, Sweet and others said.

Market momentum, however, is not enough to win the race to cap global warming under two degrees Celsius (3.6 degrees Fahrenheit), the "do-not-cross" red line set down in the Paris treaty.

An increase so far of 1 C (1.8 F) compared to pre-industrial era levels has already caused on uptick in deadly storms, droughts, wildfires and flooding.

- 'Leadership vacuum' -

National carbon-cutting pledges annexed to the Paris pact would, at best, yield an unliveable 3 C world.

On top of all this, virtually all of the climate-saving scenarios laid out by scientists depend on technologies for sucking carbon out of the air that don't even exist yet.

This suggests that political will -- at a national and global level -- remains critical for continued progress.

And that could be a problem.

"There is a real risk of a leadership vacuum," said Thomas Spencer of the Institute for Sustainable Development and International Relations in Paris.

The G20 meeting next July in Hamburg, Germany will offer the first clear clue as to whether Germany, China -- if any nation -- can step up to fill the void if the US disengages, he said.

A Trump administration hostile, or simply indifferent, to climate change action could dim the odds of preventing dangerous warming.

In the US, the auto-industry has already indicated it will try to water-down impending fuel efficiency standards, while stringent rules on capping gas-industry methane leaks are likely a dead letter.

Even if Trump doesn't do a complete about face on climate, "we are likely to see a slowing down of progress compared to what would have happened if Clinton had been elected," said Oppenheimer.

Internationally, he said, this will have repercussions.

"Countries could say, 'if the US is not going to take their (emissions reduction commitments) seriously, we're not going to either'."

Scientists point to recent red flags.

Shattered temperature records in the Arctic; evidence that Greenland's ice sheet, which could raise sea levels by six metres (20 feet), is far more sensitive to warming than thought; an unexplained surge in emissions of methane, a greenhouse gas 28 times more potent than CO2 -- all of which suggest that the margin of error has largely disappeared, they say.

"Nature will have surprises in store," Allen said.