Republican nominee Donald Trump, who has described global warming as a "hoax," said in May he would "cancel" the climate-rescue Paris Agreement if elected leader of the free world.

The pact, concluded in the French capital last December after years of haggling, came into force on Friday -- its goal to stave off worst-case-scenario global warming.

Climate scientists and NGOs have long warned of dire consequences if Trump is elected.

On Friday, President Barack Obama warned in an interview with US talk show host Bill Maher that: "All the progress we've made on climate change", among other crux issues, "is going to be on the ballot" next Tuesday.

Can Trump withdraw the US?

Obama ratified the agreement, giving it a major boost towards the threshold of endorsements required for entry into force.

The cutoff was ratification by 55 of the 197 parties to the UN's climate convention (UNFCCC), representing 55 percent of greenhouse gas emissions.

That having been achieved, it would be cumbersome, but far from impossible, for a country to withdraw from the international treaty.

Trump would have three options.

1) He could withdraw the United States from the 1992 climate convention, the framework within which the deal was negotiated.

According to the terms of the convention, withdrawal would take effect one year after notification, and any departing state "shall be considered as also having withdrawn from any protocol", such as the Paris Agreement.

2) The agreement itself makes provision for parties to withdraw, but notice can only be given three years after Friday's entry into force. The actual withdrawal would take effect a year later.

Because an executive decision, rather than a vote in the Senate, led to US accession, "a Trump administration could simply withdraw", said international relations associate professor Robert Falkner of the London School of Economics.

3) Trump could abandon or ignore the US' national emissions reduction targets, which contribute to the overall international effort. Analysts say this is his most likely course of action.

The real estate mogul has already said he would expand offshore drilling, increase natural gas production and scrap the Environmental Protection Agency's Clean Power Plan.

Can the agreement survive without the US?

America is the world's second-largest greenhouse gas polluter, representing nearly 13 percent of emissions.

Trump's threats, if carried out, would recast the United States in the role of climate villain.

The Obama administration has been an ardent champion of the Paris Agreement, a role Trump's Democratic rival, Hillary Clinton, has vowed to continue.

Washington and Beijing played a key role in helping break the deadlock between negotiating parties in the tense lead-up to last year's make-or-break Paris conference.

Obama then beat the drum for the deal's rapid, pre-election, ratification.

By Saturday, it had been ratified by 97 parties representing 67.5 percent of emissions, meaning that without the US, it would fall just short of the 55 percent emissions threshold.

At the rate by which countries are signing up, however, it is likely to pass that critical point before year-end, observers say.