Backers of a global accord to fight climate change that formally comes into force on Friday say they are confident the deal can survive any legal challenges by U.S. Republican candidate Donald Trump if he wins next week's presidential election.
Trump has threatened to reject the accord negotiated by nearly 200 governments, including that of U.S. President Barack Obama. Trump once tweeted that global warming was a concept invented by the Chinese to harm U.S. industry.
By contrast, his Democratic rival Hillary Clinton has strongly supported the plan, which enters into force after major greenhouse gas emitters the United States, China and India ratified it. The Paris Agreement aims to phase out man-made greenhouse gases in the second half of the century to avert floods, heat waves, droughts and rising sea levels.
Delegates from signatory nations meet in the Moroccan city of Marrakesh on Nov. 7-18 to start turning their many promises on tackling climate change into action and draw up a "rule book" for the sometimes fuzzily worded accord reached last December.
Adding urgency, 2016 is set to be the hottest year since records began in the 19th century.
The U.S. election, on Nov. 8, injects a large dose of political uncertainty into the gathering of environment ministers and senior officials in Marrakesh.
The relatively speedy ratification of the climate pact, a year before many predictions, was partly driven by Trump's threats to reject the accord. Some delegates played down the risk of a President Trump reneging on the deal.
"The momentum is such that it would take a heck of a climate sceptic to back pedal," said Tosi Mpanu Mpanu of the Democratic Republic of Congo, who heads the group of 48 Least Developed Countries at the Morocco negotiations.
"That's really why we have had this strong mobilization [to ratify]. If Trump wins, I don't think it will change anything."
Its entry into force gives some protection for the Paris Agreement under international law. Its Article 28 says any government wanting to quit has to wait through four years of formalities — the length of a U.S. presidential term.
If he wins, Trump is likely to focus his efforts domestically on undoing Obama's plans to cut U.S. emissions, such as from coal-fired power plants.
The Paris Agreement also helped to unlock U.N. accords this year to limit emissions from airlines and from powerful greenhouse gases used in air conditioners and refrigerators.
"There has been astonishing progress" on climate change after years of deadlock, said Andrew Steer, head of the World Resources Institute think-tank.
The global pacts contrast with more nationalist forces that helped Trump to win the U.S. Republican nomination for president and, on the other side of the Atlantic, Britain's vote in June to leave the European Union.
The Paris Agreement enters into force after passing a threshold last month of ratification by 55 nations representing 55 percent of greenhouse gas emissions. The tally is now 87 nations representing about 60 percent of emissions.
The United States alone accounts for 18 percent of global emissions. Under a Vienna Convention on the Law of Treaties, Trump could not renounce the U.S. share and then argue that the entire entry into force was invalid.
"Entry into force only goes one way. It can't be reversed," said Elliot Diringer, executive vice-president of the non-partisan Center for Climate and Energy Solutions in Washington.
But there could be legal challenges. The U.S. Republican Party platform says Obama, a Democrat who used his executive powers to ratify the agreement, should have submitted it to the Senate, where it would almost certainly have been rejected.
Governments in Marrakesh will try to bolster the pact, which sets many goals for cuts in greenhouse gas emissions by 2025 or 2030. Governments are under pressure to spell out what they will do now rather than leave it to their successors.
At its core, the Paris Agreement promises to limit global warming to "well below" 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times. But current pledges for limiting emissions are too weak and will allow temperatures to rise by perhaps 3C (5.4F) or more by 2100.
Diplomats expect it will take two years or more to write a rule book for the accord. Among difficulties, the Agreement allows all countries set their own targets for emissions.
That free-for-all design helped ensure wide support but is at odds with a need for a central set of rules to monitor and report greenhouse gas emissions from factories, cars, farms and power plants.
Irrespective of the rules, the prices of renewable energies are falling sharply, prompting a shift from fossil fuels.
U.S. climate envoy Jonathan Pershing said wind and solar energy accounted for two-thirds of all new electricity-generating capacity installed in the United States in 2015. "The U.S. solar industry now employs more people than coal," he said.
Still, Trump says he will promote the U.S. coal and oil industries to safeguard domestic jobs and national security. He says a shift from fossil fuels would force the United States to be "begging for oil again" from Middle East suppliers.