The agreement "will now be the instrument on which our future depends," said World Bank Group President Jim Yong Kim.
That's a daunting responsibility, even if the UN talks are still riding the political momentum that carried the deal – decades in the making – across the ratification finish line in record time.
Greenhouse gas emissions – which are pushing the planet into the red zone of dangerous warming – continue to climb, putting newly ambitious goals for capping rising temperatures potentially out of reach.
Even if all national carbon-cutting pledges joined to the Paris pact are kept, Earth will heat up some 3.0 degrees Celsius (5.4 degrees Fahrenheit) by century's end, a recipe for climate devastation, according to a UN Environment Programme report released Thursday, November 3.
Discussions on how to disburse $100 billion (90 billion euros) a year to poor, climate-vulnerable nations remain contentious, even as a major report estimates that the level of annual investment needed over the next 15 years in developing nations is 20 to 30 times that amount.
And hanging over the whole proceedings is the shadow of climate change denier Donald Trump, whose improbable run at the White House is gathering momentum in the campaign's final days.
"The US presidential election will loom large over the COP," said Liz Gallagher, senior advisor at climate thinktank E3G, using the acronym for the annual Conference of the Parties climate meet.
A Trump victory, most analysts agree, could cripple the Paris deal, which the Republican candidate has said he would "cancel".
A victory by his opponent Hillary Clinton – a vocal proponent of action on climate change – would surely trigger a huge, collective sigh of relief on Day Two of the 12-day conference, allowing the 15,000 attendees to get on with business.
'The battle is not won'
After the dramatic breakthrough in Paris last December, diplomats and experts are lowering expectations a year later.
"This is a catalytic COP, not a huge leap forward," said Alden Meyer, a veteran climate analyst with the Washington-based Union of Concerned Scientists.
For Laurence Tubiana, France's top climate negotiator for the Paris talks, "what is mainly at stake is setting a date for finishing the rulebook," she told Agence France-Presse.
Sweeping in scope, the Paris Agreement left more than 100 thorny issues to be worked out in nitty-gritty negotiations, from accounting methods for tracking cuts in CO2 emissions, to transparency in financing, to how to assess compensation for "loss and damages" from climate impacts.
By informal consensus, 2018 is the target for working out those rules.
More broadly, 2018 is the next critical rendezvous in the ongoing talks.
National carbon-cutting pledges that go into effect in 2020 fall seriously short of what is needed to cap global warming under 2.0C (3.6F) compared to pre-industrial era levels, the target set in Paris.
Average global temperatures have risen so far by 1.0C (1.8F) – enough to lift sea levels, trigger deadly storm surges, and unleash life-disrupting havoc on weather patterns.
"We have to plug that gap," said Tubiana. "The big battle of the next two years is how to get countries to increase their ambition."
"The battle is not won," she added.
Hottest year on record
Recent news from climate scientists is not comforting.
After two successive record-breaking years, 2016 is shaping up to be the hottest ever recorded.
And concentrations of CO2 in the atmosphere are still climbing, having passed a critical – if symbolic – threshold of 400 parts per million in 2015.
In two years' time – armed with a special report from the UN's climate science panel due in August 2018 – nations will take again take stock of the problem, and their capacity to react.
By then, most big greenhouse gas emitters will have likely presented mid-century strategies for scrubbing carbon from their national economies – a critical exercise that may avert bad decisions in future.
"We could be investing hundreds of billions of dollars in very expensive gas infrastructure that will get us to our goals for 2025 or 2030, but then will make the more important goals of 2050 impossible," said Andrew Steer, president and CEO of the World Resources Institute.
The good news is that renewable energy has surged faster and become cheaper than almost anyone imagined possible only a decade ago, accounting for 23% of energy production and attracting nearly $300 billion (260 billion euros) in investment in 2015. – Rappler.com