A day after world leaders vowed to unite in a war on climate change, negotiators at the UN talks will now get down to the nitty-gritty, where many bitterly divisive issues await.
The heads of more than 150 nations gathered in the northern outskirts of Paris on Monday (local time) in a bid to inject political momentum into what many described as the last chance to avert climate calamity.
"Never have the stakes of an international meeting been so high, because it concerns the future of the planet, the future of life," French President Francois Hollande said in an opening speech.
"The hope of all of humanity rests on all of your shoulders."
US President Barack Obama, China's Xi Jinping and many other leaders vowed their nations would strive to limit heat-trapping "greenhouse" gases that stoke global warming.
The result would be a post-2020 pact that would save Earth's climate system for generations to came, they promised.
"The future is one that we have the power to change, right here, right now," said Obama, who will on Tuesday meet with leaders of low-lying island nations to highlight his commitment to help the most climate-vulnerable.
But similar lofty promises have come crashing down to Earth during more than two decades of UN negotiations.
The UN climate process concerns the use of fossil fuels, the backbone of the world's energy supply and that means huge interests are at stake.
For years, the annual parlay has been hobbled by finger-pointing and nit-picking, riven especially by arguments between rich and poor nations over who should bear most of the carbon-curbing burden.
Those divisions were quickly exposed on Monday, as leaders of developing nations hit out at rich countries for perceived hypocrisy in making demands to use less fossil fuels after carbon-burning their way to prosperity.
"The prosperous still have a strong carbon footprint and the world's billions at the bottom of the development ladder are seeking space to grow," Indian Prime Minister Narendra Modi said.
With the debates echoing in their ears and the leaders heading home, lower-level negotiators in the 195-member UN forum have been tasked with creating such a pact by December 11.
On Tuesday morning they will start slimming down a 54-page draft text that is a labyrinth of opposing positions.
French Foreign Minister Laurent Fabius has told diplomats they have until just Saturday to iron out as many differences as they can, before ministers take over to try to resolve the most intractable of political disputes.
Points of contention include agreeing on a systematic review of emissions-curbing pledges and ramping up climate funding for poor countries so that it reaches a promised €95 billion a year by 2020, and the legal status of the accord itself.
Poorer countries are also pushing hard for rich nations to transfer clean technology to help them avoid taking the path of carbon pollution.
Many powerful nations are comfortable with aiming to limit global warming to two degrees Celsius from pre-Industrial Revolution levels, even though this goal is a long way from being achieved.
But dozens of the most vulnerable nations - small island states and poor countries in Africa - are sticking by an optional target of 1.5degC, arguing more than 1 billion people would otherwise be in peril.
Still, there is optimism that a deal can be finally be reached, as nations have already gone further in the UN process this time than ever before.
Nearly all the nations involved have submitted voluntary carbon-curbing pledges, accounting for 95 percent of greenhouse gases currently being emitted.
This still puts Earth on track for warming of 2.7-3.5degC, according to UN climate chief Christiana Figueres.
But she says this creates the architecture for more ambitious plans in the years ahead that could keep global warming below the 2degC threshold.