Paris: Politically sidelined since a 2009 UN summit almost ended in a bustup, climate change has resurfaced as a priority but faces a brutal test at talks opening in Lima on Monday.
The 12-day haggle will show what happens when high-minded global vows enter an arena where national interest rules the roost.
"Things are still fragile, but there's been good news," said French climate negotiator Laurence Tubiana -- pointing to recent political noises, and pledges of money, to protect Earth's climate system.
The meeting in the Peruvian capital is tasked with steering 196 parties toward the most ambitious climate pact in history "the matrix for managing climate issues for the next 30 years," according to Tubiana.
Due to be signed in Paris in December 2015 and take effect from 2020, the accord would roll back carbon emissions and ease the threat of misery, ecosystem damage and species loss for future generations.
Given the UN's shaky record on climate change, the challenge is vast.
The last climate treaty was the Kyoto Protocol, inked in 1997. A bid to follow it up in Copenhagen in 2009 was a near fiasco.
But in recent months, the political tone has changed, strengthening hopes that the creaking negotiations may finally yield a result.
"The broad outlines of a deal have begun to emerge and the three largest emitters have stepped forward early and encouraged others to follow suit," said Elliot Diringer of the Center for Climate and Energy Solutions (C2ES), a US thinktank.
"We're in about as good a shape in these negotiations as I think we've been for some time."
Science has been a big driver of the shift.
In a new overview, the UN's Intergovernmental Panel on Climate Change (IPCC) painted a tableau of a carbon-ravaged world just a few generations from now if emissions continue their present trajectory.
By 2100, hunger, drought, floods and rising seas would stalk humanity and spur the risk of bloodshed as nations fight for resources.
Against this grim backdrop, UN Secretary General Ban Ki-moon staged a summit in New York in September where leaders renewed oaths to fight the scourge. Hundreds of thousands of protestors rallied in cities around the world.