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.
Given the UN's shaky record on climate change, the challenge is vast.
Also Read
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.
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.