The COP24 climate summit will see nations try to agree on ways of implementing the promises they made in the 2015 Paris treaty to limit temperature rises to avert runaway global warming.
The world has already seen first-hand the effects of climate change: a crescendo of deadly wildfires, heat waves and hurricanes made more destructive by rising seas.
In a rare intervention, presidents of previous UN climate summits issued a joint statement as the talks got underway in the Polish mining city of Katowice, calling on states to take "decisive action... to tackle these urgent threats."
"The impacts of climate change are increasingly hard to ignore," said the statement, a copy of which was obtained by AFP. "We require deep transformations of our economies and societies."
But parties at the talks, which were delayed for three hours on Sunday, face tough political headwinds.
And the dust is still settling from US President Donald Trump's decision to ditch the Paris accord.
G20 leaders Saturday agreed a final communique after their summit in Buenos Aires, declaring that the Paris Agreement was "irreversible."
But it said the US "reiterates its decision to withdraw" from the landmark accord.
Even solid progress in Katowice on the Paris goals may not be enough to prevent runaway global warming, as a series of major climate reports have outlined.