The UN talks come at a crucial juncture in mankind's response to planetary warming.

The smaller, poorer nations that will bare its devastating brunt are pushing for richer states to make good on the promises they made in the 2015 Paris agreement.

In Paris three years ago, countries committed to limit global temperature rises to well below two degrees Celsius, and to the safer cap of 1.5 degrees Celsius if at all possible.

But with only a single degree Celsius of warming so far, the world has already seen a crescendo of deadly wildfires, heatwaves and hurricanes made more destructive by rising seas.

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"Climate change impacts have never been worse," Patricia Espinosa told journalists after Sunday's first negotiating session.

"This reality is telling us that we need to do much more."

In a rare intervention, presidents of previous UN climate summits issued a joint statement as the talks got underway, 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."

At the COP24 climate talks, nations must agree to a rulebook palatable to all 183 states who have ratified the Paris deal.

This is far from a given: the dust is still settling from US President Donald Trump's decision to ditch the Paris accord.

G20 leaders on 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.

The UN negotiations got off to a chaotic start in the Polish mining city of Katowice Sunday, with the opening session delayed nearly three hours by a series out last-ditch submissions from countries.

Even solid progress on the Paris goals may not be enough to prevent runaway global warming, as a series of major climate reports have outlined.

'Failure to act will be catastrophic'

Just this week, the UN's environment programme said the voluntary national contributions agreed in Paris would have to triple if the world was to cap global warming below 2C.

For 1.5C, they must increase fivefold.

While the data are clear, a global political consensus over how to tackle climate change remains elusive.

"Katowice may show us if there will be any domino effect" following the US withdrawal, said Laurence Tubiana, CEO of the European Climate Foundation and a main architect of the Paris deal.

Brazil's strongman president-elect Jair Bolsonaro, for one, has promised to follow the American lead during his campaign.

Even the most strident climate warnings -- spiralling temperatures, global sea-level rises, mass crop failures -- are something that many developed nations will only have to tackle in future.

But many other countries are already dealing with the droughts, higher seas and catastrophic storms climate change is exacerbating.

"A failure to act now risks pushing us beyond a point of no return with catastrophic consequences for life as we know it," said Amjad Abdulla, chief negotiator for the Alliance of Small Island States, of the UN talks.

A key issue up for debate is how the fight against climate change is funded, with developed and developing nations still world's apart in their demands.

Poorer nations argue that rich countries, which are responsible for the vast majority of historic carbon emissions, must help others to fund climate action.

"Developed nations led by the US will want to ignore their historic responsibilities and will say the world has changed," said Meena Ramam, from the Third World Network advocacy group.

"The question really is: how do you ensure that ambitious actions are done in an equitable way?"