(AFP) - With the direst warnings yet of impending environmental disaster still ringing in their ears, representatives from nearly 200 nations gather Sunday in Poland to firm up their plan to prevent catastrophic climate change.

The UN climate summit comes 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 (3.6 degrees Fahrenheit), and to the safer cap of 1.5C 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.

Johan Rockstrom, designated director of the Potsdam Institute for Climate Impact Research, said the talks in the Silesian mining city of Katowice were crucial in nailing down how the Paris promises will work in practice.

Delegates at the COP24 talks "cannot and will not discuss if governments worldwide must achieve rapid greenhouse gas emission reductions to limit climate risks -- but how they can do this," he said.

In Katowice, 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.

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.

- '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.

A group of over 90 independent climate scientists in October said fossil fuel use must be slashed by half in the next 12 years if we have any hope of hitting the 1.5C target.

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

Laurence Tubiana is CEO of the European Climate Foundation and, as France's top negotiator, one of the main architects of the 2015 treaty.

"Katowice may show us if there will be any domino effect" following the US withdrawal, she told AFP.

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 right now.

"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.

And Gebru Jember Endalew, chair of the Least Developed Nations group of negotiators at the COP24, said "international cooperation is the only way to address the global threat of climate change."

© Agence France-Presse