KATOWICE, Poland - With the direst warnings yet of impending environmental disaster still ringing in their ears, representatives from nearly 200 nations gathered on 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 2 C, and to the safer cap of 1.5 C if at all possible.

But with only 1 C of warming so far, the world has already seen a crescendo of deadly wildfires, heatwaves 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 under way 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. "We require deep transformations of our economies and societies."

In Katowice, nations must agree to a rule book 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.

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

For 1.5 C, they must increase fivefold.

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.

But wealthy states, led by the US, have so far resisted calls to be more transparent in how their contributions are reported－something developing nations say is vital to form ambitious green energy plans.

"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?"

AGENCE FRANCE-PRESSE