Representatives from nearly 200 countries began crunch UN climate talks in Poland today against a backdrop of dire environmental warnings and a call for action against the "urgent threats" posed by climate change.

The UN climate summit comes at a crucial juncture in the global response to planetary warming.

The smaller, poorer nations that will bear 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 2C 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.

UN General Assembly president Maria Espinosa said that mankind was "in danger of disappearing" if climate change was allowed to progress at its current rate.

"We need to act urgently, and with audacity. Be ambitious, but also responsible for the future generations," she added.

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.

G20 leaders yesterday 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.

The UN's environment programme said this week that 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 five-fold.

While the data is 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.

Incoming Brazil leader Jair Bolsonaro 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.

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.