Delegates from around the world have kicked off two weeks of United Nations talks in Poland on tackling climate change, three years since the landmark 2015 Paris deal to shift away from fossil fuels.

The COP24 climate summit opened in the mining city of Katowice on Sunday, a day earlier than originally planned because of the large number of issues that need to be resolved by December 14.

"This is a very, very important conference," Patricia Espinosa, the UN's climate chief, said.

"It also takes place in a scenario where we have clear signals about the urgency with which we need to address the issues of climate change."
Four former leaders of UN talks, including Laurent Fabius of France, who led negotiations for the Paris agreement, issued a statement urging immediate action.

"The world is at a crossroads and decisive action in the next two years will be crucial to tackle these urgent threats," they said in the joint statement.

However, political divisions were clear from the outset, with Brazil having withdrawn its offer to host the 2019 talks.

The United States, meanwhile, reiterated at the G20 summit in Argentina on Saturday its decision to withdraw from the Paris accord and Its commitment to all energy sources.

The other members of the group of industrialised nations - including the biggest polluter, China - reaffirmed their commitment to implementing the Paris deal, taking into account their national circumstances.

The talks in Katowice - at heart of Poland's coal region - precede an end-of-year deadline to produce a "rule book" to flesh out the broad details that were agreed in Paris on limiting the rise in global temperatures to between 1.5 and 2 degrees Celsius.

Delegates said that one of the trickiest issues could be monitoring emissions as the US, which cannot quit the pact until 2020, uses the talks to press for a level of detail it perceives as useful to its foreign policy dealings.

But the negotiations got off to a chaotic start, with the opening session delayed nearly three hours by a series of last-ditch submissions.

Separately, tens of thousands of people marched on Sunday in Brussels, calling on governments to respect commitments on countering climate change.

Demonstrators marched through the quarter of the Belgian capital that houses the headquarters of the European Union, with banners bearing slogans including "There is no planet B" and "Climate First, Politics Second".

A string of major climate reports have cast doubt over the entire process to avert runaway global warming, suggesting the Paris goals fall well short of what is needed.

Just last 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.

For his part, Brazil's President-elect Jair Bolsonaro vowed to follow Washington's lead during his campaign.

Many 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.