British Prime Minister Gordon Brown says failure to reach a deal at the Copenhagen climate conference may trigger an economic disaster equivalent to the combined effects of the two world wars and the Great Depression.
The talks are deadlocked over key issues including the size and verification of emissions cuts and financial aid to poorer countries.
Mr Brown is one of the first of the world leaders to arrive for the 193-nation summit, which finishes at the end of the week.
He arrived in Denmark on Tuesday, two days ahead of schedule to help broker a deal. He says the chaos caused by unchecked global warming would cost Britain up to a fifth of its national income.
Mr Brown says failure to tackle climate change would hurt Britain's standard of living, predicting a drop in national income of up to 20%.
Australian Prime Minister Kevin Rudd is also in Copenhagen now, warning the summit may fail.
Mr Rudd says there's absolutely no guarantee of success, adding he believes in telling it like it is.
China, small African nations stay united
The chief negotiator for China and the small African nations has accused Mr Rudd of lying to the Australian people about his position on climate change.
Lumumba Di-Aping represents China and the G77 group of small countries in the Copenhagen talks.
The ABC reports the G77 and China claim that the talks have broken down, degenerating into a fight between the developed and the developing world.
Mr Di-Aping accused Mr Rudd of trying to gain a strategic economic advantage by siding with the United States and the European Union at Copenhagen.
He says the talks have reached a deadlock because the developed world is not committed to helping poor countries in their efforts to adapt to and mitigate the effects of climate change.
But the White House says US President Barack Obama, who will arrive in Copenhagen later in the week, is confident of reaching a deal.
His spokesperson Robert Gibbs told a briefing the president believes an operational agreement that makes sense can be reached in Copenhagen.