The world is "doomed" unless a "historic pact" is signed between rich and poor countries, UN Secretary General Antonio Guterres said on the eve of the Cop27 climate change conference in Egypt.
The gathering of world leaders, business figures and environmentalists is being hosted in the Egyptian Red Sea resort of Sharm El Sheikh, after last year's summit in Glasgow.
Speaking to Britain's Guardian newspaper, Mr Guterres said the result of present policies and dynamics between the developed and developing world, if unchanged, would be "absolutely catastrophic".
"There is no way we can avoid a catastrophic situation, if the two [the developed and developing world] are not able to establish a historic pact," he said.
Wealthy nations have long promised to find $100 billion a year to help poor nations adopt climate-friendly policies, with the target originally meant to have been hit in 2020.
It was first promised in 2009, but has never been accomplished. A report last year suggested it would not come to fruition until 2023.
Mr Guterres also defended his use of dramatic language, saying the world is approaching "tipping points" that will "make [climate breakdown] irreversible".
"And as we are approaching those tipping points, we need to increase the urgency, we need to increase the ambition, and we need to rebuild trust, mainly trust between north and south," he said.
"Because at the present level, we will be doomed. Present policies will be absolutely catastrophic.
"And the truth is that we will not be able to change this situation if a pact is not put in place between developed countries and the emerging economies."