After a string of damning environmental reports showing mankind must drastically slash its greenhouse gas emissions to avert runaway global warming, Guterres told delegates "we are still not doing enough, nor moving fast enough".
The nearly 200 nations that signed up to the 2015 Paris climate deal must this month finalise a rulebook to limit global temperature rises to well below two Degrees Celsius, and to the safer cap of 1.5C if possible. But the rate of climate change is rapidly outstripping mankind's response.
With just one Celsius of warming so far, Earth is blighted by raging wildfires, extreme drought and mega-storms made worse by rising sea-levels.
"Even as we witness devastating climate impacts causing havoc across the world, we are still not doing enough, nor moving fast enough, to prevent irreversible and catastrophic climate disruption," Guterres said.
Some of the nations most at risk from climate change will have the chance Monday to plead the case for immediate action. Frank Bainimarama, prime minister of Fiji and president of last year's COP climate talks, said nations must act now to stave off disaster.
"Or, God forbid, (we) ignore the irrefutable evidence and become the generation that betrayed humanity," he said.
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