A report to be published Saturday by the UN's Intergovernmental Panel on Climate Change will warn that climate change is "unequivocal" and may bring "abrupt and irreversible" impacts, BBC reported.

Among the report's top-line conclusions are that climate change is "unequivocal", that humankind's emissions of greenhouse gases are more than 90% likely to be the main cause, and that impacts can be reduced at reasonable cost.

The synthesis summary finalised late on Friday strengthens the language of those earlier reports with a warning that climate change may bring "abrupt and irreversible" impacts.

Such impacts could include the fast melting of glaciers and species extinctions.

"Approximately 20-30% of species assessed so far are likely to be at increased risk of extinction if increases in global average temperature exceed 1.5-2.5C (relative to the 1980-1999 average)," the summary concludes.

Between 75 million and 250 million people are projected to have scarcer fresh water supplies than at present.

The IPCC report says the evidence of a human role in observed warming is now "unequivocal."

Banki Ki-moon, the UN chief has called for urgent action on the issue.

"I believe we are on the verge of a catastrophe if we do not act. I am not scare-mongering. But I believe we are nearing a tipping point," he is as quoted as saying in the International Herald Tribune newspaper.

The publication of the document comes before an international conference is held in Bali, Indonesia, next month to discuss the climate crisis.

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