Experts warn of 'abrupt' warming

"This is the strongest report yet by the IPCC - but says that there is still time to act," Bill Hare, an Australian climate scientist and one of the authors, told Reuters.

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.

Other potential impacts highlighted in the text include:

between 75m and 250m people projected to have scarcer fresh water supplies than at present
yields from rain-fed agriculture could be halved
food security likely to be further compromised in Africa
widespread impacts on coral reefs
Writing in the International Herald Tribune ahead of the report's release, Ban Ki-moon said the world may be nearing a tipping-point on climate change.

"We all agree. Climate change is real, and we humans are its chief cause. Yet even now, few people fully understand the gravity of the threat, or its immediacy.

"Now I believe we are on the verge of a catastrophe if we do not act."

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