Avoiding global climate chaos will require a major transformation of society and the world economy that is "unprecedented in scale," the UN said on Monday in a landmark report that warns time is running out to avert disaster.

Earth's surface has warmed one degree Celsius - enough to lift oceans and unleash a crescendo of deadly storms, floods and droughts - and is on track towards an unliveable 3C or 4C rise.

At current levels of greenhouse gas emissions, we could pass the 1.5C marker as early as 2030, and no later than mid-century, the Intergovernmental Panel for Climate Change (IPCC) reported with "high confidence".

"The report shows that we only have the slimmest of opportunities remaining to avoid unthinkable damage to the climate system that supports life as we know it," said Amjad Abdulla, the IPCC board member and chief negotiator for the alliance of small island states.

"Historians will look back at these findings as one of the defining moments in the course of human affairs."

A Summary for Policymakers of the 400-page tome underscores how quickly global warming has outstripped humanity's attempt to tame it, and outlines paradigm-shift options for avoiding the worst ravages of a climate-addled future.