The world faces "severe, widespread and irreversible" impacts of climate change without urgent action to cut emissions, a major international report has warned.
While there are solutions to climate change, experts warned there was little time before the window of opportunity to limit dangerous temperature rises closed, and that dela ying action would greatly increase costs.
Significant and swift cuts to greenhouse gases are needed, with the phasing out of fossil-fuelled power plants without technology to capture and store their carbon by the end of the century, and a shift away from investment in oil, coal and gas towards renewables.
The warning comes from the UN climate body's final report of its latest assessment on the science of climate change, which draws together three studies published in the past year.
Launching the report in Copenhagen, UN secretary general Ban Ki-moon said: "Science has spoken, there is no ambiguity in their message. Leaders must act. Time is not on our side."
But he said: "The good news is that if we act now immediately and decisively we have the means to build a better and more sustainable world.