The world's top scientists have given their clearest warning yet of the severe and irreversible effects of climate change unless emissions of greenhouse gases are virtually eliminated by the end of the century, Radio New Zealand reports.
The United Nations Intergovernmental Panel on Climate Change (IPCC) has released a major summary of its last three reports.
The IPCC report said emissions of three key greenhouse gases were at their highest in more than 800,000 years - the recent increases being mostly due to the burning of fossil fuels.
It said earth is now on a trajectory for at least 4 degrees Celcius warming by 2100 over pre-industrial times - a recipe for worsening drought, flood, rising seas and species extinctions.
Most of the world's electricity can, and must, be produced from low-carbon sources by 2050 of the world faces "severe, pervasive and irreversible" damage, it said.
UN Secretary-General Ban Ki-moon said science had spoken. “There is no ambiguity in their message. Leaders must act. Time is not on our side.”
“There is a myth that climate action will cost heavily," said Ban, “but inaction will cost much more.”
The authors say that reducing greenhouse gas emissions is crucial if global warming is to be limited to 2C - a target acknowledged in 2009 as the threshold of dangerous climate change.