The unrestricted use of fossil fuels should be phased out by 2100 if the world is to avoid dangerous climate change, a UN-backed expert panel says.
The Intergovernmental Panel on Climate Change says in a stark report that most of the world's electricity can - and must - be produced from low-carbon sources by 2050.
If not, the world faces "severe, pervasive and irreversible" damage. IPCC's Synthesis Report was published on Sunday in Copenhagen, after a week of intense debate between scientists and government officials.
It is intended to inform politicians engaged in attempts to deliver a new global treaty on climate by the end of 2015.
The report says that reducing emissions is crucial if global warming is to be limited to 2C - a target acknowledged in 2009
as the threshold of dangerous climate change.
The report suggests renewables will have to grow from their current 30% share to 80% of the power sector by 2050.
In the longer term, the report states that fossil fuel power generation without carbon capture and storage (CCS) technology would need to be "phased out almost entirely by 2100".
'Science has spoken'
The Synthesis Report summarises three previous reports from the IPCC, which outlined the causes
, the impacts
and the potential solutions to climate change
It re-states many familiar positions:
◾Warming is "unequivocal" and the human influence on climate is clear
◾The period from 1983 to 2012, it says, was likely the warmest 30 year period of the last 1,400 years
◾Warming impacts are already being seen around the globe, in the acidification of the oceans, the melting of arctic ice and poorer crop yields in many parts
◾Without concerted action on carbon, temperatures will increase over the coming decades and could be almost 5C above pre-industrial levels by the end of this century
"Science has spoken," UN Secretary-General Ban Ki-moon said. "There is no ambiguity in their message. Leaders must act. Time is not on our side."
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