World must end 'dirty' fuel use - UN
A long-awaited UN report on how to curb climate change says the world must rapidly move away from carbon-intensive fuels.
There must be a "massive shift" to renewable energy, says the study released in Berlin.
It has been finalised after a week of negotiations between scientists and government officials.
Natural gas is seen as a key bridge to move energy production away from oil and coal.
But there have been battles between participants over who will pay for this energy transition.
The report is the work of the UN's Intergovernmental Panel on Climate Change (IPCC), which was set up to provide a clear scientific view on climate change and its impacts.
The Summary for Policymakers on mitigation paints a picture of a world with carbon emissions rising rapidly.
"The high speed mitigation train needs to leave the station very soon, and all of global society will have to get on board," the IPCC's chair Rajendra Pachauri told journalists in Berlin at the launch of the report.
Dr Youba Sokono, a co-chair of the IPCC's working group 3, which drew up the report, said science has spoken.
He added that policy makers were "the navigators, they have to make decisions, scientists are the map makers".
The UK's Energy and Climate Change Secretary Ed Davey said global warming needed to be tackled using "all technologies". He told BBC News: "We can do this, we have to because it's so challenging and threatening to our economies and societies, our health, our food security. The report today shows we can do it if we have the political will."
He added that the UK government was a leader on the use of renewable energy sources, saying: "We've, for example, doubled the amount of renewable electricity in the last few years. We're likely to do better than our targets in increasing renewable electricity. But we've got to do more."
About half of all the carbon that humans have pumped into the atmosphere since 1750 has been emitted in the last 40 years.
Rates have been rising fast since 2000, despite the global economic crash.
The report points to an increased use of coal in the decade from the turn of the millennium , "reversing the longstanding trend of decarbonisation of the world's energy supply".
Driven by a global increase in population and economic activity, global surface temperature increases will be between 3.7C and 4.8C in 2100 if no new action is taken.
This is way above the 2 degree level, regarded as the point beyond which dangerous impacts of climate change will be felt.
However, the scientists involved in the report say this situation can be turned around.
"It needs a big change in the energy sector, that is undoubtedly true," said Prof Jim Skea, vice-chair of working group 3.
"One of the biggest areas that's important is getting the carbon out of electricity, so renewable energy, nuclear, fossil fuels with carbon capture and storage, that's all part of the menu if we are going to make the transition to stay under the 2 degree target."
It is not a simple task. To be sure of staying below 2 degrees, the amount of carbon in the air needs to be around 450 parts per million by 2100. To get there, emissions in 2050 need to be 40-70% lower than they were in 2010.
The IPCC says that renewables are a critical part of that pathway.
Since the last report in 2007, the scientists say that renewable energy has come on in leaps and bounds.
In 2012, renewables accounted for just over half of the new electricity generation added around the world.