World powers are running out of time to slash their use of high-polluting fossil fuels and stay below agreed limits on global warming, a draft UN study to be approved this week shows.
Government officials and top climate scientists will meet in Berlin from 7-12 April to review the 29-page draft that also estimates the needed shift to low-carbon energies would cost between 2% and 6% of world output by 2050.
It says nations will have to impose drastic curbs on their still rising greenhouse gas emissions to keep a promise made by almost 200 countries in 2010 to limit global warming to less than 2C degrees over pre-industrial times.
Temperatures have already risen by about 0.8C since 1900 and are set to breach the 2C ceiling on current trends in coming decades, UN report suggests.
"The window is shutting very rapidly on the 2 degrees target," said Johan Rockstrom, head of the Stockholm Resilience Centre, and an expert on risks to the planet from heat waves, floods, droughts and rising seas.
"The debate is drifting to 'maybe we can adapt to 2 degrees, maybe 3 or even 4'," Mr Rockstrom told Reuters.
Such rises would sharply raise risks to food and water supplies and could trigger irreversible damage, such as a melting of Greenland's ice, according to UN reports.
The draft, seen by Reuters, outlines ways to cut emissions and boost low-carbon energy, which includes renewables such as wind, hydro and solar power, nuclear power and "clean" fossil fuels, whose carbon emissions are captured and buried.
It said such low-carbon sources accounted for 17% of the world's total energy supplies in 2010 and their share would have to triple to 51% according to most scenarios reviewed.
That would displace high polluting fossil fuels as the world's main energy source by mid-century.
Reports by the Intergovernmental Panel on Climate Change (IPCC) will help governments which aim to agree a deal to slow climate change at a Paris summit in December 2015. Few nations have outlined plans consistent with staying below 2C degrees.
Another report issued by the IPCC last week in Japan showed warming already affects every continent and would damage food and water supplies and slow economic growth. It may already be having irreversible impacts on the Arctic and coral reefs.
The new draft shows that getting on track to meet the 2C goal would mean limiting greenhouse gas emissions to between 30 and 50 billion tonnes in 2030, a radical shift after a surge to 49 billion tonnes in 2010 from 38 billion in 1990.
The shift would reduce economic output by between 2-6% by 2050, because of the costs of building a cleaner energy system based on low-carbon energies that are more expensive than abundant coal, the IPCC said. Capturing carbon dioxide is also expensive, it added.
China and the United States are the top emitters.
The IPCC draft report is the third and final study in a UN series about climate change, updating findings from 2007, after the Japan report about the impacts and one in September in Sweden about climate science.
The September report raised the probability that human actions, led by the use of fossil fuels, are the main cause of climate change since 1950 to at least 95% from 90%. But opinion polls show voters are not persuaded, with many believing that natural variations are the main cause.