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 April 7 to April 12 to review the 29-page draft that also estimates that the required 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 2ÚC compared to pre-industrial times.
Temperatures have already risen by about 0.8ÚC since 1900 and are set to breach the 2ÚC ceiling on current trends in coming decades, UN reports show.
"The window is shutting very rapidly on the two degrees target," said Johan Rockstrom, head of the Stockholm Resilience Centre, and an expert on risks to the planet from heatwaves, floods, droughts and rising seas.
"The debate is drifting to 'maybe we can adapt to two degrees, maybe three or even four,'" Mr Rockstrom said.
Such rises would sharply increase risks to food and water supplies and could trigger irreversible damage, such as a meltdown of Greenland's ice, according to UN reports.
The draft outlines ways to cut emissions and boost low-carbon energy, which includes renewables such as wind, hydro-electric 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% - or quadruple by 2050, according to most scenarios reviewed.
They would displace high-polluting fossil fuels as the world's main energy source by mid-century.
Saskatchewan Power in Canada will open a $1.35 billion (£810 million) coal-fired electricity generating plant this year that will extract a million tonnes of carbon dioxide a year from its exhaust gases.
It will be the first carbon-capture and storage plant of its type.
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 2ÚC.
Another recent report by the IPCC showed warming already affects every continent and said it 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 getting on track to meet the 2ÚC goal would mean limiting greenhouse gas emissions to between 30 billion and 50 billion tonnes in 2030, a radical shift after a surge from 38 billion in 1990 to 49 billion tonnes in 2010.
The shift would reduce economic output by up to 6% by 2050 because of the costs of building clean energy systems. Capturing carbon dioxide is also expensive. China and the United States are the top emitters.
One option is to let temperatures overshoot the 2ÚC target while developing technology to extract greenhouse gases from the atmosphere, the draft says.