Government officials and top climate scientists will meet in Berlin from April 7-12 to review the 29-page draft that also estimates the needed shift to low-carbon energies would cost between two and six percent 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 2 degrees Celsius (3.6 Fahrenheit) over pre-industrial times.
Temperatures have already risen by about 0.8 C (1.4F) since 1900 and are set to breach the 2 C ceiling on current trends in coming decades, U.N. reports show.
"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 heatwaves, floods, droughts and rising seas.
"The debate is drifting to 'maybe we can adapt to 2 degrees, maybe 3 or even 4'," Rockstrom, who was not among authors of the draft, told Reuters.
Such rises would sharply raise risks to food and water supplies and could trigger irreversible damage, such as a meltdown of Greenland's ice, according to U.N. reports.
The draft 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 percent of the world's total energy supplies in 2010 and their share would have to triple - to 51 percent - or quadruple by 2050, according to most scenarios reviewed.
That would displace high polluting fossil fuels as the world's main energy source by mid-century.