At least half the world's energy supply will need to come from low-carbon sources such as wind, solar and nuclear by 2050, as part of the drastic action needed to cut greenhouse gases to relatively safe levels, a United Nations climate change assessment will say.
A leaked draft of the next report by the Intergovernmental Panel on Climate Change also warns the world is fast running out of time to make the deep cuts to emissions required to keep global warming to an average of 2 degrees - a goal that countries, including Australia, have pledged to meet through the UN.
Lord Nicholas Stern, the author of a 2006 review of the economics of climate change, has chastised Australia for being ''flaky'' on global warming.
In an interview, he said each country had to be ambitious in its approach to cutting emissions and developing a low-carbon economy because climate change was a serious global problem.
Lord Stern said Australia's target of cutting emissions by 5 per cent of 2000 levels by 2020 ''looks very small'' and the Abbott government's policy changes such as scrapping the carbon tax and its ''tone of discussion'' suggested it was not very serious about climate change.
The final version of the latest IPCC report - the third part of its fifth assessment of climate change - will be released in Berlin on Sunday. The report focuses on ways emissions caused by humans can be mitigated.
The leaked draft warns that if the world puts off deep cuts to emissions until 2030, it will make the 2-degree target significantly harder, and will limit the options for mitigation. The world has already warmed by 0.85 degrees since 1880.
The current emission reductions pledged by nations for 2020 are found to fall short of the action needed to have the best chance of meeting the target. This means deeper cuts would be required later, with higher costs and probably the need to develop technologies to draw significant amounts of carbon dioxide out of the atmosphere.
The draft says greenhouse gas emissions rose faster between 2000 and 2010 than in previous decades, driven largely by economic and population growth. Emissions were at the high end of projections, and each person living in rich countries was on average responsible for nine times more greenhouse gases than those from poorer nations.
It says most of the scenarios studied - that ensure just a 2-degree rise in warming - include a trebling, to a near quadrupling, of the share of clean energy in global supply by mid-century from 2010 levels.
That would require the share of renewable technologies, nuclear and fossil fuels using carbon capture and storage, to rise from the 2010 levels of about 17 per cent to at least 51 per cent by 2050.
The draft also says the cost of cutting emissions to safer levels would cause a loss of between 2 per cent and 6 per cent of world economic output to 2050.
A spokesman for Environment Minister Greg Hunt defended the government's climate change policies saying Australia was committed to addressing the climate change challenge with ''practical policy measures'' including the five per cent reduction target from 2000 levels by 2020, and the Coalition's direct action plan.
''The five per cent target represents serious action and is comparable with the action being taken by other countries when compared using 2005 as the benchmark starting point,'' he said.
