At least half the world's energy supply will have to come from low-carbon sources, such as renewables and nuclear, by 2050 as part of the drastic global action needed to cut greenhouse gases to relatively safe levels, a major United Nations climate change assessment will say.

A leaked draft of the next report by the Intergovernmental Panel on Climate Change, obtained by Fairfax Media, also warns the world is fast running out of time to make the cuts to emissions required to keep global warming to an average of two degrees - a goal countries, including Australia, have pledged to meet through the UN.

The emission reductions pledged by nations for 2020 are found to fall short of the action needed. Credit:Graham Tidy

The draft comes as Lord Nicholas Stern, the author of a 2006 landmark review of the economics of climate change, chastised Australia for being ''flaky'' on global warming. In an interview with Fairfax 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 such as serious and global problem.

Australia's target of cutting emissions by 5 per cent of 2000 levels by 2020 "looks very small" and Abbott government policy changes such as the scrapping the carbon tax and its "tone of discussion" suggested it was "not very serious" about climate change.