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.

Australia's emission targets "look very small": Lord Nicholas Stern. Credit:Marina Neil

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.