Andrew Macintosh, from the ANU's Centre for Climate Law and Policy, said the two climate change scenarios being modelled by the Garnaut Review and the federal Treasury have been rendered out of date by advances in climate science.

Professor Garnaut and the Government are developing two scenarios: one that would limit carbon dioxide in the atmosphere to 450 parts per million; and one that would limit it to 550 parts per million.

Mr Macintosh said much greater emission cuts than previously thought would be needed to achieve this.

He said 450ppm was "nigh on impossible", meaning there was little chance of restricting global warming to less than 2 degrees.

Leading climate researcher Dr Michael Raupach, of the CSIRO, confirmed the 450ppm target might be out of reach: "With any scenario except the most aggressive emissions reduction scenarios, exceeding 2 degrees is extremely likely."

The Intergovernmental Panel on Climate Change has identified 2 degrees as the threshold above which the risk of extreme climate change impacts becomes high.

Mr Macintosh said dramatic action would be needed to keep greenhouse levels between 500 and 550 ppm, and warming below 3 degrees. Above that, there was a greatly increased risk of irreversible climate change.

Professor Garnaut has said modelling for the two scenarios was already running late and would not be ready for Friday's report, and modelling additional scenarios would delay the process even further.