To make a fair contribution to global efforts, Australia must cut emissions by at least 27%, and it can't achieve cuts of that scale solely through domestic action, according to a report commissioned by WWF.
The report by consultancy Ecofys says Australia needs to cut emissions to between 27% and 41% below 2000 levels by 2020, depending on how the global burden is shared and when the nation begins making cuts.
The recommendations are more stringent than those proposed by Professor Ross Garnaut, who said a global effort to keep warming below 2°C would likely require Australia to make a 25% cut by 2020 and 90% reduction by 2050.
Cuts of this magnitude "go beyond what is technically realisable in Australia domestically", the Ecofys report says.
Consequently, the nation will need to rely partly on buying international carbon credits, or will need to support offshore abatement activities through some other means, it says.
Australia fast spending its carbon 'budget'
The Ecofys report warns that Australia has already emitted "a substantial share" of the amount of carbon it is entitled to emit between 1990 and 2100.
According to an associated WWF statement, the results mean that Australia "has already 'spent' at least two-thirds of what would be considered a reasonable carbon budget that is consistent with keeping global warming below two degrees".
"If annual emissions remain at their current level, the entire budget would be used up in a little over a decade, if not sooner," it says.
WWF says that, given Australia's abatement action to date, it will need to make cuts "somewhere in the range of 27% to 34% below 2000 levels by 2020" if it is to be considered by other countries to be making a fair contribution.
"While in theory it could be possible for Australia to adopt a relatively weak 2020 target and make this up by adopting much stricter targets in the future, this approach may lack credibility, as it would imply unrealistically deep targets in the post-2020 period," it says.
The analysis excludes emissions associated with land use change from both the global budget and from its calculation of Australia's budget.
