This is not necessarily due to flaws in the Government's scheme, but because the amount of heat-trapping greenhouse gases in the atmosphere has already passed safe limits.

The most ambitious outcome now is to restrict carbon dioxide in the atmosphere to about 450 parts per million - a figure calculated to give the world an even chance of avoiding the more catastrophic climate change scenarios for the second half of this century, said Matthew England, a lead author with the United Nations' Intergovernmental Panel on Climate Change.

If the Government ties the scheme to the upper level of its targets for 2020 - a 25 per cent cut in carbon emissions - and other nations make equivalent cuts, then the 450 goal endorsed by most climate scientists could be achieved under the ETS.

If it ties the scheme to the lower end of its target range, a 5 per cent reduction over the next decade, Australia will find it hard to play a part in reaching the 450 goal.

"The whole world is watching what we do here and it could have real ramifications for their own efforts - this is the reason why Australia is important," Professor England said.

Many environment groups are calling for carbon dioxide in the atmosphere to be cut and stabilised at 350 parts per million, but that goal is out of reach for the foreseeable future. The current ratio is 387 parts per million.

"To stabilise emissions at 350 is a laudable goal because there is a much larger chance of averting dangerous climate change," Professor England said.

"But I think that, unfortunately, it's a goal which is no longer achievable - 350 is history. At this stage 450 is looking incredibly difficult. But it's not as if 450 guarantees the survival of the Greenland ice sheet."

The Prime Minister, Kevin Rudd, has supported a global target of 450 parts per million.