Rule changes for measuring forestry emissions proposed at the Copenhagen climate summit are estimated to allow wealthy nations to release an extra 236 million tonnes of carbon dioxide into the atmosphere from burning fossil fuels while still meeting their emissions reduction targets.

The shift involves changing the baseline year against which current emissions are measured. By using a higher baseline figure - a prediction of average annual future forestry emissions - they can emit more while still meeting their target.

Sean Cadman, a forest and climate consultant working for the Wilderness Society, said the wealthy were trying to ''disappear'' huge real emissions from greenhouse gas accounts by manufacturing a false baseline. It meant Australian could emit an extra 6 per cent of emissions based on 1990 levels while still meeting its target.

''This would weaken the effort that is required to reduce emissions from fossil fuel use and the land sector,'' Mr Cadman said. ''The critical thing is these are policy choices - Australia could instead protect native forests and have a real saving of emissions.''

He said the world only knew the impact of the proposed changes because the G77 bloc of developing countries had demanded data be released.

''Hopefully now we have the opportunity to have this particular loophole closed,'' he said.

Environmentalists said the loophole was another example of Australia fighting for treatment of forestry emissions that minimised the effort it had to make to meet a target.

Under the Kyoto Protocol the so-called ''Australia clause'' allowed Canberra to earn credits for the massive drop in emissions from land-clearing after 1990.

It means it may meet its Kyoto target of an 8 per cent increase in emissions between 1990 and 2012 despite fossil fuels emissions increasing by about 30 per cent.

Australian Conservation Foundation climate change campaigner Tony Mohr said Australia's position on land and forestry emissions was under scrutiny.

''The Australian Government needs to make a clear commitment that they won't try and get a last-minute fiddle on the rules that gives us a free kick on land-use emissions,'' he said.