Flannery said the Intergovernmental Panel on Climate Change (IPCC) report will show that greenhouse gas in the atmosphere in mid-2005 had reached about 455 parts per million of carbon dioxide equivalent -- a level not expected for another 10 years.

"We thought we'd be at that threshold within about a decade," Flannery told Australian television late on Monday.

"We thought we had that much time. But the new data indicates that in about mid-2005 we crossed that threshold," he said.

"What the report establishes is that the amount of greenhouse gas in the atmosphere is already above the threshold that could potentially cause dangerous climate change."

Flannery, from Macquarie University and author of the climate change book "The Weather Makers", said he had seen the raw data which will be in the IPCC Synthesis Report.

He said the measurement of greenhouse gas in the atmosphere included not just carbon dioxide, but also nitrous oxide, methane and hydrofluorocarbons (HFCs). All these gases were measured and then equated into potentially one gas to reach a general level.

"They're all having an impact. Probably 75 percent is carbon dioxide but the rest is that mixed bag of other gases," he said.

COLLISION COURSE

Flannery said global economic expansion, particularly in China and India, was a major factor behind the unexpected acceleration in greenhouse gas levels.

"We're still basing that economic activity on fossil fuels. You know, the metabolism of that economy is now on a collision course, clearly, with the metabolism of our planet," he said.

The report adds an urgency to international climate change talks on the Indonesian island of Bali in December, as reducing greenhouse gas emissions may no longer be enough to prevent dangerous climate change, he said.

UN environment ministers meet in December in Bali to start talks on a replacement for the Kyoto Protocol on curbing climate change that expires in 2012.

"We can reduce emissions as strongly as we like -- unless we can draw some of the standing stock of pollutant out of the air and into the tropical forests, we'll still face unacceptable levels of risk in 40 years time," he said.

Flannery suggested the developed world could buy "climate security" by paying villages in countries like Papua New Guinea not to log forests and to regrow forests.

"That 200 gigatonnes of carbon pollutant, the standing stock that's in the atmosphere, is there courtesy of the industrial revolution, and we're the beneficiaries of that and most of the world missed out," he said.