Humans have already released half the total carbon dioxide emissions permissible before the planet is at risk of warming to dangerous levels, a draft United Nations scientific assessment says.
The final draft of a major assessment by the Intergovernmental Panel on Climate Change suggests that keeping warming to 2 degrees - regarded as a guard-rail against the worst impacts of climate change - will require deep global emissions cuts in coming decades.
Under the future emissions scenarios considered by the IPCC, only the most stringent would keep the world within the remaining CO2 allowance for 2 degrees. It would mean an average global emission cut of 50 per cent by mid-century on 1990 levels, and possibly require removal of carbon dioxide from the atmosphere by 2100.
Dr Malte Meinshausen, senior researcher at the Potsdam Institute for Climate Impact Research, said it also meant global emissions would need to fall to almost zero by 2070.
He said many studies showed it was technically and economically feasible. ''It is a question of political will whether the technologies that we have are implemented to the extent needed.''
Next Friday the IPCC will release the first part of its fifth global assessment of climate science in Stockholm. It was prepared over six years by hundreds of climate scientists.
The draft report lifts the IPCC's scientific certainty that human activity - such as burning fossil fuels - caused more than half the warming since the middle of last century to a 95 per cent confidence.
It also says warming has slowed in the past 15 years to 0.05 degrees a decade - below the long-term average of 0.12 degrees since 1951 - but the first 10 years of the 21st century were the warmest since 1850. All up, Earth's surface has warmed on average 0.89 degrees since 1901.
For the first time, the IPCC draft also includes an estimate of the total cumulative CO2 emissions that can be released since pre-industrial times to give the world above a 66 per cent chance of keeping warming below 2 degrees.
The draft estimates total human CO2 emissions need to be limited to about 3670 billion tonnes of carbon dioxide to keep below 2 degrees. By 2011, between 1688 and 2312 billion tonnes of CO2 had been released, about half the total allowance.
The budget does not take into account other planet-warming gases such as methane and nitrous oxide. Nor does it consider potential emissions from thawing permafrost or methane ice. ''If it is taken as a policy indication of where the carbon budget should be then we should certainly have to aim for something substantially lower than that because we have other greenhouse gases [than CO2],'' Dr Meinshausen said.
If the world proceeds along the highest future emissions path, a further 6183 billion tonnes of CO2 would be released by 2100, the draft says, meaning potential temperature rises of 2.6 to 4.8 degrees by century's end.
Environment Minister Greg Hunt would not comment on the IPCC report until it is released. But he was confident the world would deal with climate change.
''The reason I am most confident is because the Chinese and the Americans who are the central part of any agreement both have a very strong view,'' he said.
''The most heartening development in the past two years has been China's growing commitment to action from its paramount leadership.''