Australian scientists say the climate change debate is over and it's time to drastically cut carbon emissions after a major UN report unequivocally declared global warming is real and a significant threat if left unchecked.
The latest assessment of the climate science by the Intergovernmental Panel on Climate Change (IPCC) has concluded there's a 95 per cent likelihood that humans are responsible for global warming.
"In science, 95 per cent, most of the time, is as good as it gets," senior CSIRO research scientist and lead IPCC report author Pep Canadell told AAP.
Australia is the highest emitter of CO2 on a per capita basis, and environment groups are pressuring the federal government to act on this wake up call and commit to greater emissions cuts.
The IPCC warns the world is on track for a dangerous rise in global temperatures, with concentrations of greenhouse gases in the atmosphere at levels not seen in at least 800,000 years.
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There's more conclusive evidence that the planet is warming than in the IPCC's last report in 2007, and if emissions continue to rise there will be further unprecedented changes in the climate system.
Sea levels are predicted to rise further, the ice caps to recede even more, and the ocean to become more acidic as it absorbs even more excess CO2.
Professor Will Steffen from the Climate Council warned temperatures could rise between 3.5 and 5.5 degrees by 2100, well above the two-degree "guardrail" most countries agree is dangerous.
"The IPCC report is saying quite clearly we're on a road that is going to lead us way beyond what most scientists and indeed most countries consider the limit limit of climate change we can tolerate," he told AAP.
Importantly, the IPCC for the first time included a carbon budget in its assessment, which indicates how much carbon can be released into the atmosphere if the world is to avoid two degrees of warming.
That budget is about 800 billion tonnes, but worryingly about half of that has already been exhausted and 10 billion tonnes are being released every year.
Professor Canadell said emissions had tripled this decade compared to the last, but needed to be slashed by 60 per cent by 2050 if there's to be any hope of staying within that carbon budget.
"We cannot wait 10 more years or 15 more years," he said.
Professor Steffen went even further, saying to stay within budget most of the world's known fossil fuel reserves would have to stay in the ground.
Environment Minister Greg Hunt said the federal government supported the climate science and vowed the coalition would ensure Australia meets its 5 per cent emissions reduction target by 2020.
But environment groups say much deeper cuts - in the vicinity of 25 per cent - are needed if Australia is going to contribute its fair share to the huge task identified by the UN climate body.
"If the rest of the world followed that (5 per cent) kind of path, then we'd be looking towards a four degree increase in global temperature," deputy head of the Climate Institute Erwin Jackson told AAP.
Australian Greens leader Christine Milne said Mr Hunt was "plain wrong" to suggest the 5 per cent target was consistent with the drastic cuts required to stay within the carbon budget.
"He will be laughed out of the UNFCCC (United Nations Framework Convention on Climate Change) plenary if he even tries to spin that line," senator Milne said.
The Climate Change Authority is due next month to deliver a draft recommendation for Australia's emissions reduction targets, but the coalition has vowed to uphold its promise to abolish the agency.
