Global warming is real, man made, and could cause the world's sea level to rise a metre by the end of the century, much higher than previously thought, according to the federal government's Climate Commission.
Melbourne faces extreme and more frequent flooding, while higher sea levels are already "bad news" for fragile areas including Kakadu and the Great Barrier Reef, says Commissioner Will Steffen.
It also says this year's Queensland and Victorian flooding ''raised the question of a possible link between the floods and human-induced climate change''.
"A plausible estimate of the amount of sea-level rise by 2100 compared to 2000 is 0.5 to one metre," the report says.
''While a sea-level rise of 0.5 metre … may not seem like a matter for much concern, such modest levels of sea-level rise can lead to unexpectedly large increases in the frequency of extreme high sea-level events,'' it said.
The sea-level forecast is higher than the Intergovernmental Panel on Climate Change's top range of 0.18m to 0.76m.
Commissioner Will Steffen, a Canberra-based climate scientist, made the assessment after surveying the existing literature and speaking to experts.
"Some people may take issue with that - but that's my judgment," Professor Steffen said ahead of the report's release at Parliament House.
The report states that even a rise of 0.5m could lead to an increase in extreme flooding events for coastal areas of Sydney and Melbourne "by factors of 1000 or 10,000 for some locations".
The global sea level has risen by about 20cm since the 1880s.
But the rises aren't uniform - they vary according to ocean currents and the local conditions on the land.
In Australia, sea levels are rising fastest on the northern coastline.
Around Arnhem Land it's rising by more than seven millimetres a year while the global average is 3.2mm.
Prof Steffen notes that's bad news for Kakadu.
"It is low-lying and already we're seeing some salt water intrusion into some of the fresh water wetlands," he said.
When it comes to damage to the Great Barrier Reef, the Australian National University academic argues the report contains "solid data that says we are indeed starting to see some of these affects on calcifying organisms".
The commission, established by Labor to spruik the case for tackling dangerous climate change, is also calling for a fresh approach to reducing carbon emissions.
It suggests that, rather than focusing on interim targets based on percentage cuts, governments should commit to emitting no more than an agreed carbon dioxide "budget" by 2050.
This so-called budget approach would allow greater flexibility and encourage investment in the most-effective technologies rather than quick-fix solutions.
Prof Steffen said he hoped the report would refocus the political debate on the risks posed by climate change.
"The costs of not doing something about climate change will almost surely be far, far greater than the costs of doing something about it," he said.
When it comes to taking action, the report suggests focusing on limiting emissions to an agreed global budget.
"The strategic challenge (then) changes from whether the 2020 target is a five per cent, 25 per cent or 40 per cent reduction against a particular baseline to how do we implement the transition to a low- or no-carbon economy by 2050 with the least economic and social cost while staying within the budget," the report states.
For humanity to have a 75 per cent chance of limiting temperature rises to two degrees Celsius it would need to emit no more than one trillion tonnes of carbon dioxide between 2000 to 2050.
Some 30 per cent of that budget has already been spent.
The report acknowledges the difficulty with this approach would be allocating the global budget to individual countries.
But Prof Steffen points out the new method "really focuses attention on the endgame which is to decarbonise economies by the middle of the century".
The report argues this would encourage investment decisions to be taken from a long-term perspective.