Global warming is on track to hit 1.5C in the early 2030s and the world needs to slam on the fossil fuel brakes now to limit the damage.
The United Nations' Intergovernmental Panel on Climate Change has delivered a "reality check" about what the world is doing to itself.
Even under a best-case scenario, global warming of at least 1.5C above pre-industrial levels is anticipated within 20 years.
Climate scientists expect 1.5C to be reached early next decade.
The IPCC's Valerie Masson-Delmotte labelled the panel's sixth assessment a "reality check".
The worst of five scenarios modelled shows 1.6C of warming within 20 years and a planet that's 4.4C hotter by the end of the century.
Australian climate scientist and report author Blair Trewin says the world is seeing conditions "outside the range of human experience".
"The climate system is in a state that it has not been for at least centuries to millennia and changing at a rate unprecedented at least for the last 2000 years," he told reporters on Monday.
Without immediate, rapid and large-scale reductions, it would not be possible to limit warming to close to 1.5C or even 2C.
Many changes - especially when it came to the ocean, ice sheets and global sea level - were already locked in and irreversible for centuries or even millennia.
But CSIRO scientist and fellow report author Pep Canadell said the world still had a chance to determine its long-term climate future.
"We may not be (in control) any more over the next few decades, but we're still in control about where we want to take the future of this planet," he said.
It was less of a cliff than a slippery slope.
"We try to slam the brakes as hard as we can. And every decimal of a degree that we are avoiding, it is a win for us and a win for the planet," Dr Canadell said.
"But the important thing to understand is that there's no bottom end ... to how much damage we can create."
A best-case scenario was in line with 1.6C of warming between 2041 and 2060.
Under this trajectory, warming would then dip back below 1.5C by the century's end.
Other scenarios had the planet on track for between 1.8C and 3.6C of warming by 2081 and 2100, depending on the rate of greenhouse gas emissions pumped into the atmosphere.
Human activity had already caused about 1.1C of warming compared with pre-industrial levels.
Greenhouse gas emissions were in concentrations not seen for at least 800,000 years.
The global sea level had risen 20cm already and was on track for an additional increase of between 30cm and a metre or more by 2100.
In Australia, land temperatures were around 1.4C warmer.
Weather extremes such as fires, floods, droughts, cyclones and coral bleaching had increased, intensified and would continue to do so.
Australia remains uncommitted to achieving net zero emissions by 2050.
But the Morrison government has repeatedly said it wants to reach this as soon as possible and preferably by that time.
