Global warming is on track to hit 1.5C next decade, many climate change consequences are irreversible, and the world needs to hit the fossil fuel brakes now to limit the damage.
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The United Nations' Intergovernmental Panel on Climate Change has delivered a reality check about what the world is doing to itself.
Even under the best-case scenario modelled, global warming of at least 1.5C above pre-industrial levels is anticipated within 20 years.
Climate scientists expect 1.5C to be reached in the early 2030s.
Nobody is safe from climate change and it's getting worse faster, the UN's Environment Programme executive director Inger Andersen warns.
"Climate change is here, now. We are also here now. If we don't act, who will," she told the global launch of the IPCC's sixth assessment on Monday.
Countries' existing emissions reduction pledges equate to about 2.1C of warming.
This isn't enough to avoid food production loss, worsening heat extremes and fires, continued sea level rise, and potential refugee crises.
Some changes - to ice sheets, deep ocean temperatures, acidification - are already locked in and irreversible for centuries or even millennia.
But immediate, rapid, deep and sustained cuts to greenhouse gas emissions can slow and even stop worse-case outcomes.
Without such action, the world cannot hope to limit warming to close to 1.5C or even 2C.
The worst of five scenarios modelled by the IPCC shows 1.6C of warming within 20 years and a planet that's likely 4.4C hotter by the end of the century.
Australian scientist and IPCC report author Blair Trewin says the pace and extent of climate change means 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 a media briefing ahead of the report's launch.
But there is still some hope.
CSIRO scientist and fellow report author Pep Canadell says the world hasn't yet run out of time to determine the extent of its long-term climate future.
"We may not be (in control) anymore over the next few decades, but we're still in control about where we want to take the future of this planet," he said.
"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.
"But the important thing to understand is that there's no bottom end ... to how much damage we can create."
The best-case scenario modelled is 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 have the planet on track for between 1.8C and 3.6C of warming by 2081 and 2100.
Human activity has already caused about 1.1C of warming since between 1850 and 1900.
CO2 concentrations are their highest in at least two million years, while sea level rise is occurring at the fastest rate in at least 3000 years.
Already, sea level rise has reached 20cm. It's tracking to increase between 30cm and a metre or more by 2100.
Arctic sea ice is at the lowest level in at least 1000 years.
Globally, each additional half degree of warming will cause more frequent and intense extreme heat, drought and extreme rainfall.
At 2C, heat extremes will reach tolerance thresholds for agriculture and human health more frequently.
In Australia, land temperatures are already around 1.4C warmer.
Weather extremes such as fires, floods, droughts, cyclones and coral bleaching have increased, intensified and will continue to do so.
The Morrison government has not committed to net zero emissions by 2050.
Emissions Reduction Minister Angus Taylor said the government wants to reach net zero as soon as possible and preferably within that time frame.
He said overcoming the challenges of climate change is a shared responsibility.
"When it comes to emissions reduction, our record is one of delivery and achievement that Australians can be proud of," he said.
Australian Associated Press
