UN Secretary-General Antonio Guterres has described the latest report from the Intergovernmental Panel on Climate Change as a 'code red for humanity'.
The report released on Monday warns the world is dangerously close to runaway warming - and that humans are "unequivocally" to blame.
Already, greenhouse gas levels in the atmosphere are high enough to guarantee climate disruption for decades if not centuries, the scientists conclude.
The UN chief urged an immediate end to coal energy and other high-polluting fossil fuels.
"The alarm bells are deafening," Guterres said in a statement. "This report must sound a death knell for coal and fossil fuels, before they destroy our planet."
The IPCC report comes just three months before a major UN climate conference in Glasgow, Scotland, where nations will be under pressure to pledge ambitious climate action and substantial financing.
Drawing on more than 14,000 scientific studies, the report gives the most comprehensive and detailed picture yet of how climate change is altering the natural world -- and what still could be ahead.
Unless immediate, rapid and large-scale action is taken to reduce emissions, the report says, the average global temperature will likely cross the 1.5-degree Celsius warming threshold within the next 20 years.
Reacting to the findings, governments and campaigners expressed alarm.
"The IPCC report underscores the overwhelming urgency of this moment," US climate envoy John Kerry said in a statement. "The world must come together before the ability to limit global warming to 1.5 degrees Celsius is out of reach."
"We have all the evidence we need to show we are in a climate crisis," said three-time IPCC co-author Sonia Seneviratne, a climate scientist at ETH Zurich. "Policy makers have enough information. You can ask: Is it a meaningful use of scientists' time, if nothing is being done?"
"Every bit of warming matters," said IPCC co-author Ed Hawkins, a climate scientist at the University of Reading in Britain. "The consequences get worse and worse as we get warmer."
Greenland's ice sheet is "virtually certain" to continue melting. Oceans will keep warming, with surface levels rising for centuries to come.
It's too late to prevent these particular changes. The best the world can do is to slow them down so that countries have more time to prepare and adapt.
But even to slow climate change, the report says, the world is running out of time.
"We have already changed our planet, and some of those changes we will have to live with for centuries and millennia to come," said IPCC co-author Joeri Rogelj, a climate scientist at Imperial College London.
The question now is how many more irreversible changes we avoid.
"We still have choices to make." he said.
Australian Associated Press