By Nina Chestney and Andrea Januta

The U.N. panel on climate change sounded a dire warning on Monday, saying 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, scientists warn in a report from the Intergovernmental Panel on Climate Change (IPCC).

That’s on top of the deadly heat waves, gargantuan hurricanes and other weather extremes that are happening now, and are likely to become more severe.

Describing the report as a "code red for humanity", U.N. Secretary-General António Guterres urged an end to the use of coal and other highly 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 U.N. climate conference known as COP26 in Glasgow, Scotland, where nations will be under pressure to pledge much more ambitious climate action, and substantial financing to go with it.

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 could still be ahead.

Unless immediate, rapid and large-scale action is taken to reduce emissions, the report says, the average global temperature is likely to cross the 1.5-degree Celsius (2.7 degrees Fahrenheit) warming threshold within the next 20 years.

So far, nations’ pledges to cut emissions have been inadequate for bringing down the level of greenhouse gases accumulated in the atmosphere.

Governments and campaigners reacted to the findings with alarm.

British Prime Minister Boris Johnson, whose country will host the climate conference, said the next decade would be "pivotal" to securing the future of our planet:

"I hope today’s IPCC report will be a wake-up call for the world to take action now, before we meet in Glasgow in November for the critical COP26 summit."

IRREVERSIBLE CHANGE

Emissions “unequivocally caused by human activities” have already pushed the average global temperature up 1.1C from its pre-industrial average -- and would have raised it 0.5C further without the tempering effect of pollution in the atmosphere, the report says.

That means that, even as societies move away from fossil fuels, temperatures will be pushed up again by the loss of those airborne pollutants.

Scientists warn that a rise of more than 1.5C above the preindustrial average could trigger runaway climate change with catastrophic impacts, such as heat so intense that people die just from being outdoors.

Any further warming will also boost the intensity and frequency of heat extremes and heavy rainfall, as well as droughts in some regions. Because temperatures fluctuate from year to year, scientists measure climate warming in terms of 20-year averages.

"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 who doubts she will sign up for a fourth report. "Policy makers have enough information. You can ask: Is it a meaningful use of scientists' time, if nothing is being done?"

The 1.1C warming already recorded has been enough to unleash disastrous weather https://tmsnrt.rs/3wcycMk. This year, heat waves killed hundreds in the Pacific Northwest and smashed records around the world. Wildfires fuelled by heat and drought are sweeping away entire towns in the U.S. West, releasing record carbon dioxide emissions from Siberian forests, and driving Greeks to flee their homes by ferry.

"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, and raising the sea level, which will continue to rise for centuries to come as the oceans warm and expand.

It is already 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.

“We are now committed to some aspects of climate change, some of which are irreversible for hundreds to thousands of years,” said IPCC co-author Tamsin Edwards, a climate scientist at King’s College London. “But the more we limit warming, the more we can avoid or slow down those changes.”

‘WE STILL HAVE CHOICES TO MAKE’

Even to slow climate change, the report says, the world is running out of time.

If the world drastically cuts emissions in the next decade, average temperatures could still be up 1.5C by 2040 and possibly 1.6C by 2060 before stabilising.

If the world does not cut emissions dramatically, but continues the current trajectory, the rise could be 2.0C by 2060 and 2.7C by the century’s end.

The earth has not been that warm since the Pliocene Epoch roughly 3 million years ago -- when humanity's first ancestors were appearing, and the oceans were 25 metres (82 feet) higher than they are today.

It could get even worse, if warming triggers feedback loops that release even more climate-warming carbon emissions -- such as the melting of Arctic permafrost or the dieback of global forests. Under these high-emissions scenarios, Earth could broil at temperatures 4.4C above the preindustrial average by 2081-2100.

“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, he said, was how many more irreversible changes could be avoided.

"We still have choices to make."