Since the last Ice Age ended nearly 12,000 years ago and human civilisations developed, the Earth's long-term average global air temperature has never varied by more than 1.5 degrees above a stable 14 degrees Celsius (57 degrees Fahrenheit), scientists say.
But within a decade, fossil fuel emissions accumulating in the atmosphere and trapping more of the sun's energy are expected to drive global temperatures above that 1.5C (2.7F) warming mark.
What happens then?
In a September report in the journal Science, scientists said irreversible planetary "tipping points" - from melting of the Greenland ice sheet, setting in motion 7 metres (23 feet) of sea level rise, to the release of heat-accelerating methane trapped in melting permafrost - are likely to be triggered.
That could have dramatic implications for human and other life on the planet, as extreme weather surges, seas rise and food and water security weaken, Intergovernmental Panel on Climate Change (IPCC) scientists warn.
"I can say with a high degree of certainty that civilisations can thrive in a 14-degree world - but nobody can tell at any degree of certainty that we can thrive at (much higher temperatures) because we've never been there," said Johan Rockström, director of the Potsdam Institute for Climate Impact Research, at COP27 in Egypt.
Some countries at the UN climate talks have reportedly suggested the now heavily endangered 1.5C goal is unrealistic and should be removed from any agreement in Egypt, but others - including the low-lying Maldives - have insisted it remain.
How close is the world to passing 1.5C of warming, the more ambitious temperature limit set in the 2015 Paris Agreement?
Here is what Rockström, a leading climate scientist, thinks:
HOW MUCH HAS THE WORLD WARMED ALREADY?
IPCC scientists agree that temperatures have now risen at least 1.2C above pre-industrial times - but warming is not spread evenly around the globe.
On land, temperatures have already risen about 1.5C - more than over the bigger, cooler oceans - and the northern hemisphere is particularly warm, with Europe having seen about 2C of warming and the Arctic 3C.
"The amplification gets stronger the further north you go," Rockström explained.
But while temperatures are measured at a range of places around the planet and by a range of agencies - from the US National Oceanic and Atmospheric Administration (NOAA) to Britain's Met Office Hadley Centre, for example - significant data gaps still exist.
"You can't measure every pixel on earth. We don't measure much in Antarctica or the Arctic," Rockström said. That suggests, "we're underestimating temperature rise because we don't have all the data point", he added.
Europe and the Arctic are warming so fast in part because more of the planet's land - which is warming faster than its oceans - is in the northern hemisphere and because of how the planet tilts toward the sun.
But Europe's success in clearing up its air pollution is also playing a role, Rockström said.
HOW DOES POLLUTION HOLD DOWN TEMPERATURES?
In more polluted places such as major cities in China and India, a mask of haze effectively reflects some of the sun's energy back into space, like "small microscopic mirrors," the scientist said.
Air pollution around the world is likely holding down global average temperature readings by about 0.3C, IPCC scientists say.
That means the planet may effectively "already have 1.5C in the pipeline" but is not showing the effects yet, Rockström said.
That is worrying, he said, as many countries try to move swiftly to green renewable energy that would provide cheaper and cleaner power - and also clean up those choking skies.
"If we would clean up the air in Mumbai and New Delhi and Beijing, we would get an immediate warming pulse," Rockström said.
IS THE 1.5C WARMING LIMIT IN THE PARIS AGREEMENT ALREADY LOST? HOW IMPORTANT IS IT TO HOLD ONTO IT?
At the current rate people are burning fossil fuels, only seven to eight years remain before the 1.5C limit is passed. To stick to the target, countries, companies and individuals would have to cut their emissions by half every decade until 2050.
That is a daunting task with global emissions today still rising, which suggests an overshoot of the target is now likely, Rockström said.
The key, he said, will be to make that overshoot as small as possible - hopefully just fractions of a degree - and to try to swiftly pull temperatures back down again.
HOW MIGHT TEMPERATURES BE BROUGHT BACK DOWN?
Machines that pull carbon dioxide from the air so it can be stored underground have been developed - but the cost of running them remains very high.
A cheaper way to lower temperatures would be to keep the planet's remaining forests and other natural systems healthy, as they naturally absorb carbon dioxide released by burning fossil fuels, helping buffer temperature rise.
Nature has absorbed more than half – 56 percent - of the fossil fuel emissions emitted since 1750, about half of that on land and half in oceans, Rockström said.
That huge buffering of what might otherwise now be runaway climate change amounts to "the biggest subsidy in the world economy", he said.
But as deforestation and other land degradation rises in many places around the world, the ability of nature to absorb climate pollution is falling.
If the world overshoots the 1.5C temperature limit, healthy ecosystems combined with a halt to fossil fuel use could over time help absorb pollution and bring down temperatures again.
"But this will only work if we don't destroy nature," Rockström said.
IS IT TIME TO ABANDON THE 1.5C LIMIT, IF IT'S UNLIKELY TO BE MET?
Dropping the 1.5C limit for a higher one would dramatically increase the risks of food and water shortages, conflict, deadly extreme weather and other crises, Rockström and other scientists have said, and likely push the world past dangerous tipping points from which there is no possibility of return.
To remain safe on the planet, "1.5C is a temperature threshold we cannot drop," Rockström said.
Beyond that, "what's certain is there's a big risk and we don't want to take that risk."