The surface of the Earth almost certainly will not heat between four and five degrees Celsius by the year 2100, according to a study published Wednesday that, if correct, cancels the worst-case predictions for UN climate change.
A revised calculation of how greenhouse gases increase the planet's temperature reduces the range of possible end-of-the-century results by more than half, the researchers said in the report, published in the journal Nature .  "Our study almost rules out very low and very high climate sensitivities," said lead author Peter Cox, a professor at the University of Exeter.
How effectively the world radically reduces CO2 and methane emissions, improves energy efficiency and develops technologies to eliminate CO2 from the air will determine whether climate change is still manageable or will trigger a maelstrom of human misery.
But the uncertainty about how hot things will come also derives from the inability of scientists to clear up a very simple question: how much will the average surface temperature of the Earth increase if the amount of CO2 in the atmosphere doubles?
That "known unknown" is called equilibrium climate sensitivity (ECS), and for the last 25 years the UN Intergovernmental Panel on Climate Change (IPCC) – the last authority on climate science – has been established in a range from 1.5 C to 4.5 C (2.7 to 8.1 degrees Fahrenheit).
Cox and colleagues, using a new methodology, have reached a much narrower range: 2.2 C to 3.4 C, with a better estimate of 2.8 C (5 F).
If necessary, exclude the most destructive apocalyptic scenarios.
"These scientists have produced a more accurate estimate of how the planet will respond to rising CO2 levels," said Piers Forster, director of the International Priestley Climate Center at the University of Leeds.
Gabi Hegerl, a climate scientist at the University of Edinburgh who, like Forster, did not participate in the research, added: "Having a lower likelihood of a very high sensitivity is reassuring.
" There was a sensitivity Very high it is extremely difficult to limit climate change in accordance with the Paris objectives. "
Pressure still in
The historic Paris climate agreement in 2015 called for limiting global warming in" very much below "of 2 C compared to a preindustrial reference point, and pursuing efforts for a ceiling of 1.5 C.
The findings should not be seen as a way to alleviate the need to address climate change, warned the authors and other experts.
"We will still see significant warming and impacts in this century if we do not increase our ambition to reduce CO2 emissions," said Forster.
Even a 1.5 C will have consequences.
With only one degree Celsius of warming so far, the Earth is already facing a crescent of climate impacts including deadly droughts, erratic rainfall and storm surges fattened by rising seas.
A world of 3.5 C, scientists say, could pull the fiber of civilization.
Since industrialization took off at the beginning of the 19th century, CO2 concentrations in the atmosphere have increased by almost half, from 280 parts per million to 407 parts per million.
So far, attempts to reduce the elusive equilibrium climate sensitivity have focused on the historical temperature record.
In contrast, Cox and his colleagues "considered annual fluctuations in global temperature," said Richard Allan, climate scientist at the University of Reading.
In analyzing the response of short-term changes in temperature to "pushes and bumps" in the climate system, he explained, they were able to exclude results that would have resulted in devastating increases of 4 C or more by 2100.  A wild card not taken into consideration by the new model is the possibility of rapid changes in climate caused by the planet itself.
"In fact, there is evidence that the climate system can undergo abrupt changes or turning points", Cox told AFP
The collapse of the Gulf Stream, the melting of the carbon-rich permafrost or the melting of ice sheets in Greenland and Antarctica, any of which could quickly change the equation, and not in favor from the earth.
Future climate change revealed by current climate variations
Peter M. Cox et al. Emergent Restriction in the Sensitivity to the Equilibrium Climate from the Variability of the Global Temperature, Nature (2018). DOI: 10.1038 / nature25450