Worst-case global warming scenarios not credible: Study
PARIS - Agence France-Presse
A revised calculation of how greenhouse gases drive up the planet’s temperature reduces the range of possible end-of-century outcomes by more than half, researchers said in the report, published in the journal Nature.
How effectively the world slashes CO2 and methane emissions, improves energy efficiency, and develops technologies to remove CO2 from the air will determine whether climate change remains manageable or unleashes a maelstrom of human misery.
But uncertainty about how hot things will get also stems from the inability of scientists to nail down a very simple question: By how much will Earth’s average surface temperature go up if the amount of CO2 in the atmosphere is doubled?
That “known unknown” is called equilibrium climate sensitivity (ECS), and for the last 25 years the U.N.’s Intergovernmental Panel on Climate Change (IPCC) -- the ultimate authority on climate science -- has settled on a range of 1.5 C to 4.5 C (2.7 to 8.1 degrees Fahrenheit).
Cox and colleagues, using a new methodology, have come up with a far narrower range: 2.2 C to 3.4 C, with a best estimate of 2.8 C (5 F).
If accurate, it precludes the most destructive doomsday scenarios.
“These scientists have produced a more accurate estimate of how the planet will respond to increasing CO2 levels,” said Piers Forster, director of the Priestley International Centre for Climate at the University of Leeds.
Gabi Hegerl, a climate scientist at the University of Edinburgh who, like Forster, did not take part in the research, added: “Having lower probability for very high sensitivity is reassuring.“Very high sensitivity would have made it extremely hard to limit climate change according to the Paris targets.”
“We will still see significant warming and impacts this century if we don’t increase our ambition to reduce CO2 emissions,” said Forster.
Even a 1.5 C increase will have consequences.
.A 3.5 C world, scientists say, could pull at the fabric of civilization.
Since industrialization took off in the early 19th century, CO2 concentrations in the atmosphere have increased by nearly half, from 280 parts per million to 407 parts per million.
Up to now, attempts to narrow down the elusive equilibrium climate sensitivity have focused on the historical temperature record.
By analyzing the responsiveness of short-term changes in temperature to “nudges and bumps” in the climate system, he explained, they were able to exclude the outcomes that would have resulted in devastating increases of 4 C or more by 2100.
One wild card not taken into consideration by the new model is the possibility of rapid shifts in climate brought on by the planet itself.
“There is indeed evidence that the climate system can undergo abrupt changes or ‘tipping points’,” Cox told AFP.
The collapse of the gulf stream, the thawing of carbon-rich permafrost, or the melting of ice sheets on Greenland and Antarctica -- any of these could quickly change the equation, and not in the Earth’s favor.