Last updated January 18, 2018 at 5:09 pm
New research rules out the most pessimistic warming scenarios.
A revised calculation of how carbon dioxide drives global warming has led to a new estimate of the end-of-century temperature increase range.
While the new study, published in the journal Nature, rules out the upper extremes, it also says that the optimists also have it wrong.
“Our study all but rules out very low and very high climate sensitivities,” lead author Peter Cox, a professor at the University of Exeter in the UK, was quoted as saying.
Although the greenhouse effect that allows CO2 to capture and retain heat in the atmosphere is well understood, exactly how much the climate warms in response to increasing units of CO2 is not. This is the ‘sensitivity’ of the climate to CO2.
What if the carbon dioxide concentration doubled?
Equilibrium Climate Sensitivity (ECS) is the global mean warming that would occur if the atmospheric carbon dioxide concentration doubled, once the atmosphere and ocean temperatures come into equilibrium.
So if the CO2 concentration were to double from the pre-industrial levels of 280 parts per million by volume (ppmv) to 560 ppmv, how much would the temperature rise?
The ‘likely range’ of ECS has been stated as 1.5-4.5 °C by the Intergovernmental Panel on Climate Change (IPCC) for more than 25 years.
Climate contrarians argue that sensitivity is low, that even if CO2 doubles there would be minimal impact on temperatures.
‘The mainstream view has got it right’
If climate sensitivity is high then it would make it more difficult to avoid the 2 °C limit of global warming required by the Paris Agreement.
Peter Cox and colleagues publish a new estimate for ECS based on the observed historical variability in temperature, rather than the overall warming trend. Their estimate puts ECS at 2.2 – 3.4 °C, reducing the uncertainty in the existing IPCC estimate.
Professor Steven Sherwood is ARC Laureate Fellow at the ARC Centre for Climate System Science and UNSW Climate Change Research Centre says the new findings confirm the view of most climate scientists.
“If their approach is right it basically means the mainstream view has been correct: there is essentially no chance that contrarians have been right, but also hardly any chance of the worst-case scenario.
“Both the lowest and highest previous predictions are contradicted by this new approach.”
Professor Sherwood states there are some caveats to this study in how natural variability due to El Niño, decadal variations, volcanic eruptions and air pollutants was incorporated into the estimate.
“But one thing all the studies agree on is that climate sensitivity is not very low, i.e., it is above 2 °C per doubling.”
This research appeared in Nature.