Earth may be less sensitive to CO2 than our worst fears

Just how much the temperature of our planet will rise in response to increasing levels of CO2 in the atmosphere is still a matter of debate, but a new study suggests the most optimistic and the most pessimistic predictions may be wide of the mark.

In research published in Nature, British scientists used historical records to estimate Earth’s equilibrium climate sensitivity (ECS) – the average temperature increase caused by a doubling of atmospheric CO₂.

“Climate contrarians say it’s not sensitive, while some models say it is very sensitive; most scientists figure it’s unlikely to be too far from a middle value [around 3 degrees Celsius per doubling of CO₂],” Steven Sherwood from the ARC Centre for Climate System Science told the Australian Science Media Centre. 

“Middle implies about 4 degrees Celsius of warming by 2100 if we continue on a high-emissions trajectory, which would not be good,” he added. “Low sensitivity would imply less to worry about, while high sensitivity would be really alarming.”

The new study estimates Earth’s ECS at between 2.2° degrees Celsius and 3.4 degrees Celsius.

Previous estimates from the Intergovernmental Panel on Climate Change suggested the ETS lay between 1.5 degrees Celsius and 4.5 degrees Celsius.

“Climate sensitivity has been a hard nut to crack,” said Piers Forster from the University of Leeds in the UK. “For nearly four decades, we atmospheric scientists have had only a rough idea of how much the Earth’s temperature will warm as carbon dioxide levels rise.”

The new findings suggest an ECS of 4.5 degrees Celsius can probably be ruled out, and it’s equally unlikely that an ECS of 1.5 degrees Celsius –  the most optimistic prediction – is correct.

“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,” said Sherwood. “Both the lowest and highest previous predictions are contradicted by this new approach.”

So how confident can we be that the approach of the new study is correct?

“Questions still remain,” according to Gabi Hegerl from the University of Edinburgh. “The paper may not be the last word, but the result is interesting and also fits well to other lines of evidence on climate sensitivity”.

Sherwood agreed: “It is not clear whether to put more weight on this study, or the previous ones suggesting even higher sensitivity,” he said. “But one thing all the studies agree on is that climate sensitivity is not very low, i.e., it is above 2 degrees Celsius per doubling.”

And what does this mean for the fight against climate change? 

Sherwood heralded the “good news of further reducing the chance of the worst-case possibilities”.

And the University of Reading’s Richard Allan was also optimistic: “Rather than warming being inconsequential or catastrophic, as some have suggested, we can be sure societies are facing a dangerous temperature rise, but one which we still have time to fix,” he said.

Prepared by the Australian Science Media Centre and used here with permission.