Australasian Science: Australia's authority on science since 1938

Climate Uncertainty Reduced

By Stephen Luntz

A new way of estimating the uncertainty of the Earth’s average temperature for the rest of this century anticipates warming very similar to predictions by the Intergovernmental Panel on Climate Change (IPCC) while narrowing the uncertainty.

However, the modelling uses only known processes, leaving out tipping points such as a sudden release of methane from permafrost.

“The IPCC used 20 models and estimated uncertainty from the range,” says Prof David Karoly of the University of Melbourne. However, the models did not consider the way the carbon cycle is affected by interactions with the oceans and land surface. Extra uncertainty was added to allow for this.

Karoly and Dr Roger Bodman of Victoria University used a simpler model but added these carbon cycle factors. They then used not only 20th century temperature observations to constrain their model, as is standard, but past observations of carbon dioxide variations.

By considering these neglected factors Karoly and Bodman have shown that we are only likely to exceed a 6°C temperature increase by 2100 if human-induced warming triggers a runaway effect they could not model. However, the pair also reveal that it is even less likely than previously recognised that temperatures will remain within the globally agreed target of 2°C of pre-industrial levels without substantial action to stem emissions.

“The model was not constrained to produce the same median warming as the IPCC’s average,” Karoly says, but the findings on this point were effectively identical. The research was published in Nature Climate Change.

Substantial uncertainty remains, and while Karoly and Bodman say this can be reduced with further research they say it cannot be eliminated entirely. Karoly says: “Uncertainty in climate sensitivity depends largely on aerosols. We know a lot about the variation in greenhouse gases over the last century, but not about the aerosol forcing.”

Since aerosols increase clouds and cool the planet, the planet must be extremely sensitive to warming from carbon dioxide to have overcome this and warmed anyway. Without knowing the amount of aerosols and how effective they have been we cannot know how sensitive the Earth is to a specific increase in carbon dioxide.

While almost two-thirds of the remaining uncertainty was attributed to specific unknown factors, more than one-third came from the interaction between these factors, making it even harder to resolve.

“This study ultimately shows why waiting for certainty will fail as a strategy,” Bodman says. “Some uncertainty will always remain, meaning that we need to manage the risks of warming with the knowledge we have.”