Estimates of how the global average temperature will ultimately respond to rising carbon dioxide levels are as good as they’re going to get, a new study says, which suggests that policy makers should change strategies to combat global warming, scientists say.

This ultimate reaction of Earth's climate system to global warming is called climate sensitivity. This measure answers the question, "If you doubled carbon dioxide in the atmosphere [from pre-industrial levels] and then waited for the system to adjust, how much temperature change would you have?" said study leader Gerard Roe of the University of Washington in Seattle.

"So a climate with a higher sensitivity would have a large temperature change for a given increase in carbon dioxide, and a less sensitive system would have a weaker temperature change," Roe explained.

The most recent report from the Intergovernmental Panel on Climate Change puts the Earth's climate sensitivity at between 2 and 4.5 degrees Celsius (3.6 to 8.1 Fahrenheit), with a small chance that it could be higher.

Despite advances in computer modeling and improved understanding of climate processes, this range has remained essentially unchanged since Svante Arrhenius first calculated it in 1896, Roe told LiveScience.

Roe's study, detailed in the Oct. 26 issue of the journal Science, aimed to find out why scientists had been unable to refine this range, despite all the advances made in climate science and experts' certainty that Earth's climate is warming.

Amplifying uncertainty

"Our question was whether it was telling us something fundamental about the system—if it's a real, sort of underlying reason why that's not been narrowing the uncertainty, and it turns out that, that there is an underlying reason," Roe said.

That reason is that the climate system is what Roe describes as an "amplifier of uncertainty." Every feedback in the climate system that results from rising carbon dioxide levels and temperatures (for example, the formation of clouds) has a certain amount of uncertainty associated with it as to how large or small that feedback will be.

And as these internal processes of the climate system propagate and amplify the response to greenhouse gases, the uncertainties associated with them are also amplified, like the proverbial butterfly flapping its wings and creating a hurricane half way around the world.

What this means in practical terms is that the 2- to 4.5-degree Celsius range isn't likely to be narrowed down anytime soon, but as Myles Allen of the University of Oxford, who wasn't involved in the study, said in an accompanying essay in Science, this problem is of little matter if policy makers frame mitigation strategies in the right way.

Effect on policy

Most mitigation strategies have been framed in terms of stabilizing carbon dioxide at a certain level, but this would mean that the ultimate response of the global average temperature could fall anywhere within the IPCC range or be higher, Allen told LiveScience.

A better strategy, he said, would be to pick a temperature target and adjust carbon dioxide concentration levels as we observe the response of the climate system—if the climate sensitivity turned out to be higher (or we observed more warming), we could further reduce emissions to keep the temperature rise down.

Part of the reasoning to this strategy is that systems with higher sensitivities take longer to realize the full effects of climate changes (because they are less efficient at getting rid of the extra heat in the system)—so a climate sensitivity of 8 degrees Celsius would likely take hundreds of years longer to realize than a sensitivity of 2 degrees Celsius. Because we don't know exactly what the sensitivity is and whatever it is, it is likely to take hundreds of years to realize, climate sensitivity isn't an important measure when considering strategies for the next 100 years.

"It's this measure of the ultimate adjustment of the system, so it takes awhile to get there," Roe said. "And so that's an important difference between these figures of, say, what the temperature might be in 2100 versus this other measure, this measure of climate sensitivity."

Allen and Roe caution that the uncertainty inherent to the measure of climate sensitivity doesn't mean that the conclusions of climate science are uncertain. Scientists may not know the exact amount of warming that will occur, but they are certain that warming will, and is, occurring.

"The fact that there will be some warming is a practical certainty, as certain as anything in science actually," Roe said.