Climate sensitivity to rising CO2 levels still uncertain
October 25, 2007

Climate sensitivity to increasing concentrations of greenhouse gases is still largely uncertain and researchers will likely not be able to further refine their estimates on future climate scenarios, say two University of Washington scientists writing in this week's issue of the journal Science.

The scientists, Dr. Gerard Roe and Dr. Marcia Baker, "believe the uncertainty remains so high because the climate system itself is very sensitive to a variety of factors, such as increased greenhouse gases or a higher concentration of atmospheric particles that reflect sunlight back into space," according to a statement from the University of Washington.

"Uncertainty and sensitivity have to go hand in hand. They're inextricable," said Gerard Roe, an associate professor of Earth and space sciences. "We're used to systems in which reducing the uncertainty in the physics means reducing the uncertainty in the response by about the same proportion. But that's not how climate change works."

The researchers say that processes in the climate system can amplify or subdue the response of climate to rising carbon dioxide concentrations. For example, a warmer atmosphere will hold more water vapor -- itself a greenhouse gas -- amplifying the effect on temperature caused by the original rise in carbon dioxide concentrations. Roe and Baker say such effects could boost global temperatures by 5 degrees Fahrenheit or more over the projected 2.2-degree rise in temperatures resulting from a doubling of pre-Industrial Revolution CO2 levels.

To improve forecasts, Roe and Baker have devised an equation to help climate modelers "understand built-in uncertainties so that the researchers can get meaningful results after running a climate model just a few times, rather than having to run it several thousand times and adjust various climate factors each time".

"It's a yardstick against which one can test climate models," Roe said.

The new equation suggests that while possible, extreme climate change forecast by some models, is unlikely to occur. Nevertheless, the researchers say that reducing emissions is important to reducing the risk of worst-case scenarios.

"If all we do is stabilize concentrations, then we will still be risking the highest temperature change shown in the models," said Roe.

This article is based on news releases from Science and the University of Washington.

CITATION: (October 25, 2007).

Climate sensitivity to rising CO2 levels still uncertain.