"Climate researchers often use a scenario approach," said Klaus Keller, assistant professor of geosciences at Penn State University. "Nevertheless, scenarios are typically silent on the question of probabilities."

The Intergovernmental Panel on Climate Change -- now in its third round of climate assessment -- uses models that include scenarios of human climate forcing drive. The forcing scenarios are, the researchers say, overconfident.

"One key question is which scenario is likely, which is less likely and which they can neglect for practical purposes," said Keller. "At the very least, the scenarios should span the range of relevant future outcomes. This relevant range should also include low-probability, high-impact events."

The researchers say current practice neglects a sizeable fraction of low probability events and results in biased outcomes.

Keller; Louis Miltich, graduate student; Alexander Robinson, Penn State research assistant; and Richard Tol, senior research officer at the Economic and Social Research Institute in Dublin, Ireland, presented their study last week in San Francisco during the annual meeting of the American Geophysical Union.

Related Links