Scientists: Ancient, Modern Evidence Suggest Limits To Magnitude Of Future Global Warming

Instrumental readings made during the past century offer ample evidence that carbon dioxide and other "greenhouse gases" in the atmosphere are warming Earth's climate, a team led by Duke University scientists reported on April 20. By analyzing indirect evidence of temperature fluctuations over six previous centuries, the team also found that the magnitude of future global warming will likely fall well short of current highest predictions.

In making their deductions, the researchers ran some 1,000 computer simulations, covering 1,000 years, that took into account a range of modern and ancient climate records. Modern records are based on thermometer readings, while measurements derived from such sources as tree rings and ice cores served as markers of warm and cold spells over prior centuries.

The investigators evaluated the data using an "energy balance model" that they describe as a slimmed-down version of the heavy-duty computer models typically used to analyze climate trends. It is the model's streamlined nature that enabled the researchers to perform such large numbers of simulations over such a long period in such detail, they said.

The group used thousands of different versions of this model, each version varying in some of its properties, in order to determine which variants best matched actual observations. One key property that varied was what the researchers termed "sensitivity" -- that is, how much the simulations' temperatures would change in response to increasing greenhouse gas levels.

"What I can say very confidently is that the present-day sensitivity is not zero, meaning that there is a positive, warming response to greenhouse gases," said climate analyst Gabriele Hegerl, an associate research professor at Duke's Nicholas School of the Environment and Earth Sciences (http://www.env.duke.edu). "Our work also substantially reduces the probability of very high climate sensitivities."

Hegerl is lead author of the study, published on April 20 in the journal Nature. Her co-authors are Thomas Crowley, Duke's Nicholas Professor of Earth Systems Science; William Hyde, a former Nicholas School research scientist now at the University of Toronto; and David Frame, a researcher at the University of Oxford.

Many scientists expect that the level of carbon dioxide in the atmosphere will sometime this century reach double the levels that were present during pre-industrial times. Because carbon dioxide traps outgoing heat energy similarly to the glass in a greenhouse, the additional human-created outputs of the gas -- mostly from fossil-fuel burning -- are expected to warm Earth's climate. The key question is: by how much?

The commonly accepted range for how much average global temperatures will rise in response to a doubling of atmospheric carbon dioxide is between 1.5 degrees and 4.5 degrees Celsius, according to the researchers. But some observational studies, they noted, suggest the possibility that average temperatures might rise more than 9 degrees Celsius.

However, the new study -- using "reconstructions" of Northern Hemisphere temperatures since the year 1270 -- indicates a 90 percent probability that a doubling of carbon dioxide levels will result in temperature increases of between 1.5 degrees Celsius and 6.2 degrees Celsius, the team reported.

In turn, the study showed a reduced likelihood that the actual maximum increase will exceed 4.5 degrees Celsius -- "from 36 percent to 15 percent or less," the researchers said. A 4.5 degree Celsius increase is the highest maximum currently predicted by the international Intergovernmental Panel on Climate Change.

Gabriele Hegerl: http://www.env.duke.edu/people/faculty/hegerl.html

This article originally appeared in the 04/01/2006 issue of Environmental Protection.

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