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Latest Forecast: Stand By for a Warmer, But Not Scorching, World

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Science  21 Apr 2006:
Vol. 312, Issue 5772, pp. 351a
DOI: 10.1126/science.312.5772.351a

While newly climate-conscious news reporters seek signs of apocalyptic change in hungry polar bears and pumped-up hurricanes, evidence-oriented researchers are working to nail down some numbers. They are concerned with climate sensitivity: how much a given increase in atmospheric carbon dioxide will warm the world. If it's extremely high, continued emissions of greenhouse gases could ignite a climatic firestorm. If it's very low, they might merely raise the global thermostat a notch or two.

Now two new studies that combine independent lines of evidence agree that climate sensitivity is at least moderately strong—moderate enough so that a really scorching warming appears unlikely. Even with the most conservative assumptions, says climate researcher Chris E. Forest of the Massachusetts Institute of Technology in Cambridge, the studies cool the maximum warming. And the reinforced low end of the range, he says, means continued emissions will fuel a substantial warming in this century.

The new studies use a technique called Bayesian statistics to gauge how adding new information improves past estimates of climate sensitivity. Most previous estimates used only a single line of evidence, such as how climate warmed as greenhouse gases increased during the 20th century or how climate cooled right after the debris from a major volcanic eruption shaded the planet. Lately, such analyses have tended to support a 25-year-old guess about climate sensitivity: If the concentration of CO2 were to double, as is expected by late in the 21st century, the world would warm between a modest 1.5°C and a hefty 4.5°C (Science, 13 August 2004, p. 932). The low end of that range looked fairly firm; the negligible warming claimed by greenhouse contrarians looked very unlikely. But no one was sure about the high end. Some studies allowed a real chance that doubling CO2 could raise temperatures by 7°C, 9°C, or even 11°C (Science, 28 January 2005, p. 497).

Sharpening the odds.

Analyzing how climate forces changed temperature in the past yields a wide range for climate sensitivity (left), but combining independent data sets (right) narrows the range.


The two new studies rein in those soaring upper limits for climate sensitivity while reinforcing the substantial lower limit. Climate modeler Gabriele Hegerl of Duke University in Durham, North Carolina, and colleagues started with Northern Hemisphere temperatures between 1270 and 1850 extracted from records such as tree rings. In those preindustrial times, volcanoes, the waxing and waning of the sun, and natural variations in greenhouse gases were changing temperature. Hegerl and her colleagues then combined the preindustrial temperature response to those climate forcings with the global response in the 20th century to volcanoes, rising greenhouse gases, and thickening pollutant hazes. In this week's issue of Nature, they report a 5% probability that climate sensitivity is less than 1.5°C and a 95% chance that it's less than 6.2°C. That's still pretty high, but a far cry from 9°C or 11°C.

In a similar study published on 18 March in Geophysical Research Letters, climate modelers James Annan and Julia Hargreaves of the Frontier Research Center for Global Change in Yokohama, Japan, found the same lower limit of 1.5°C and a 95% upper limit of 4.5°C. They combined published 20th century warming data with records of coolings after recent volcanic eruptions and estimates of chilling in the depths of the latest ice age.

“Combining multiple lines of evidence is certainly the way to go,” says Forest. An extremely high climate sensitivity “is probably less likely than we thought a year ago,” agrees climate researcher Reto Knutti of the National Center for Atmospheric Research in Boulder, Colorado. More importantly, “we start to see a much better agreement on the lower bound,” says Knutti. “We can be pretty sure the changes will be substantial” by the end of the century, he says.

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