Climate Less Sensitive to Greenhouse Gases Than Predicted, Study Says

John Roach
for National Geographic News
April 19, 2006
How sensitive is Earth's climate? Sufficient to warm by at least several
degrees in response to greenhouse gas pollution but perhaps not as
sensitive as some scientists have feared, according to a new study.

Climate sensitivity is a measure of how much the global temperature will warm in response to greenhouse gas emissions, explained Gabriele Hegerl, a climate scientist at Duke University in Durham, North Carolina.

The measure specifically estimates how the climate might respond to a doubling of the amount of carbon dioxide in the atmosphere. Carbon dioxide is a greenhouse gas that humans release by burning fossil fuels in cars and power plants.

"If [climate sensitivity is] high, we have a strong response not only to carbon dioxide but to any greenhouse gas. If it's low, we have a weak response. So we would really like to know what it is," Hegerl said.

Hegerl and her team measured climate sensitivity by studying temperature changes in the Northern Hemisphere over the past 700 years.

The study's results refute recent research suggesting that the climate may be susceptible to extreme increases in temperature. But Hegerl cautions that the findings do not diminish the threat of global warming.

"[The finding] means the climate does react significantly to greenhouse gases," she said.

"In other words," she added, "we have really detected greenhouse warming, and we are really concerned it is not small."

(See National Geographic magazine's "Global Warning: Signs From Earth.")

Climate Sensitivity

In 1979 meteorologist Jules Charney made the first modern estimate of climate sensitivity using two climate models. He concluded that the Earth would warm within a range of 2.7º to 8.1ºF (1.5º to 4.5ºC) if carbon dioxide in the atmosphere doubled.

That range has become the generally accepted value of climate sensitivity and is used in international climate-change research.

But some estimates over the past five years have suggested that the temperature change may be much higher.

Several studies have found that the temperature change may be higher than 16.2ºF (9ºC). One estimate put it at 19.8ºF (11ºC).

To obtain their estimate, Hegerl and her colleagues used reconstructions of the climate in the Northern Hemipshere over the past 700 years, which were made using data about ancient volcanic eruptions, changes in solar radiation, and greenhouse gas levels.

They then used a simple computer model to determine what type of climate conditions led to those temperatures, tweaking different variables such as volcanic ash and solar radiation.

On the basis of this model, they found a climate sensitivity of 2.2º to 15.5ºF (1.2º to 8.6ºC).

In addition, the team used a different model using climate data from only the 20th century and came up with a similar result.

"We have two lines of evidence, basically," Hegerl said.

When the team combined the two estimates, the researchers found that the climate sensitivity range narrowed even further to 2.7º to 11.2ºF (1.5º to 6.2ºC).

These findings are similar to the conclusions reached by Charney in 1979, rather than the more extreme ranges estimated in recent studies.

The results are reported in tomorrow's issue of the journal Nature.

Controversial Results

Gavin Schmidt is a climate modeler with the NASA Goddard Institute for Space Studies in New York. He said the study is the first to formally confirm scientists' assumptions about climate sensitivity.

"Basically no one really believes that those really high sensitivities [measured in the past five years] are possible," he said.

But Michael Schlesinger, a professor of meteorology at the University of Illinois at Urbana-Champaign and co-author of a 2001 paper on climate sensitivity, is wary of the new study's methodology.

In particular, Schlesinger questions the effects of comparing climate sensitivity based on the 20th century record with another estimate based on a 700-year record.

"If they chose a different [record]—if they chose our [study] or any others—and had done this, I do not see how [the range] gets narrowed," Schlesinger said.

"I'm not comfortable with the results."

On the other hand, James Annan, a climate modeler with the Frontier Research Center for Global Change in Kanagawa, Japan, said the sensitivity range should be narrowed even further, based on the results of his own work published in March.

"If they had looked at a greater range of evidence, then the limits would have been even tighter," he said.

Regardless of the numbers, most scientists seem to agree that the study confirms that the climate remains susceptible to global warming.

The top of the range found in Hegerl's study is higher than that found in 1979, Schlesinger points out.

"[That] means climate sensitivity is larger than we thought for 30 years," he said. "So the problem is worse than we thought. This doesn't give us any solace."

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