CO2 sensitivity possibly less than most extreme projections

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A new study in the journal Science suggests that the global climate may be less sensitive to carbon dioxide fluctuations than predicted by the most extreme projections, and maybe slightly less than the best estimates of the Intergovernmental Panel on Climate Change.

Andreas Schmittner, a climate scientist at Oregon State University in Corvallis, Ore., and lead author on the new study, notes that, while man-made global warming is happening and tiny changes in global average temperatures can have huge and deleterious effects, the atmosphere may not be as sensitive to carbon dioxide change as has been reported.

“We used paleoclimate data to look at climate sensitivity to CO2 doubling in the atmosphere, and we are coming up with a somewhat lower value,” says Schmittner.

A 2007 IPCC report addressed the climate sensitivity to carbon dioxide, estimating that air near the surface of the earth would warm 2 to 4.5 degrees Celsius with a doubling of atmospheric carbon dioxide from pre-Industrial (pre-1850) standards. The mean value of that estimate was 3.0 degrees. Thus, if the amount of carbon dioxide in the atmosphere doubled from the amount in 1850, we’d expect a 3 degree C rise in temperature.


Schmittner’s study, however, took a longer view.

“We looked at the paleoclimatic record from the Last Glacial Maximum, which was 19,000-23,000 years before the present. At that time, the planet was much colder than today: There were huge ice sheets over Canada and Northern Europe; the sea levels were much lower, 120 meters lower than today; and C02 levels were also lower, were at 185 ppm. Other greenhouse gas concentrations in the atmosphere were also lower, and there was more dust in the atmosphere,” he said.

Researchers involved with this study compiled large data sets of land and ocean surface temperature reconstructions from that Last Glacial Maximum, when the last ice age was at its peak, and then ran them through the climate models they’ve been using for years, inserting a range of sensitivity numbers from near zero degrees to as high as 8 degrees. Their results showed that temperatures didn’t change as much as would be predicted using the most dire sensitivity numbers. Some independent studies have suggested that carbon dioxide sensitivity might be 10 degrees or higher.

“In fact, a climate sensitivity of more than 6 [degrees] would completely freeze over the planet,” Schmittner pointed out, referring to the ice age. Which, of course, didn’t happen. The ice sheets and glaciation only reached so far toward the equator and then stopped. “So, from that observation alone that it was pretty clear to me that those high climate sensitivities are out of the question, as they are virtually impossible.”

“The best-fitting models had a climate sensitivity of about 2.3-2.4. So that is slightly less than the IPCC best estimate of 3.”

Applying those findings to the future atmosphere, a doubling of carbon dioxide from 1850 levels might mean a rise of 2.4 degrees, rather than 10 or more.

Schmittner points out, however, that there are uncertainties associated with the climate modeling he was using. For instance, the study was unable to take into account changes in clouds on the absorption on sunlight. He expects that the range of climate sensitivities found by the study would expand if cloud changes could be figured in.

Still, Schmittner notes, tiny numbers mean enormous changes. “The temperature reconstructions, they hold a cautionary tale for us,” he says, commenting that even with glaciers covering much of the earth, the ocean temp only went up 2 degrees C.

“If we look at model projections for the future, that suggests temperature changes on the global average of 2-4 degrees are possible. Now, that’s in the similar range to what we had between the Last Glacial Maximum and today. These numbers sound small, but some regions change very dramatically.”


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