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New study: Climate not as sensitive to carbon dioxide as some fear

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Issue date: 
November 24, 2011
Publisher Name: 
SciGuy, Eric Berger
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Let’s see if an intriguing new paper in Science, released this afternoon, can break through your somnolent Turkey Day afternoon.

Led by Oregon climate scientist Andreas Schmittner the paper suggests the rate of warming from a doubling of atmospheric carbon dioxide may be less than the most dire estimates.

Specifically, if atmospheric carbon dioxide levels are to double from pre-industrial levels of 280 ppm to 560 ppm, the researchers say Earth’s average temperature is likely to rise from 1.7 to 2.6 Celsius degrees, a decrease from the previous, accepted range of 2 to 4.5 degrees.

Here’s what the research group did to reach this conclusion:

They compiled land and ocean surface temperature reconstructions from the Last Glacial Maximum about 20,000 years ago and created a global map of those temperatures. They then compared those values with those of climate models. There was a big difference, suggesting the models were overcooking the sensitivity of temperatures to carbon dioxide levels.

“If these paleoclimatic constraints apply to the future, as predicted by our model, the results imply less probability of extreme climatic change than previously thought,” Schmittner said.

As a result the group’s best estimation of climate sensitivity for a doubling of carbon dioxide levels is 2.4 Celsius degrees, which is 20 percent below the IPCC’s best estimate of 3 Celsius degrees.

Model projections from various warming scenarios. (IPCC 2007).

Is this significant? I asked Andy Dessler, a climate scientist at Texas A&M University, for his thoughts. His reply:

Ordinarily, when something is published in Science, you expect it to be a significant advance/revision of our prior knowledge. I’m not sure that’s the case here. The conclusion about the climate sensitivity is pretty consistent what most climate scientists think, so I don’t think this paper will change what people think.

My sense is that most scientists consider the very high end of the sensitivity range (greater than 4°C) to be pretty unlikely (although it cannot be ruled out), and the most likely value for climate sensitivity is around, probably slightly below, 3°C.

In other words, I was not terribly worried about runaway climate change before this.  After all, we know that the Earth’s had much higher CO2 in the past (and the temperature were correspondingly much higher), and the Earth did not turn into Venus.

To me, the real effect of this paper will be to really impair the credibility of the more extreme environmentalists who have been saying the planet faces certain doom from climate change.

I am thinking about such efforts as Bill McKibben’s 350 campaign, in which he asserts that 350 ppm is the most important number in the world. Such environmentalists assert that the planet will warm as much as 6 Celsius degrees with a doubling of atmospheric carbon dioxide levels.

That’s a big number and doubtless would have catastrophic consequences for the planet. This is not in dispute. But scientists are now telling us this is not going to happen.

Clearly climate change is nonetheless worth being concerned about, and the levels of carbon dioxide and other greenhouse gases we’re putting into the atmosphere will have consequences now, and more serious consequences for our children.

But it is time to rein in the 6-degrees, world-is-ending rhetoric. This is a serious issue that will require serious solutions. Just as denial of any human role in a warming climate has no place in reality, neither does overstating the case for political action.


Extpub | by Dr. Radut