Study: CO2 may not warm the planet as much as thought

The climate may be less sensitive to carbon dioxide than we thought – and temperature rises this century could be smaller than expected. That's the surprise result of a new analysis of the last ice age. However, the finding comes from considering just one climate model, and unless it can be replicated using other models, researchers are dubious that it is genuine.

As more greenhouse gases enter the atmosphere, more heat is trapped and temperatures go up – but by how much? The best estimates say that if the amount of carbon dioxide in the atmosphere doubles, temperatures will rise by 3 °C. This is the "climate sensitivity".

But the 3 °C figure is only an estimate. In 2007, the Intergovernmental Panel on Climate Change (IPCC) said the climate sensitivity could be anywhere between 2 and 4.5 °C. That means the temperature rise from a given release of carbon dioxide is still uncertain.

There have been several attempts to pin down the sensitivity. The latest comes from Andreas Schmittner of Oregon State University, Corvallis, and colleagues, who took a closer look at the Last Glacial Maximum around 20,000 years ago, when the last ice age was at its height.

They used previously published data to put together a detailed global map of surface temperatures. This showed that the planet was, on average, 2.2 °C cooler than today. We already know from ice cores that greenhouse gas levels in the atmosphere at the time were much lower than they are now.

Schmittner plugged the atmospheric greenhouse gas concentrations that existed during the Last Glacial Maximum into a climate model and tried to recreate the global temperature patterns. He found that he had to assume a relatively small climate sensitivity of 2.4 °C if the model was to give the best fit.

When the time comes, and they can no longer hold the fort, they will simply stuff the whole issue down the memory hole and, like the mad hatter's teaparty, just move on to the next place where they can create a whole new mess out of nothing.

The memory hole just received a giant plug by the release of the Climategate II e-mails with almost 200,000 more to be decrypted. This is the second "walking back" story I have read this morning so now we know somebody's are getting scared, and I would wager they are mostly politicians.

Have to agree that it is much more likely that they will drop it down the memory hole and change the subject than admit they were wrong.

Puts me in mind of the story of "Operation Bodyguard" - the phantom "army" under disgraced General Patton which "was going to invade directly across the English Channel and strike at Calais." I had known that story for a generation or more, before I read a book which discussed its ramifications in a little more detail. It never occurred to me to realize that that phantom "army" was supposedly of fantastic size - more like a Russian army than anything we had any thoughts of actually mustering. That was why that threat pinned all of the German tanks at Calais. In reality we had no ability to throw so much force at any particular point that we could have established a beachhead against the kind of force the Germans had at Calais.
But the reason "Bodyguard" is relevant to this discussion is the aftermath of the Normandy landing. It was establishment "logic" - call it "settled science" that we were going to launch a huge offensive at Calais. Then we invaded Normandy. Fine, you would think that the whole German high command would say, "Wow, we've been tricked!" - not in English, of course - and dedicate all their resources to opposing the breakout from the Normandy beachhead. Rommel, of course, saw the implication of Normandy instantly - he knew that a successfully established Allied beachhead anywhere on the French coast was a dagger pointed at the heart of the Reich, and vehemently argued for the movement of resources to attack our beachhead as vigorously as possible. But no - the supposed Bodyguard thrust just hadn't happened yet. The Allies didn't announce that Bodyguard was a ruse, and although they couldn't conceal the Normandy landing, the idea of Calais had its own inertia. Once the establishment had committed to Calais, it was hard to speak of anything else.
The fact that Normandy was the real deal just gradually became the reality on the ground, over the course of a month or so. There never came a single moment when everyone announced in unison that the Emperor had no clothes on.

a green garden pea dropped in an empty 5 gal bucket

How about 100% of the atmosphere visualized in a monetary format...a dollar equals one percent:

Imagine $100.00 in pennies in rows on a large table. That's 10,000 pennies.

Paint four of the pennies green. (CO2 = 4/100ths of a percent)

Using the 4 green pennies, form a layer to trap heat!

Scatter them throughout the "atmosphere"...they can't even see each other.

Water vapor is a larger greenhouse factor than CO2

During the summer, humidity is likely to be a much bigger factor affecting temperatures than CO2. During the winter, with cold air being able to hold less moisture, CO2 may be a bigger factor.

The assumption paraded to the public is that increased CO2 would lead to hotter summers, and this is bad. But if the net effect of CO2 was to have MILDER WINTERS, it would be harder to get people upset over it.

Ice age constraints on climate sensitivity

Response and media coverage

All in all, this is an interesting paper and methodology, though we think it slightly underestimates the most likely sensitivity, and rather more seriously underestimates the chances that the sensitivity lies at the upper end of the IPCC range. Some other commentaries have come to similar conclusions: James Annan (here and here), and there is an excellent interview with Nathan Urban here, which discusses the caveats clearly. The perspective piece from Gabi Hegerl is also worth reading.

Unfortunately, the media coverage has not been very good. Partly, this is related to some ambiguous statements by the authors, and partly because media discussions of climate sensitivity have a history of being poorly done. The dominant frame was set by the press release which made a point of suggesting that this result made “extreme predictions” unlikely. This is fair enough, but had already been clear from the previous work discussed above. This was transformed into “Climate sensitivity was ‘overestimated’” by the BBC (not really a valid statement about the state of the science), compounded by the quote that Andreas Schmittner gave that “this implies that the effect of CO2 on climate is less than previously thought”. Who had previously thought what was left to the readers’ imagination. Indeed, the latter quote also prompted the predictably loony IBD editorial board to declare that this result proves that climate science is a fraud (though this is not Schmittner’s fault – they conclude the same thing every other Tuesday).

The Schmittner et al. analysis marks the insensitive end of the spectrum of climate sensitivity estimates based on LGM data, in large measure because it used a data set and a weighting that may well be biased toward insufficient cooling. Unfortunately, in reporting new scientific studies a common fallacy is to implicitly assume a new study is automatically “better” than previous work and supersedes this. In this case one can’t blame the media, since the authors’ press release cites Schmittner saying that “the effect of CO2 on climate is less than previously thought”. It would have been more appropriate to say something like “our estimate of the effect is less than many previous estimates”.

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