The idea is straight-forward and, by this point, quite well-known: because the molecular structure of CO2 allows it to absorb infrared radiation (i.e. heat) as shown in Figure 1, an increase of CO2 in the Earth’s atmosphere will force a delay in the escape of longwave, infrared radiation back out into space. This results in the greenhouse effect, and along with sufficient levels of atmospheric oxygen, is one of the main reasons why planet Earth is habitable for human and animal life. 

 

(Fig. 1. Different vibrational modes of CO2 molecule from infrared absorption.)

So when Lindzen and Choi (2009) published a paper in Geophysical Research Letters  (GRL) last August suggesting that the effect of CO2 in the atmosphere is – for simplicity’s sake – overstated, some AGW critics were quick to jump on it. Can you blame them? Here’s a study that looks at Earth Radiation Budget Experiment (ERBE) data from NASA and finds that current model predictions are too sensitive. In other words, if we were to double the concentration of atmospheric CO2, it would have much less of a warming effect on the atmosphere than the AGW crowd would have you believe – amounting to a lot of environmental and political fuss for nothing. 

Except that chinks are now starting to appear in the armour of this paper. 

The first blow came last November when former NASA scientist Roy Spencer weighed in on the issue. He commended the authors on their methodology, agreeing that if you want to investigate the link between outgoing radiative change and temperature change, it makes sense to focus on periods of time where a temperature change is actually taking place. However, Spencer was unable to replicate their results under the same parameters, and was less forgiving with the study’s conclusions: 

“So, while I tend to agree with the Lindzen and Choi position that the real climate system is much less sensitive than the IPCC climate models suggest, it is not clear to me that their results actually demonstrate this.” 

The second look comes courtesy of the RealClimate blog with some guest commentary by the authors of a study (Trenberth et al., 2010) published in the past February issue of GRL. They ask the most obvious question of all: 

“Why would such a significant finding have gone undiscovered when these feedbacks are widely studied and recognised as central to the projections of climate change?” 

Indeed, if CO2 really is much less senstive to climate that we have thought all this time, why has it taken so long to figure that out? Climate models may be relatively new on the scene, but the greenhouse properties of CO2 have been known for over 100 years.

So what gives? A lot, apparently, considering the number of objections Trenberth et al. (2010) have to the paper concerning everything from results that are not robust, to misinterpretations, to comparing results to existing, incomplete, ‘test’ models – assumedly never intended for such rigorous attention. In the GRE paper they have this to say:

“While [Lindzen and Choi] adopt a procedure to avoid one of these pitfalls, they fail to recognize and account for several others, they do not account for external forcings, and their use of a limited tropical domain is especially problematic. Moreover their results do not stand up to independent testing.”

Now I’ll be the first to admit I don’t have the personal experience to back up my reaction on this, but nevertheless, having read  a number of scientific papers the past four years, them’s fighting words. Lindzen and Choi clearly have their work cut out for them if they want to continue down their present path. 

References

Lindzen, R. S., and Y.‐S. Choi, 2009. On the determination of climate feedbacks from ERBE data. Geophysical Research Letters, vol., 36, L16705. 

Trenberth, K. E., J. T. Fasullo, C. O’Dell, and T. Wong, 2010. Relationships between tropical sea surface temperature and top‐of‐atmosphere radiation. Geophysical Research Letters, vol. 37, L03702.

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