Simple observational proof of the greenhouse effect of carbon dioxide
Posted by Ari Jokimäki on April 19, 2010
Recently, I showed briefly a simple observational proof that greenhouse effect exists using a paper by Ellingson & Wiscombe (1996). Now I will present a similar paper that deepens the proof and shows more clearly how different greenhouse gases really are greenhouse gases. I’ll highlight the carbon dioxide related issues in their paper.
Walden et al. (1998) studied the downward longwave radiation spectrum in Antarctica. Their study covers only a single year so this is not about how the increase in greenhouse gases affects. They measured the downward longwave radiation spectrum coming from atmosphere to the surface during the year (usually in every 12 hours) and then selected three measurements from clear-sky days for comparison with the results of a line-by-line radiative transfer model.
First they described why Antarctica is a good place for this kind of study:
Since the atmosphere is so cold and dry (<1 mm of precipitable water), the overlap of the emission spectrum of water vapor with that of other gases is greatly reduced. Therefore the spectral signatures of other important infrared emitters, namely, CO2, O3, CH4, and N2O, are quite distinct. In addition, the low atmospheric temperatures provide an extreme test case for testing models
Spectral overlapping is a consideration here because they are using a moderate resolution (about 1 cm-1) in their spectral analysis. They went on further describing their measurements and the equipment used and their calibration. They also discussed the uncertainties in the measurements thoroughly.
They then presented the measured spectra in similar style than was shown in Ellingson & Wiscombe (1996). They proceeded to produce their model results. The models were controlled with actual measurements of atmospheric consituents (water vapour, carbon dioxide, etc.). The model is used here because it represents our theories which are based on numerous experiments in laboratories and in the atmosphere. They then performed the comparison between the model results and the measurements. Figure 1 shows their Figure 11 where total spectral radiance from their model is compared to measured spectral radiance.
The upper panel of Figure 1 shows the spectral radiance and the lower panel shows the difference of measured and modelled spectrum. The overall match is excellent and there’s no way you could get this match by chance so this already shows that different greenhouse gases really are producing a greenhouse effect just as our theories predict. Walden et al. didn’t stop there. Next they showed the details of how the measured spectral bands of different greenhouse gases compare with model results. The comparison of carbon dioxide is shown here in Figure 2 (which is the upper panel of their figure 13).
The match between the modelled and measured carbon dioxide spectral band is also excellent, even the minor details track each other well except for couple of places of slight difference. If there wouldn’t be greenhouse effect from carbon dioxide or if water vapour would be masking its effect, this match should then be accidental. I see no chance for that, so this seems to be a simple observational proof that carbon dioxide produces a greenhouse effect just as our theories predict.
Walden, V. P., S. G. Warren, and F. J. Murcray (1998), Measurements of the downward longwave radiation spectrum over the Antarctic Plateau and comparisons with a line-by-line radiative transfer model for clear skies, J. Geophys. Res., 103(D4), 3825–3846, doi:10.1029/97JD02433. [abstract]