AGW Observer

Observations of anthropogenic global warming

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.


Figure 1. The measured spectral radiance compared to the results of a line-by-line radiative transfer model (Walden et al., 1998, Figure 11).

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).


Figure 2. The measured spectral radiance within carbon dioxide spectral band compared to the results of a line-by-line radiative transfer model (Walden et al., 1998, Figure 13 upper panel).

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.

References

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]

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4 Responses to “Simple observational proof of the greenhouse effect of carbon dioxide”

  1. In a couple of places I have seen mention that naturally occurring CO2 has a different “footprint” than CO2 produced from burning fossil fuels – carbon 14 v. carbon 12.

    I’m not a scientist and so much of this is as murky as a foreign language. Do you know of any research showing this distinction? It seems like a very good answer to those who claim humans cannot make much of an impact in the atmosphere.

  2. Ari Jokimäki said

    Yes, there’s some research showing the distinction. Here:

    http://agwobserver.wordpress.com/2009/10/31/papers-on-anthropogenic-carbon-dioxide-observations/

    There are several methods by which we can distinguish the fossil fuel CO2 from natural CO213C/12C ratio, 14C/12C ratio among others. I suggest the Levin & Hesshaimer (2000) for the 14C issue.

    14C is the best way to distinguish fossil fuel CO2 from modern plant CO2, I think. 14C, a.k.a. radiocarbon is exceptional among naturally occurring carbon isotopes in that it is not stable. If you have radiocarbon somewhere it decays by itself in some thousands of years (radiocarbon dating is based on this). There’s always some 14C in the atmosphere because it is produced by cosmic rays entering Earth’s atmosphere and causing some chemical reactions which produce 14C. So, when a plant is taking carbon dioxide from atmosphere the “footprint” of the plant’s carbon always contains some radiocarbon. When we burn recently killed plants, the released carbon dioxide contains radiocarbon because it hasn’t decayed yet. Fossils are millions of years old, so they don’t have any radiocarbon left. Burning fossil fuels therefore releases carbon dioxide that doesn’t have any radiocarbon and burning lot of fossil fuels shows in atmosphere so that the radiocarbon concentration decreases.

    There is, however, a disturbing factor here. The nuclear bombs produce lot of radiocarbon and we have tested lot of nuclear bombs after 1950. That means that after 1950 the atmospheric radiocarbon concentration is spoiled by the bomb radiocarbon, so we have usable records of this only up to that time.

  3. Magnus W said

    Hi Ari,

    Just wonderd if you have seen this: http://climaterealists.com/index.php?id=5540&linkbox=true&position=2

    and know more about the subject?

    (got a mail?)

  4. Ari Jokimäki said

    Hi Magnus!

    Yes, I know something about it but not much details because the person in question haven’t provided any details. Key sentence there is “he intends to publish research results in Nature magazine June issue”. As far as I know, the work hasn’t even been submitted there yet, and knowing the high paper rejection rate of Nature…

    It seems to me that this is just a case of people seeking publicity and a newspaper looking for headlines.

    Of course, we here have already seen what the spectral evidence shows on the AGW and it’s a lot different picture than Kauppinen and/or Turun Sanomat paints.

    Edited to add: By the way, I think the translation of the news article got the word “sue” wrong. I think “challenge” would be more correct. I would have to check this, but I’m quite sure they were not going to sue anybody. Seeing that there are some Finnish words in the article, it seems that this is a google translator produced translation. Finnish doesn’t translate well automatically.

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