Simple observational proof of greenhouse effect
Posted by Ari Jokimäki on March 10, 2010
I have seen people claiming that warmer world and more carbon dioxide is better because who wouldn’t like warmth and carbon dioxide is such a life gas (despite of the science).
I have seen people claiming that climate sensitivity is lower than thought so even if carbon dioxide has caused some warming, it won’t cause catastrophic warming (despite of the science).
I have seen people claiming that it’s not carbon dioxide that is causing warming so any actions on carbon dioxide emissions are useless (despite of the science).
I even have seen people claiming that the greenhouse effect does not exist…
The last one of these claims is why I’m writing this thing now.
Ellingson & Wiscombe (1996) gave a description of the Spectral Radiance Experiment (SPECTRE) and some initial results of the measurements. SPECTRE is a surface-based experimental field program which has a goal to “…establish a reference standard against which to compare models and also to drastically reduce the uncertainties in humidity, aerosol, etc., which radiation modelers had invoked in the past to excuse disagreements with observations.”
They are measuring the atmospheric longwave emission, i.e. the thermal radiation that atmosphere emits and which then arrives to the Earth’s surface. More specifically, they are measuring the spectrum of that emission. This is where it gets interesting. From the spectrum of thermal radiation it is possible to detect the influences of different greenhouse gases. If carbon dioxide sends thermal radiation, it has certain frequencies characteristic to carbon dioxide and as spectrum shows the emission strength at different frequencies, the spectrum shows directly if carbon dioxide is emitting thermal radiation.
In the greenhouse effect, the sunlight warms the surface of the Earth and then warm surface emits thermal radiation to the space. In atmosphere, however, there are greenhouse gases which absorb some of the emitted thermal radiation at certain frequencies (spectral bands) and then send some of it back at same spectral bands to the surface of the Earth. So, if there is a greenhouse effect caused by carbon dioxide, we should see atmosphere emitting thermal radiation at carbon dioxide’s spectral bands.
Ellingson & Wiscombe (1996) show examples of the measurements they have made. One of them is presented in Figure 1 (Fig. 3 of Ellingson & Wiscombe, 1996). There we can see at which frequencies atmosphere is emitting thermal radiation to the surface of the Earth. The effects of different greenhouse gases have been marked (CO2 is carbon dioxide, H2O is water vapor, O3 is ozone, CH4 is methane, N2O is nitrous oxide).
Figure 1. The figure 3 of Ellingson & Wiscombe (1996) showing one of their measurements of downward longwave radiation coming from atmosphere to the surface of the Earth. Measurements were made in Wisconsin, December 1991.
All we need to do is to take a peek at the Fig. 1 to see if greenhouse effect exists or not. See the effects of different greenhouse gases there? That’s the greenhouse effect.
Ellingson & Wiscombe (1996), Bulletin of the American Meteorological Society, Volume 77, Issue 9, “The Spectral Radiance Experiment (SPECTRE): Project Description and Sample Results”, [abstract, full article]