Accounting for Greenhouse Gases
The heating effect due to greenhouse gases comes about because molecules only interact with very specific wavelengths of electromagnetic radiation. The Sun mostly provides energy for heating in the form of visible light, but at these relatively small wavelengths (l « 10~7 m) there is little direct absorption by atmospheric molecules. Hence, sunlight can penetrate through a planet's atmosphere to heat the ground. The planet, however, being much cooler than the Sun, characteristically radiates its energy back into space at much longer, infrared wavelengths (l « 10~5 m). The greenhouse-heating
H2O CO2 CF4
H2O CO2 CF4
Figure B.1. Wavelength absorption bands corresponding to various greenhouse gases. Thick lines represent strong absorption bands, whereas thin lines represent weak absorption regions. The height of the absorption bands in the diagram is schematic and not intended to indicate relative absorption strengths. The wavelength axis is plotted on a logarithmic scale. Diagram based upon data published by Marinova et al. Journal of Geophysical Research, 110, E03002 (2005).
effect now comes into play, since molecules in the atmosphere can readily absorb energy at infrared wavelengths.
The so-called absorption bands over which molecules absorb energy can be mapped out in the laboratory, or they can be determined through detailed quantum mechanical calculations. Figure B.1 shows a comparison of the positions of the absorption bands for several greenhouse gases with respect to the blackbody radiation curves for the Earth and Mars.
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