Now that we know about Earth's radiation balance, we can learn to evaluate the importance of a greenhouse gas based on the three criteria that we discussed earlier.
- Concentration of the gas: If more of the gas is present, more molecules of the gas are able to absorb IR radiation.
- Where in the IR spectrum and how strongly the gas absorbs IR radiation: The Earth does not emit IR radiation equally at all wavelengths in the IR spectrum. Therefore, the region of the IR spectrum in which a gas absorbs IR radiation affects its importance as a greenhouse gas. Also, if the gas absorbs IR radiation very strongly, it will have a greater effect on Earth's radiation balance.
- How long the gas stays in the atmosphere: Gases can only absorb IR radiation as long as they are present in the atmosphere. If a gas is present in the atmosphere for a longer time it will absorb more IR radiation over the course of its atmospheric lifetime.
Open the IR Windows Learning Tool. Click on the carbon dioxide molecule to display its IR spectrum. The peaks in the spectrum show where the molecule absorbs IR radiation (expressed in wavenumbers). Next, select the Black Body Curve from the 'Display Options' menu to display the wavenumbers of IR radiation emitted by the Earth. Use the information displayed in the Learning Tool to answer the following questions.
In what region of the IR spectrum (in wavenumbers) does the Earth emit radiation? How does this compare to the region in which carbon dioxide absorbs IR radiation? How do you think this affects the significance of carbon dioxide as a greenhouse gas?
The Earth emits radiation between approximately 400 and 1600 cm-1. Carbon dioxide absorbs IR radiation at about 700, 2400 and 3600 cm-1. Therefore, carbon dioxide's absorption at 700 cm-1 is directly within Earth's emission spectrum. This suggests that carbon dioxide is a significant greenhouse gas as it is able to absorb some of the IR radiation which is emitted by the Earth.