Chlorofluorocarbons are stable in the troposphere and can only be broken down by UV light when they rise into the stratosphere. Two of the most common CFCs are CFC-11 and CFC-12. Their atmospheric lifetimes and global warming potentials are given below.

Although CFC-12 and CFC-11 have low concentrations in the atmosphere, approximately 0.000533 ppm and 0.000240 ppm respectively, their GWPs ensure that they will affect Earth's radiation balance.

Question: Why does CFC-12 have a much higher global warming potential than methane (CH₄)?

Recall that the GWP of a gas depends on both the atmospheric lifetime of the gas and the region of the infrared spectrum in which the greenhouse gas absorbs.

First of all, CFC-12 has a much longer atmospheric lifetime than CH₄. CFC-12 has an atmospheric lifetime of 100 years, while CH₄ has an atmospheric lifetime of only 12 years. This explains in part why CFC-12 has a much higher GWP than CH₄.

Also, consider the regions in the IR spectrum in which CFC-12 and CH₄ absorb. Open the IR Windows learning tool and turn on the Earth's black body curve. Now click on the IR spectrum of CFC-12. Where do the absorption peaks in CFC-12's spectrum occur?

Now turn on the IR spectra of CO₂ and H₂O. Notice how the regions in which CFC-12 absorbs infrared radiation are located in an IR window region.

Turn on methane's IR spectrum and compare its absorption peaks to those of CFC-12. Notice that CFC-12 absorbs IR radiation within the peak of Earth's emission spectrum as well as in the IR window region. CH₄ absorbs in the IR window region but not directly within the peak of Earth's emission spectrum.

Both the long atmospheric lifetime of CFC-12 and the region in which it absorbs make CFC-12 an extremely potent greenhouse gas. Because its absorption peak occurs near the height of Earth's emission spectrum and in the middle of the IR window region, this gas absorbs wavelengths of radiation that would otherwise escape into space.

Question: Why does CFC-11 have a high global warming potential?