In addition to their concentrations in the atmosphere there are two other important factors related to greenhouse gases. The first is the average time each gas remains in the atmosphere. As noted above, for example, carbon dioxide is cycled in and out of the atmosphere by photosynthesis and oxidation. Each of the greenhouse gases has an average atmospheric lifetime:
CFCs 60-100 years
Remind students that these persistence figures tell us something about the chemical reactivity of the compounds. The less reactive a compound is the longer it remains in the atmosphere. This is one of the important intersections of climate change and chemistry. The longer a gas remains in the atmosphere the greater its influence. Note that even if efforts to eliminate the excess production of greenhouse gases were enacted now, the effect of these gases would continue well into the future.
The second factor to be considered when “rating” the influence of greenhouse gases is the degree to which they absorb the infrared radiation passing through the atmosphere. Some gases absorb IR radiation much better than others. That is, some gases are more efficient at absorbing IR radiation. So each of the greenhouse gases is assigned an efficiency value called the “Global Warming Potential” (GWP), which is a comparison of the efficiency of a gas relative to CO2 over a time span of 100 years. For example, a gas with a GWP of 20 is 20 times more efficient at retaining heat than CO2 over a 100 year time period.