The atmospheric lifetime of a gas also plays a role in determining the importance of a greenhouse gas. Atmospheric lifetime refers to the average length of time that molecules of a specific type of gas remain in the atmosphere. If molecules of a particular greenhouse gas remain in the atmosphere for a very long time, the substance will absorb a large amount of IR radiation over the time it remains in the atmosphere.

The atmospheric lifetime of a gas is highly dependent upon the reactivity of the molecules of the gas in the presence of electromagnetic radiation and other substances found in the atmosphere. Chemical reactivity is another property that is not the same for different gases. Some gases react very rapidly in the atmosphere and have short lifetimes, while other gases are so unreactive that they remain in the atmosphere for centuries.

Information concerning the atmospheric lifetime of some of the major greenhouse gases is contained in the IR Windows Learning Tool. Click on the Scaled IR Spectra option and take a look at the table that appears in the top right corner. Which greenhouse gas has the shortest atmospheric lifetime? Which one has the longest?

Your Turn

Atmospheric lifetime is an important factor governing the effect of greenhouse gases on climate. Based solely on the data concerning atmospheric lifetime given in the IR Windows Learning Tool, which of the greenhouse gases shown would be the most important greenhouse gas? Note: In reality, the gas with the longest atmospheric lifetime may not be the most important greenhouse gas because other factors also affect the importance of a greenhouse gas.