Greenhouse Gas Lifetimes in the Atmosphere
The insidious characteristic of most greenhouse gases is that they have long lifetimes in the atmosphere, as measured by their "half life" (the time for half of an initial amount released to be removed from the atmosphere by natural processes). Carbon dioxide has a half life of about 120 years, methane 10.5 years, nitrous oxide 132 years, and the CFCs 16 to more than 500 years, as shown in the accompanying table. So, for instance, of the 20,000 kg of carbon dioxide you put into the atmosphere in 2004, 10,000 kg will still be contributing to enhanced greenhouse warming in the year 2124, 5,000 kg will be remaining in 2244, 2,500 kg in 2361, ..., 1 kg in the year 3684. At least 1 kg of carbon dioxide you put into the atmosphere this past year will contribute to enhanced greenhouse warming for the next 1,680 years!
From these examples, you can see that there are two factors that combine to determine the "global warming potential" (GWP) of a greenhouse gas: (1) radiation absorbing capacity, and (2) lifetime in the atmosphere. We will come back to GWP in a later lecture.