The atmospheric lifetime of a greenhouse gas refers to the approximate amount of time it would take for the anthropogenic increase (i.e. increase due to human behavior) to an atmospheric pollutant concentration to return to its natural level. That can happen as a result of either being converted to another chemical compound or being taken out of the atmosphere via a so-called sink. The lifetime depends on the pollutant’s sources and sinks as well as its reactivity.

The lifetime of a pollutant is often considered together with the mixing of pollutants in the atmosphere –a long lifetime will allow the pollutant to mix throughout the atmosphere. Average lifetimes can vary from about a week ( e.g. sulfate aerosols, small particles in a gas) to more than a century (e.g. carbon dioxide). The chart below shows the atmospheric lifetime of four common greenhouse gases.

In the graph you see that carbon dioxide is hanging around in the atmosphere for quite a long time after we emit it, longer than other greenhouse gases like methane. But you may have heard people talking about methane being 20 times or even 50 times stronger than carbon dioxide. Such statements are quite misleading without further clarification. In fact, that’s the reason why scientist have come up with something called the Global Warming Potential, which looks at the overall effect of a greenhouse gas over the timespan of 100 years after it has been emitted. Even though methane has disappeared after 12-15 years, the net effect is still 23 times stronger than carbon dioxide!