Breathe in. Breathe out. You just emitted about a sextillion (that’s a one with 21 zeros behind it) carbon dioxide molecules. In a thousand years 25 percent, or about 250 quintillion, of those molecules will still be floating around in the earth’s atmosphere. Once it is emitted, in other words, carbon dioxide has a very, very, long atmospheric lifetime.
Other greenhouse gases, like methane (CHâ‚„), have a much shorter lifetime. On average, a CHâ‚„ molecule only lasts about 10 or 12 years in the atmosphere before it is broken down.
The interactive graph below represents the persistence, or lifetime, of greenhouse gases in the atmosphere. The Y axis is the fraction of the original “impulse” that remains, so changing the absolute amount of gas released in the slider on the left will not change it. The X axis shows the time that has elapsed since the original emission in years. Suppose you were to release one gigatonne of COâ‚‚ (GtCOâ‚‚). At that moment (year 0) one GtCOâ‚‚ would be in the atmosphere. Each year thereafter, some of that COâ‚‚ would be eliminated. After 20 years, for example, only 0.6 GtCOâ‚‚ would remain.
COâ‚‚ is a very stable molecule. It is primarily eliminated from the atmosphere by plants, which use it for photosynthesis, and by absorption in the ocean. Set the years slider on the left to 1,000 and you will see that even after a millennium, about 25% of that original impulse remains in the atmosphere.
Methane, on the other hand, breaks down relatively quickly. As it circulates in the atmosphere, a series of chemical reactions transform it into other molecules including COâ‚‚. After only a few decades, most of it is gone. The different atmospheric lifetimes of the different greenhouse gases have important implications for climate mitigations.
If you’d like to examine it in more detail, the parameters and equations used to calculate the “impulse response functions” for these gases comes from the IPCC here, here, and here.