Atmospheric Lifetime and GWP Compared to CO₂

Figure 2, from the IPCC report, gives the atmospheric lifetime and GWP compared to carbon dioxide for various gases over several different time horizons.

From this table you can see that methane, a short-lived species, contributes 62 times as much as carbon dioxide to the Global Warming potential over twenty years, but over longer periods of time its impact decreases because of its shorter lifetime (relative to carbon dioxide). Contrast this with CFC-13, which has a lifetime of 400 years: its GWP increases as the time horizon of consideration increases. This points out the importance of knowing the lifetime of these chemicals that we put into the atmosphere.

A byproduct of increased methane in the stratosphere is an increase in stratospheric water vapor, which arises due to oxidation of methane. So even after stratospheric methane is destroyed, its contribution to global warming continues through the presence of water vapor.