Global warming potential is calculated by comparing the change in radiative forcing caused by 1 kg of a greenhouse gas to that of 1 kg of CO2, which is assigned an arbitrary value of 1. Below is a table showing the GWP of methane over various time scales.
| Gas | Atmospheric Lifetime | Global Warming Potential Time Horizon | ||
|---|---|---|---|---|
| (Years) | 20 Years | 100 Years | 500 Years | |
| Carbon Dioxide (CO2) | variable | 1 | 1 | 1 |
| Methane (CH4) | 12 | 72 | 25 | 7.5 |
Question: Why does the GWP of methane decrease over time?
There are three main sinks that remove CH4 from the atmosphere: UV destruction in the stratosphere, oxidation by bacteria in the soils and reaction with hydroxyl radicals in the atmosphere. A molecule of CH4 reacts with a hydroxyl radical (oOH):
This begins a series of reactions that can be summarized by the equation below:
Over time, much of the methane that is originally released is either removed from the atmosphere or converted into greenhouse gases with lower global warming potentials (CO2 or H2O). As a result, the GWP of atmospheric methane decreases over time.
Question: Can you think of two ways in which the addition of methane might increase the concentration of water vapour in our atmosphere?
Hint: Consider feedback effects.