Can somebody please explain this statement (about the greenhouse effect)?

"Methane is a potent gas and over a period of 20 years has a 72 times greater effect on warming the atmosphere than carbon dioxide. Due to my limited english, i am failing to understand how timeline has anything to do with the effect of a gas on the greenhouse effect. I think it has to do with the half life of the gas in the atmoshpere. Maybe methane is broken down more readily than CO2 and the whole thing means that if they remained in the atmosphere for the same amount of time, methane will do a greater damage. Am i right ?

"Methane has a large effect (24 times as strong as carbon dioxide per unit mole) for a brief period (having an estimated lifetime of 8.9±0.6 years in the atmosphere), whereas carbon dioxide has a small effect for a long period (over 100 years). Because of this difference in effect and time period, the global warming potential of methane over a 20-year time period is 72."[ Wikipedia, quoting Jain, Briegleb et al ].

Simplified, that means:

• $\ce{CH4}$ has a shorter half-life in the atmosphere than $\ce{CO2}$, reducing its effect as a green-house gas over time, but

• $\ce{CH4}$ is so much more potent in retaining heat than $\ce{CO2}$ that this more than compensates for its short lifetime, making $\ce{CH4}$ a stronger green-house gas over the long term.

• And so if I understand correctly even though methane is present as $0.00018$% and $\ce{CO2}$ at 0.04% the 72 times is equivalent to $\approx 0.01$% $\ce{CO2}$, so a slight increase in methane in the future can be very significant. – porphyrin Jun 15 '17 at 8:13
• Lets say that we have 100 cm3 of methane and 100 cm3 of CO2. Suppose that methane decomposes in 5 seconds, CO2 in 60 seconds , that methane is more potent than CO2 but not potent enough to have a greater effect on global warming over the course of 60 seconds. If it remained in the atmosphere for 60 seconds then it would do the greater overall effect on global warming. Is this model correct ? (Assume that the half lives of both methane and CO2 are as stated above and that all the other ir-realistic values are true). – Max white Jun 15 '17 at 20:06
• Remember that the time quoted, ~9 yr, is the half-life of methane. The half-life of CO2, ~27 yr, is given at euanmearns.com/the-half-life-of-co2-in-earths-atmosphere-part-1. You'd need to calculate the exponential decay of each these greenhouse gases and then multiply the CH4 by ~24 to account for its greater effectiveness in retaining IR. – DrMoishe Pippik Jun 15 '17 at 20:35
• Sir you are talking to someone with fairly basic understanding in this area so it would help if we used a simple model like the one above to discuss this subject. Is my model (and the result which i obtained from it) correct ? Note : My result from that model is that CO2 has a greater overall effect in one minute because : 1- Methane decays more quickly than CO2. 2- Methane isnt potent enough to have a greater overall effect in one minute. But in the first five seconds, methane was causing the greater effect.is this true ? – Max white Jun 16 '17 at 1:04
• No, it's wrong. To understand why, you need to learn about gradual, exponential decay. In your model, the methane does not disappear in 5 seconds, it's halved in five seconds. – DrMoishe Pippik Jun 18 '17 at 2:50