Can somebody please explain this statement (about the greenhouse effect)?
"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),[12] 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.