Compare methane and carbon dioxide in terms of per-molecule warming potential and atmospheric lifetimes.

• A. Methane has higher short-term radiative efficiency per molecule but remains about a decade in the atmosphere; CO2 persists for centuries.
• B. Methane has lower efficiency but longer lifetime; CO2 is less persistent.
• C. Methane and CO2 have identical radiative efficiency and lifetimes.
• D. Methane persists for millennia; CO2 clears in days.

Answer: (B)

Comparing methane and carbon dioxide hinges on two properties: per-molecule warming potential and atmospheric lifetime. Methane is a much more potent greenhouse gas on a per-molecule basis, meaning each methane molecule traps more infrared radiation than a molecule of CO2. But methane doesn’t stay in the atmosphere for long—the typical lifetime is about a decade. CO2, on the other hand, is far less potent per molecule, but it persists for centuries to millennia as it cycles through the atmosphere, oceans, and land reservoirs. So the best way to think about it is that methane delivers a strong, short-lived warming punch, while CO2 delivers a weaker punch per molecule but accumulates and influences climate for a very long time.