Which statement best describes methane's role in the climate system?

• A. Nitrous oxide has a shorter lifetime than methane but lower radiative forcing.
• B. Carbon dioxide is the only greenhouse gas with a short lifetime.
• C. Methane is a potent greenhouse gas with relatively short atmospheric lifetime, contributing significantly to radiative forcing.
• D. Water vapor is the only greenhouse gas that matters for climate policy.

Answer: (C)

The climate impact of a gas depends on how effectively it traps heat and how long it stays in the atmosphere. Methane is notable because it absorbs infrared radiation very efficiently, giving it a strong heat-trapping effect per molecule. At the same time, it remains in the atmosphere for a relatively short time—on the order of about a decade—so its influence is especially strong in the near term but declines faster than gases with longer lifetimes, like carbon dioxide.

Because of this combination—high radiative efficiency and short lifetime—methane contributes a significant amount to radiative forcing, even though its atmospheric concentration is much lower than CO2. This is why methane is considered a potent short-lived climate pollutant and a major near-term driver of climate change.

The other statements don’t fit as well: nitrous oxide lasts much longer than methane, so its lifetime isn’t shorter; carbon dioxide has a long, not short, lifetime; and water vapor is a major greenhouse gas but its amount in the atmosphere is controlled by temperature and climate feedbacks, not directly by emissions policy.