Which greenhouse gas currently contributes the largest fraction of anthropogenic radiative forcing and what is notable about its lifetime?

• A. Methane; it persists for millennia.
• B. Ozone; lifetime is not relevant.
• C. Nitrous oxide; it has the shortest lifetime.
• D. Carbon dioxide; it has a long atmospheric lifetime (centuries to millennia).

Answer: (D)

Long-lived greenhouse gases set the long-term warming picture because they stay in the atmosphere for very long times. Carbon dioxide earns the largest share of human-caused radiative forcing not by being the most potent per molecule, but by accumulating and persisting for a very long period. Its atmospheric lifetime spans centuries to millennia, so CO2 emitted today continues to influence warming for many generations. The removal processes—deep ocean uptake, carbonate chemistry, and rock weathering—work slowly, meaning CO2 stays in the air far longer than other greenhouse gases. In comparison, methane is much more potent but short-lived (about a dozen years), and nitrous oxide persists for roughly a century, so their contributions to forcing don't accumulate to the same long-term level as CO2.