Which greenhouse gas is most responsible for radiative forcing due to its abundance in the atmosphere?

• A. CO2
• B. CH4
• C. N2O
• D. Halocarbons

Answer: (A)

Radiative forcing depends on two things: how much of a gas is in the atmosphere and how strongly it absorbs infrared radiation. Carbon dioxide is the most abundant greenhouse gas in the modern atmosphere and it stays there for a very long time, so it accumulates year after year. That combination—high concentration plus long lifetime—means CO2 contributes the largest amount of energy imbalance, making it the dominant driver of radiative forcing today.

Other gases are indeed powerful per molecule. Methane and nitrous oxide have higher radiative efficiency, but they exist at much lower concentrations and methane, in particular, has a shorter atmospheric lifetime. Halocarbons are extremely effective per molecule, but their concentrations are tiny compared to CO2. Taken together, their smaller amounts and lifetimes mean their total forcing is smaller than that of CO2, despite their potency.

So, the biggest influence on radiative forcing from atmospheric greenhouse gases at present comes from the most abundant long-lived gas: carbon dioxide.