C or D since “1.5.2 Other Greenhouse Gases and Aerosols The next most important greenhouse gas is methane (CH4), which has increased from 0.8 ppm before the industrial revolution to about 1.9 ppm in 2020. This might seem like a small increase, particularly compared to the 130-ppm increase in carbon dioxide, but methane is a far more powerful greenhouse gas on a permolecule basis—each kilogram of methane traps as much heat as 28 kilograms of carbon dioxide.”