What is the basic composition of Earth's atmosphere and the role of greenhouse gases?

• A. Atmosphere is about 78% nitrogen and 21% oxygen; greenhouse gases trap infrared radiation.
• B. Atmosphere is 50% oxygen and 50% nitrogen; greenhouse gases have no effect.
• C. Atmosphere consists mainly of carbon dioxide and argon; no greenhouse effect.
• D. Atmosphere is mostly water vapor only; greenhouse gases do not affect temperature.

Answer: (A)

The basic idea is that Earth's atmosphere is dominated by nitrogen and oxygen, making up roughly 78% and 21% of the air, with much smaller amounts of other gases such as argon, carbon dioxide, neon, helium, and methane. Greenhouse gases—water vapor, carbon dioxide, methane, nitrous oxide, and ozone among others—have a special role: they absorb infrared radiation that the Earth’s surface emits after absorbing sunlight. When these gases absorb heat, they re-emit infrared energy in all directions, including back toward the surface. That trapping of heat keeps the lower atmosphere and the surface warmer than they would be otherwise, which is the greenhouse effect that maintains a habitable climate. The statement that the atmosphere is mostly nitrogen and oxygen and that greenhouse gases trap infrared radiation captures both the dominant composition and the heat-trapping role of these gases. The other options misstate the major components or deny this heat-trapping effect.