What determines whether a gas is a 'greenhouse gas'?

• A. Its ability to absorb and re-emit longwave infrared radiation.
• B. Its ability to reflect visible light.
• C. Its chemical reactivity in the stratosphere.
• D. Its absence from the atmosphere.

Answer: (A)

Greenhouse gases shape Earth's climate because they can absorb longwave infrared radiation emitted by the surface and then re-emit that energy in all directions, including back toward the surface. This trapping of heat increases the time energy stays in the lower atmosphere, warming the surface. The key factor is infrared absorption in the wavelengths Earth emits, not how much visible light is reflected (that affects sunlight in different ways but doesn’t define the greenhouse effect). Chemical reactions in the stratosphere or whether the gas is present or absent don’t define a greenhouse gas, though presence is necessary for any greenhouse effect to occur. So, the defining property is the ability to absorb and re-emit longwave infrared radiation.