How are greenhouse gases defined?

Greenhouse gases are defined as gases that trap heat in the atmosphere. This phenomenon occurs because these gases, such as carbon dioxide, methane, and nitrous oxide, absorb and emit infrared radiation. When the Earth’s surface absorbs sunlight, it warms up and then emits heat in the form of infrared radiation. Greenhouse gases in the atmosphere capture some of this outgoing radiation, which helps to keep the planet warm enough to support life.

The greenhouse effect is a natural process, and without it, the Earth would be much colder and inhospitable. This is essential for maintaining a stable climate and supporting ecosystems. The accumulation and increased concentration of these gases due to human activities can enhance the greenhouse effect, leading to global warming and climate change.

The other choices highlight characteristics or roles of gases that are not relevant to the direct definition of greenhouse gases. For example, while some air pollutants can contribute to air quality issues, not all greenhouse gases are classified as pollutants. Additionally, gases that allow sunlight to pass through do not trap heat and are thus not considered greenhouse gases, and gases that cool the planet's surface contradict the primary role of greenhouse gases in warming the atmosphere.