Here's why:

* Sunlight: The Sun emits a wide range of radiation, including visible light, ultraviolet radiation, and infrared radiation.

* Earth's Atmosphere: Some of this radiation is absorbed by the atmosphere, some is reflected back into space, and some reaches the Earth's surface.

* Infrared Radiation: The Earth's surface absorbs solar radiation and warms up. It then emits this absorbed energy as infrared radiation.

* Greenhouse Gases: Certain gases in the atmosphere, known as greenhouse gases (e.g., carbon dioxide, methane, nitrous oxide), are good absorbers of infrared radiation. These gases trap the infrared radiation, preventing it from escaping back into space.

* Warming Effect: The trapped infrared radiation warms the Earth's surface and lower atmosphere, creating the greenhouse effect.

So, while the Sun emits a wide spectrum of radiation, it's the infrared radiation emitted by the warmed Earth that is primarily trapped by greenhouse gases.