Earth's surface emits most of its thermal energy as what kind of radiation?

Earth’s surface emits most of its thermal energy as longwave radiation. Because the surface is relatively cool compared with the Sun, its outgoing heat is concentrated mainly in the infrared part of the electromagnetic spectrum.

Sunlight arriving at Earth is dominated by shortwave radiation, including visible light and shorter infrared wavelengths. Land and ocean surfaces absorb part of that energy, warm up, and then emit thermal radiation at longer wavelengths. This distinction between incoming shortwave energy and outgoing longwave energy is central to Earth’s radiation budget.

Greenhouse gases such as water vapour, carbon dioxide, methane, and ozone absorb some of the outgoing longwave radiation. They then emit infrared radiation in all directions, including back toward the surface. Some radiation escapes directly through the atmospheric window, but much is absorbed and re-emitted before reaching space.

“Longwave radiation” is often used interchangeably with terrestrial radiation or thermal infrared radiation in climate science. It does not mean that Earth emits only one wavelength, and it is not the same as ultraviolet or cosmic radiation. The exact spectrum varies with surface temperature, atmospheric composition, clouds, and the emitting material.