A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Earth's climate is controlled by the balance between incoming solar radiation and outgoing terrestrial radiation. The atmosphere is partially transparent to incoming sunlight but absorbs and re-radiates outgoing infrared radiation back to the surface, creating the natural greenhouse effect that makes Earth habitable. An imbalance where more radiation is absorbed than escapes (due to increased greenhouse gases) leads to net warming; understanding this budget is fundamental to climate science.
From your study of solar radiation and Earth's energy balance, you know that the Sun delivers about 1,361 watts per square meter to the top of the atmosphere (the solar constant). But Earth is a sphere, so this energy is spread over four times the area that intercepts it, giving an average input of roughly 340 W/m². Of this incoming shortwave radiation, about 30% is immediately reflected back to space by clouds, ice, and bright surfaces — this fraction is Earth's albedo. The remaining ~240 W/m² is absorbed by the surface and atmosphere, warming the planet. For Earth's temperature to remain stable, exactly 240 W/m² must be radiated back to space as outgoing longwave (infrared) radiation. When incoming and outgoing fluxes balance, the planet is in radiative equilibrium.
If Earth had no atmosphere, this balance would produce a surface temperature of about −18°C — far too cold for liquid water. The reason our actual average surface temperature is around +15°C is the greenhouse effect, which you encountered as a prerequisite. The atmosphere is largely transparent to incoming solar radiation (visible light passes through easily), but greenhouse gases — water vapor, CO₂, methane, and others — absorb outgoing infrared radiation emitted by the warm surface. These gases then re-radiate energy in all directions, including back toward the ground. This downwelling longwave radiation is an additional energy input to the surface beyond direct sunlight, raising the surface temperature by about 33°C above what bare radiative equilibrium would predict.
The full energy budget includes more than just radiation. The surface also loses energy through latent heat flux (evaporation of water, which carries energy into the atmosphere where it is released during condensation) and sensible heat flux (direct warming of air in contact with the ground). These non-radiative transfers move about 100 W/m² from surface to atmosphere, which is why the surface radiative budget alone would overestimate surface warming. The atmosphere, in turn, radiates this energy to space from its upper layers. The key insight is that the planet radiates to space primarily from an effective emission height several kilometers up, where the temperature is cold enough to emit the required 240 W/m². Adding greenhouse gases raises this emission height, where it is colder, temporarily reducing outgoing radiation and creating a radiative imbalance — more energy comes in than goes out, and the system warms until a new equilibrium is reached at a higher temperature. This is the fundamental mechanism of anthropogenic climate change: human emissions shift the radiative balance, and Earth's temperature adjusts until outgoing radiation once again matches incoming.