The Earth is warmed by sunlight and cooled by infrared heat it sends back to space. Follow the energy both ways — shortwave sunlight arriving, part of it reflected, and the infrared that greenhouse gases catch and send back down.
The greenhouse effect is the way certain gases in the atmosphere slow the escape of heat from the Earth. It is best understood as a balance. Energy arrives from the Sun, and energy leaves the Earth for space; the temperature settles where the two match. Greenhouse gases change how easily the outgoing energy gets away.
Sunlight is shortwave radiation. The clear atmosphere lets most of it through, so it is absorbed mainly by the ground and the oceans, which warm up. A warm surface then sends energy back out, but as infrared radiation. Greenhouse gases absorb part of that infrared and re-emit it in every direction, including back down.
Yellow beams mark the incoming sunlight and the small part reflected back to space. The grey band is the atmosphere, with greenhouse gas molecules drawn inside it. Red arrows rising to the top are infrared heat escaping to space; orange arrows pointing down are the back-radiation that keeps the surface warm.
Because the effect depends on a balance, changing how much greenhouse gas is in the air shifts that balance and changes the temperature the surface settles at. The same physics also explains the climates of other planets — it is not unique to Earth.
Almost all of the Earth's energy comes from the Sun. It arrives as shortwave sunlight, which crosses the clear atmosphere with very little absorbed on the way, and reaches the surface.
Not all sunlight is absorbed. Bright surfaces — clouds, snow, ice and pale ground — reflect a share of it straight back to space. The reflected part never warms the planet.
The land and oceans take in the remaining sunlight and warm up. This is why the ground feels hot on a sunny day even when the air stays cool: the energy is absorbed at the surface first.
A warm object gives off energy. The warmed surface radiates it outward as infrared, which is invisible long-wavelength radiation. Left alone, this infrared would carry the energy straight out to space.
Molecules such as carbon dioxide, methane and water vapour absorb infrared. When one absorbs some, it re-emits energy in all directions, so part of it travels back down and warms the surface again. This downward share is the back-radiation.
Because some heat is sent back, the surface has to be warmer before the energy leaving the top of the atmosphere balances the energy coming in. That higher temperature is the greenhouse effect at work: not a lid, just a slower escape of heat.
It is the way certain gases in the atmosphere slow the escape of heat. Sunlight warms the surface, the warm surface radiates infrared, and greenhouse gases absorb some of it and re-emit it in all directions, including back down. The surface therefore settles at a higher temperature than it would without those gases.
Only as an analogy. A glass house keeps warm air from blowing away; the atmosphere instead traps energy by absorbing and re-emitting infrared radiation. The name comes from the similarity in effect, not from the same mechanism.
No. Energy still leaves; the gases only slow it down and hold the surface at a higher temperature. For a steady climate, the energy escaping to space still has to match the energy arriving from the Sun.
Incoming sunlight is shortwave radiation, which the clear atmosphere mostly lets through, so it is absorbed mainly at the ground and the oceans. The air is warmed from below, and greenhouse gases then act on the infrared the warm surface sends up.
Yes. Water vapour, carbon dioxide and other natural greenhouse gases have always been present, and they keep the Earth warm enough to be habitable. What changes the climate is how the balance shifts when the amount of greenhouse gases changes.