How does the greenhouse effect actually work?

The greenhouse effect is a natural process in which certain gases let the Sun's energy reach the ground but slow the escape of heat back to space. Sunlight warms the surface, the surface gives off infrared radiation, and greenhouse gases absorb much of that radiation and send it out again in all directions. Some of it returns to the surface, so the planet stays warmer than it would be otherwise. As [niwa.co.nz](https://niwa.co.nz/atmosphere/what-greenhouse-effect) puts it, the effect warms the surface and lower atmosphere because these substances let solar energy through but impede energy passing from Earth back into space. [1]

To keep temperatures stable over the long term, incoming solar energy must be balanced by an equal amount of heat radiated to space. According to [csiro.au](https://www.csiro.au/en/research/environmental-impacts/climate-change/Climate-change-QA/How-do-greenhouse-gases-warm-the-planet), about 29 per cent of incoming sunlight is reflected by clouds, the atmosphere and the ground, about 23 per cent is absorbed by the atmosphere, and about 48 per cent reaches the surface and warms it. [2]

The warmed ground then emits infrared radiation, which we feel as heat. [ugc.berkeley.edu](https://ugc.berkeley.edu/background-content/greenhouse-effect/) explains that without greenhouse gases this heat would pass directly to space. With them, most of the long-wave radiation is absorbed and re-radiated many times before it leaves. [3]

The main greenhouse gases are carbon dioxide, water vapour, methane and nitrous oxide. [scied.ucar.edu](https://scied.ucar.edu/learning-zone/how-climate-works/greenhouse-effect) notes that these molecules have three or more atoms, held loosely enough that they vibrate when they absorb heat. Nitrogen and oxygen, which make up most of the atmosphere, cannot absorb heat in this way. [4]

Wavelength matters too. The [climate.mit.edu](https://climate.mit.edu/ask-mit/how-do-greenhouse-gases-trap-heat-atmosphere) explainer quotes MIT's Jesse Kroll saying that carbon dioxide's most important absorption is light of about 15 microns. Incoming sunlight has much shorter wavelengths, so CO2 does not block it, but the longer-wavelength infrared that Earth re-emits is absorbed. Water vapour does not absorb efficiently around 15 microns, so CO2 captures light that would otherwise escape most easily. The same page adds that a real greenhouse works differently: its glass stops warm air from exchanging heat with colder air outside, although the end result is similar. [5]

The natural effect makes Earth habitable. Both [niwa.co.nz](https://niwa.co.nz/atmosphere/what-greenhouse-effect) and [csiro.au](https://www.csiro.au/en/research/environmental-impacts/climate-change/Climate-change-QA/How-do-greenhouse-gases-warm-the-planet) give an average surface temperature without an atmosphere or greenhouse gases of about -18 °C, against roughly 14 to 15 °C today. [1] [2]

Adding more greenhouse gases strengthens the effect. NIWA describes how extra gases absorb more infrared radiation, so the surface and lower atmosphere warm until incoming and outgoing radiation balance again. CSIRO links the enhancement over the past two centuries to burning fossil fuels, agriculture and deforestation. Berkeley adds that clouds play a role: thick low clouds can reflect sunlight, while high clouds can intensify the effect by re-radiating heat from the surface. [1] [2] [3]