The greenhouse effect is the process by which heat accumulates in the earth's lower atmosphere instead of being released out into space. This process occurs naturally and keeps the earth warm enough to sustain life. Without the natural greenhouse effect the surface of the planet would be about 33°C colder on average, and most of the globe would be frozen.
The greenhouse effect arises due to the fact that the sun's radiant energy passes relatively freely through the atmosphere to heat the earth's surface, but much of the heat emitted from the earth is absorbed by and retained within the atmosphere.
Heat radiated from the sun has a different wavelength and energy content from heat radiated from the earth because the sun is much hotter than the earth (the temperature of the sun's outer layer is about 6000°C). The sun emits 'shortwave radiation' (mostly in the visible, ultraviolet and near infrared bands) due to its high temperature, whereas the heat emitted from the earth is in the form of 'longwave radiation' (in the infrared band). The amount of energy in this longwave radiation corresponds to the unique energy absorption bands of various gases in the earth's atmosphere, including water vapour, carbon dioxide, and other greenhouse gases. This means that while they are relatively transparent to the sun's shortwave radiation, these gases absorb longwave radiation emitted from the earth, and re-release it back into the atmosphere when they return to their low-energy state. This retains energy within the surface-atmosphere system and leads to heating of the system. The process is illustrated in the figure below.
Global energy balance
Source: Bureau of Meteorology, The greenhouse effect and climate change, Bureau of Meteorology, 2003, p. 7.
The gases most responsible for absorbing this longwave radiation are water vapour, CARBON DIOXIDE, NITROUS OXIDE, METHANE, and CHLOROFLUOROCARBONS. With the exception of chlorofluorocarbons, these gases occur naturally in the atmosphere in very small concentrations. Even in such small quantities, however, they exert a marked warming effect. As noted above, this natural warming makes the earth habitable, keeping it about 33°C warmer than it would otherwise be, given its distance from the sun. Recent human activities, in particular the burning of carbon-containing fuels, and agricultural practices that release methane, have significantly increased the concentration of these gases in the atmosphere. The additional warming by the atmosphere is termed the enhanced greenhouse effect.
The mechanism giving rise to the greenhouse effect in the earth's atmosphere is different from the mechanism that retains heat within a greenhouse or glasshouse. In the latter case, it is the isolation of air within the greenhouse that prevents heat loss through convection, and to a lesser extent, by conduction. Reflection of infrared energy by the glass is a very minor contributor.
The existence of the greenhouse effect has been known for a long time. The Swedish chemist Svante Arrhenius was the first to quantify the greenhouse effect when he published a paper in 1896 discussing the mean temperature of the ground being influenced by the presence of heat-absorbing gases in the atmosphere. The notion of heat absorption by gases was put forward even earlier by scientists such as Tyndall and Fourier. Whilst the greenhouse effect may be relatively new to policy-makers and the media, it has been known and investigated in the scientific community for well over a century.
Bureau of Meteorology, The greenhouse effect and climate change, Bureau of Meteorology, 2003.