| The greenhouse effect, discovered by Joseph | | | | downwards. The upward infrared flux emitted |
| Fourier in 1824 and first investigated | | | | by the surface must balance not only the |
| quantitatively by Svante Arrhenius in 1896, | | | | absorbed solar flux but also this downward |
| is the process in which the emission of | | | | infrared flux emitted by the atmosphere. The |
| infrared radiation by an atmosphere warms a | | | | surface temperature will rise until it |
| planet's surface. The name comes from an | | | | generates thermal radiation equivalent to the |
| incorrect analogy with the warming of air | | | | sum of these two incident radiation streams. |
| inside a greenhouse compared to the air | | | | |
| outside the greenhouse. The Earth's average | | | | A more realistic picture taking into account |
| surface temperature is about 25°C warmer | | | | the convective and latent heat fluxes is |
| than it would be without the greenhouse | | | | somewhat more complex. But the following |
| effect [1]. In addition to the Earth, Mars | | | | simple model captures the essence. The |
| and especially Venus have greenhouse effects. | | | | starting point is to note that the opacity of |
| | | | the atmosphere to infrared radiation |
| In common usage, "greenhouse effect" may | | | | determines the height in the atmosphere from |
| refer either to the natural greenhouse effect | | | | which most of the photons emitted to space |
| due to naturally occurring greenhouse gases, | | | | are emitted. If the atmosphere is more |
| or to the enhanced (anthropogenic) greenhouse | | | | opaque, the typical photon escaping to space |
| effect which results from gases emitted as a | | | | will be emitted from higher in the |
| result of human activities (see also global | | | | atmosphere, because one then has to go to |
| warming, scientific opinion on climate change | | | | higher altitudes to see out to space in the |
| and attribution of recent climate change). | | | | infrared. Since the emission of infrared |
| | | | radiation is a function of temperature, it is |
| The basic mechanism | | | | the temperature of the atmosphere at this |
| | | | emission level that is effectively determined |
| The Earth receives energy from the Sun in the | | | | by the requirement that the emitted flux |
| form of radiation. The Earth reflects about | | | | balance the absorbed solar flux. |
| 30% of the incident solar flux; the remaining | | | | |
| 70% is absorbed, warming the land, atmosphere | | | | But the temperature of the atmosphere |
| and oceans. | | | | generally decreases with height above the |
| | | | surface, at a rate of roughly 6.5 °C per |
| To the extent that the Earth is in a steady | | | | kilometer on average, until one reaches the |
| state, the energy stored in the atmosphere | | | | stratosphere 10-15 km above the surface. |
| and ocean does not change in time, so energy | | | | (Most infrared photons escaping to space are |
| equal to the absorbed solar radiation must be | | | | emitted by the troposphere, the region |
| radiated back to space. Earth radiates energy | | | | bounded by the surface and the stratosphere, |
| into space as black-body radiation, which | | | | so we can ignore the stratosphere in this |
| maintains a thermal equilibrium. This | | | | simple picture.) A very simple model, but one |
| thermal, infrared radiation increases with | | | | that proves to be remarkably useful, involves |
| increasing temperature. One can think of the | | | | the assumption that this temperature profile |
| Earth's temperature as being determined by | | | | is simply fixed, by the non-radiative energy |
| the infrared flux needed to balance the | | | | fluxes. Given the temperature at the emission |
| absorbed solar flux. | | | | level of the infrared flux escaping to space, |
| | | | one then computes the surface temperature by |
| The visible solar radiation heats the | | | | increasing temperature at the rate of 6.5 |
| surface, not the atmosphere, whereas most of | | | | °C per kilometer, the environmental lapse |
| the infrared radiation escaping to space is | | | | rate, until one reaches the surface. The more |
| emitted from the upper atmosphere, not the | | | | opaque the atmosphere, and the higher the |
| surface. The infrared photons emitted by the | | | | emission level of the escaping infrared |
| surface are mostly absorbed by the atmosphere | | | | radiation, the warmer the surface, since one |
| and do not escape directly to space. | | | | then needs to follow this lapse rate over a |
| | | | larger distance in the vertical. While less |
| reason this warms the surface is most easily | | | | intuitive than the purely radiative |
| understood by starting with a simplified | | | | greenhouse effect, this less familiar |
| model of a purely radiative greenhouse effect | | | | radiative-convective picture is the starting |
| that ignores energy transfer in the | | | | point for most discussions of the greenhouse |
| atmosphere by convection (sensible heat | | | | effect in the climate modeling literature. |
| transport) and by the evaporation and | | | | |
| condensation of water vapor (latent heat | | | | The term "greenhouse effect" is a source of |
| transport). In this purely radiative case, | | | | confusion in that actual greenhouses do not |
| one can think of the atmosphere as emitting | | | | warm by this same mechanism |
| infrared radiation both upwards and | | | | |