Greenhouse effect
The greenhouse effect occurs when greenhouse gases in a planet's atmosphere insulate the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source as in the case of Jupiter, or from its host star as in the case of the Earth. In the case of Earth, the Sun emits shortwave radiation (sunlight) that passes through greenhouse gases to heat the Earth's surface. In response, the Earth's surface emits longwave radiation that is mostly absorbed by greenhouse gases. The absorption of longwave radiation prevents it from reaching space, reducing the rate at which the Earth can cool off.
This article is about the atmospheric phenomenon causing planetary warming. For the general heating or cooling of Earth's surface, see Earth's energy budget. For other uses, see Greenhouse (disambiguation).
Without the greenhouse effect, the Earth's average surface temperature would be about −18 °C (−0.4 °F),[1][2] which is less than Earth's 20th century average of about 14 °C (57 °F), or a more recent average of about 15 °C (59 °F).[3][4] In addition to naturally present greenhouse gases, burning of fossil fuels has increased amounts of carbon dioxide and methane in the atmosphere.[5][6] As a result, global warming of about 1.2 °C (2.2 °F) has occurred since the Industrial Revolution,[7] with the global average surface temperature increasing at a rate of 0.18 °C (0.32 °F) per decade since 1981.[8]
All objects with a temperature above absolute zero emit thermal radiation. The wavelengths of thermal radiation emitted by the Sun and Earth differ because their surface temperatures are different. The Sun has a surface temperature of 5,500 °C (9,900 °F), so it emits most of its energy as shortwave radiation in near-infrared and visible wavelengths (as sunlight). In contrast, Earth's surface has a much lower temperature, so it emits longwave radiation at mid- and far-infrared wavelengths.[6] A gas is a greenhouse gas if it absorbs longwave radiation. Earth's atmosphere absorbs only 23% of incoming shortwave radiation, but absorbs 90% of the longwave radiation emitted by the surface,[9] thus accumulating energy and warming the Earth's surface.
The existence of the greenhouse effect, while not named as such, was proposed as early as 1824 by Joseph Fourier.[10] The argument and the evidence were further strengthened by Claude Pouillet in 1827 and 1838. In 1856 Eunice Newton Foote demonstrated that the warming effect of the sun is greater for air with water vapour than for dry air, and the effect is even greater with carbon dioxide.[11][12] The term greenhouse was first applied to this phenomenon by Nils Gustaf Ekholm in 1901.[13][14]
Definition
The greenhouse effect on Earth is defined as: "The infrared radiative effect of all infrared absorbing constituents in the atmosphere. Greenhouse gases (GHGs), clouds, and some aerosols absorb terrestrial radiation emitted by the Earth’s surface and elsewhere in the atmosphere."[15]: 2232
The enhanced greenhouse effect describes the fact that by increasing the concentration of GHGs in the atmosphere (due to human action), the natural greenhouse effect is increased.[15]: 2232
Terminology
The term greenhouse effect comes from an analogy to greenhouses. Both greenhouses and the greenhouse effect work by retaining heat from sunlight, but the way they retain heat differs. Greenhouses retain heat mainly by blocking convection (the movement of air).[16][17] In contrast, the greenhouse effect retains heat by restricting radiative transfer through the air and reducing the rate at which thermal radiation is emitted into space.[5]
Basic formulas
Effective temperature
A given flux of thermal radiation has an associated effective radiating temperature or effective temperature. Effective temperature is the temperature that a black body (a perfect absorber/emitter) would need to be to emit that much thermal radiation.[71] Thus, the overall effective temperature of a planet is given by
Related effects on Earth
Negative greenhouse effect
Scientists have observed that, at times, there is a negative greenhouse effect over parts of Antarctica.[83][84] In a location where there is a strong temperature inversion, so that the air is warmer than the surface, it is possible for the greenhouse effect to be reversed, so that the presence of greenhouse gases increases the rate of radiative cooling to space. In this case, the rate of thermal radiation emission to space is greater than the rate at which thermal radiation is emitted by the surface. Thus, the local value of the greenhouse effect is negative.