Greenhouse effect

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." 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. == 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). 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. ==History of discovery and investigation==

The existence of the greenhouse effect, while not named as such, was proposed as early as 1824 by Joseph Fourier. The effect was more fully quantified by Svante Arrhenius in 1896, who made the first quantitative prediction of global warming due to a hypothetical doubling of atmospheric carbon dioxide. The term greenhouse was first applied to this phenomenon by Nils Gustaf Ekholm in 1901. ==Measurement==

Matter emits thermal radiation at a rate that is directly proportional to the fourth power of its temperature. Some of the radiation emitted by the Earth's surface is absorbed by greenhouse gases and clouds. Without this absorption, Earth's surface would have an average temperature of . However, because some of the radiation is absorbed, Earth's average surface temperature is around . Thus, the Earth's greenhouse effect may be measured as a temperature change of . Thermal radiation is characterized by how much energy it carries, typically in watts per square meter (W/m). Scientists also measure the greenhouse effect based on how much more longwave thermal radiation leaves the Earth's surface than reaches space. Currently, longwave radiation leaves the surface at an average rate of 398 W/m, but only 239 W/m reaches space. Thus, the Earth's greenhouse effect can also be measured as an energy flow change of 159 W/m. The greenhouse effect can be expressed as a fraction (0.40) or percentage (40%) of the longwave thermal radiation that leaves Earth's surface but does not reach space. Whether the greenhouse effect is expressed as a change in temperature or as a change in longwave thermal radiation, the same effect is being measured. == Role in climate change ==

'' article estimated future damages from past emissions to be at least an order of magnitude larger than historical damages from the same emissions. Strengthening of the greenhouse effect through additional greenhouse gases from human activities is known as the enhanced greenhouse effect. this increase in radiative forcing from human activity has been observed directly, and is attributable mainly to increased atmospheric carbon dioxide levels. is produced by fossil fuel burning and other activities such as cement production and tropical deforestation. Measurements of from the Mauna Loa Observatory show that concentrations have increased from about 313 parts per million (ppm) in 1960, passing the 400 ppm milestone in 2013. The current observed amount of exceeds the geological record maxima (≈300 ppm) from ice core data. Over the past 800,000 years, ice core data shows that carbon dioxide has varied from values as low as 180 ppm to the pre-industrial level of 270 ppm. Paleoclimatologists consider variations in carbon dioxide concentration to be a fundamental factor influencing climate variations over this time scale. == Energy balance and temperature ==

Incoming shortwave radiation spectrum for direct light at both the top of Earth's atmosphere and at sea level Hotter matter emits shorter wavelengths of radiation. As a result, the Sun emits shortwave radiation as sunlight while the Earth and its atmosphere emit longwave radiation. Sunlight includes ultraviolet, visible light, and near-infrared radiation. and absorbs the rest (240 W/m). Carbon dioxide is understood to be responsible for the dip in outgoing radiation (and associated rise in the greenhouse effect) at around 667 cm−1 (equivalent to a wavelength of 15 microns). Each layer of the atmosphere with greenhouse gases absorbs some of the longwave radiation being radiated upwards from lower layers. It also emits longwave radiation in all directions, both upwards and downwards, in equilibrium with the amount it has absorbed. This results in less radiative heat loss and more warmth below. Increasing the concentration of the gases increases the amount of absorption and emission, and thereby causing more heat to be retained at the surface and in the layers below. a bit lower than the effective surface temperature. This value is warmer than Earth's overall effective temperature. Energy flux Energy flux is the rate of energy flow per unit area. Energy flux is expressed in units of W/m2, which is the number of joules of energy that pass through a square meter each second. Most fluxes quoted in high-level discussions of climate are global values, which means they are the total flow of energy over the entire globe, divided by the surface area of the Earth, . The fluxes of radiation arriving at and leaving the Earth are important because radiative transfer is the only process capable of exchanging energy between Earth and the rest of the universe. The temperature of a planet depends on the balance between incoming radiation and outgoing radiation. If incoming radiation exceeds outgoing radiation, a planet will warm. If outgoing radiation exceeds incoming radiation, a planet will cool. A planet will tend towards a state of radiative equilibrium, in which the power of outgoing radiation equals the power of absorbed incoming radiation. Earth's energy imbalance is the amount by which the power of incoming sunlight absorbed by Earth's surface or atmosphere exceeds the power of outgoing longwave radiation emitted to space. Energy imbalance is the fundamental measurement that drives surface temperature. A UN presentation says "The EEI is the most critical number defining the prospects for continued global warming and climate change." Earth's energy imbalance (EEI) was about 0.7 W/m in 2015, indicating that Earth as a whole was accumulating thermal energy and is in a process of becoming warmer. decrease with height in the atmosphere. == Effect of lapse rate ==

Lapse rate In the lower portion of the atmosphere, the troposphere, the air temperature decreases (or "lapses") with increasing altitude. The rate at which temperature changes with altitude is called the lapse rate. On Earth, the air temperature decreases by about 6.5 °C/km (3.6 °F per 1000 ft), on average, although this varies. Emission temperature and altitude . In the chart, emission temperatures range between Tmin and Ts. "Wavenumber" is frequency divided by the speed of light). Greenhouse gases make the atmosphere near Earth's surface mostly opaque to longwave radiation. The atmosphere only becomes transparent to longwave radiation at higher altitudes, where the air is less dense, there is less water vapor, and reduced pressure broadening of absorption lines limits the wavelengths that gas molecules can absorb. Greenhouse gases that were largely transparent to incoming solar radiation are more absorbent for some wavelengths in this range. == Infrared absorbing constituents in the atmosphere ==

Greenhouse gases A greenhouse gas (GHG) is a gas which contributes to the trapping of heat by impeding the flow of longwave radiation out of a planet's atmosphere. Greenhouse gases contribute most of the greenhouse effect in Earth's energy budget. and act as greenhouse gases. Most gases whose molecules have two different atoms (such as carbon monoxide, ), and all gases with three or more atoms (including and ), are infrared active and act as greenhouse gases. (Technically, this is because when these molecules vibrate, those vibrations modify the molecular dipole moment, or asymmetry in the distribution of electrical charge. See Infrared spectroscopy.) Effect on surface cooling: Longwave radiation flows both upward and downward due to absorption and emission in the atmosphere. These canceling energy flows reduce radiative surface cooling (net upward radiative energy flow). Latent heat transport and thermals provide non-radiative surface cooling which partially compensates for this reduction, but there is still a net reduction in surface cooling, for a given surface temperature. Thin cirrus clouds can have a net warming effect. Clouds can absorb and emit infrared radiation and thus affect the radiative properties of the atmosphere. == 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. Thus, the overall effective temperature of a planet is given by :T_\mathrm{eff} = (\mathrm{OLR}/\sigma)^{1/4} where OLR is the average flux (power per unit area) of outgoing longwave radiation emitted to space and \sigma is the Stefan-Boltzmann constant. Similarly, the effective temperature of the surface is given by :T_\mathrm{surface,eff} = (\mathrm{SLR}/\sigma)^{1/4} where SLR is the average flux of longwave radiation emitted by the surface. (OLR is a conventional abbreviation. SLR is used here to denote the flux of surface-emitted longwave radiation, although there is no standard abbreviation for this.) Sometimes the greenhouse effect is quantified as a temperature difference. This temperature difference is closely related to the quantities above. When the greenhouse effect is expressed as a temperature difference, \Delta T_\mathrm{GHE}, this refers to the effective temperature associated with thermal radiation emissions from the surface minus the effective temperature associated with emissions to space: :\Delta T_\mathrm{GHE} = T_\mathrm{surface,eff} - T_\mathrm{eff} :\Delta T_\mathrm{GHE} = \left(\mathrm{SLR}/\sigma\right)^{1/4} - \left(\mathrm{OLR}/\sigma\right)^{1/4} Informal discussions of the greenhouse effect often compare the actual surface temperature to the temperature that the planet would have if there were no greenhouse gases. However, in formal technical discussions, when the size of the greenhouse effect is quantified as a temperature, this is generally done using the above formula. The formula refers to the effective surface temperature rather than the actual surface temperature, and compares the surface with the top of the atmosphere, rather than comparing reality to a hypothetical situation. The temperature difference, \Delta T_\mathrm{GHE}, indicates how much warmer a planet's surface is than the planet's overall effective temperature. Radiative balance . Evaporation and convection partially compensate for this reduction in surface cooling. Low temperatures at high altitudes limit the rate of thermal emissions to space. Earth's top-of-atmosphere (TOA) energy imbalance (EEI) is the amount by which the power of incoming radiation exceeds the power of outgoing radiation: :\mathrm{EEI} = \mathrm{ASR} -\mathrm{OLR} where ASR is the mean flux of absorbed solar radiation. ASR may be expanded as :\mathrm{ASR} = (1-A) \,\mathrm{MSI} where A is the albedo (reflectivity) of the planet and MSI is the mean solar irradiance incoming at the top of the atmosphere. The radiative equilibrium temperature of a planet can be expressed as :T_\mathrm{radeq} = (\mathrm{ASR}/\sigma)^{1/4} = \left[(1-A)\,\mathrm{MSI}/\sigma \right]^{1/4} \;. A planet's temperature will tend to shift towards a state of radiative equilibrium, in which the TOA energy imbalance is zero, i.e., \mathrm{EEI} = 0. When the planet is in radiative equilibrium, the overall effective temperature of the planet is given by :T_\mathrm{eff} = T_\mathrm{radeq}\;. Thus, the concept of radiative equilibrium is important because it indicates what effective temperature a planet will tend towards having. If, in addition to knowing the effective temperature, T_\mathrm{eff}, we know the value of the greenhouse effect, then we know the mean (average) surface temperature of the planet. This is why the quantity known as the greenhouse effect is important: it is one of the few quantities that go into determining the planet's mean surface temperature. Greenhouse effect and temperature Typically, a planet will be close to radiative equilibrium, with the rates of incoming and outgoing energy being well-balanced. Under such conditions, the planet's equilibrium temperature is determined by the mean solar irradiance and the planetary albedo (how much sunlight is reflected back to space instead of being absorbed). The greenhouse effect measures how much warmer the surface is than the overall effective temperature of the planet. So, the effective surface temperature, T_\mathrm{surface,eff}, is, using the definition of \Delta T_\mathrm{GHE}, :T_\mathrm{surface,eff} = T_\mathrm{eff} + \Delta T_\mathrm{GHE} \;. One could also express the relationship between T_\mathrm{surface,eff} and T_\mathrm{eff} using or . So, the principle that a larger greenhouse effect corresponds to a higher surface temperature, if everything else (i.e., the factors that determine T_\mathrm{eff}) is held fixed, is true as a matter of definition. Note that the greenhouse effect influences the temperature of the planet as a whole, in tandem with the planet's tendency to move toward radiative equilibrium. == Misconceptions ==

. Among those who do not believe in the greenhouse effect, there is a fallacy that the greenhouse effect involves greenhouse gases sending heat from the cool atmosphere to the planet's warm surface, in violation of the second law of thermodynamics. However, this idea reflects a misunderstanding. Radiation heat flow is the net energy flow after the flows of radiation in both directions have been taken into account. Radiation heat flow occurs in the direction from the surface to the atmosphere and space, The downward thermal radiation simply reduces the upward thermal radiation net energy flow (radiation heat flow), i.e., it reduces cooling. == Simplified models ==

. Data as of 2007. Simplified models are sometimes used to support understanding of how the greenhouse effect comes about and how this affects surface temperature. Atmospheric layer models The greenhouse effect can be seen to occur in a simplified model in which the air is treated as if it is single uniform layer exchanging radiation with the ground and space. Slightly more complex models add additional layers, or introduce convection. Equivalent emission altitude One simplification is to treat all outgoing longwave radiation as being emitted from an altitude where the air temperature equals the overall effective temperature for planetary emissions, T_\mathrm{eff}. Some authors have referred to this altitude as the effective radiating level (ERL), and suggest that as the concentration increases, the ERL must rise to maintain the same mass of above that level. This approach is less accurate than accounting for variation in radiation wavelength by emission altitude. However, it can be useful in supporting a simplified understanding of the greenhouse effect. Earth's overall equivalent emission altitude has been increasing with a trend of /decade, which is said to be consistent with a global mean surface warming of /decade over the period 1979–2011. == Related effects on Earth ==

Negative greenhouse effect Scientists have observed that, at times, there is a negative greenhouse effect over parts of Antarctica. 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. Runaway greenhouse effect == Bodies other than Earth ==

In the Solar System, apart from the Earth, at least two other planets and a moon also have a greenhouse effect. Venus The greenhouse effect on Venus is particularly large, and it brings the surface temperature to as high as . This is due to its very dense atmosphere which consists of about 97% carbon dioxide. Due to its high pressure, the CO2 in the atmosphere of Venus exhibits continuum absorption (absorption over a broad range of wavelengths) and is not limited to absorption within the bands relevant to its absorption on Earth. this idea is still largely accepted. The planet Venus experienced a runaway greenhouse effect, resulting in an atmosphere which is 96% carbon dioxide, and a surface atmospheric pressure roughly the same as found underwater on Earth. Venus may have had water oceans, but they would have boiled off as the mean surface temperature rose to the current . Mars Mars has about 70 times as much carbon dioxide as Earth, but experiences only a small greenhouse effect, about . The same radiative transfer calculations that predict warming on Earth accurately explain the temperature on Mars, given its atmospheric composition. While the gases N2 and H2 ordinarily do not absorb infrared radiation, these gases absorb thermal radiation on Titan due to pressure-induced collisions, the large mass and thickness of the atmosphere, and the long wavelengths of the thermal radiation from the cold surface. This means that greenhouse gases are able to absorb more wavelengths in the lower atmosphere than they can in the upper atmosphere. On other planets, pressure broadening means that each molecule of a greenhouse gas is more effective at trapping thermal radiation if the total atmospheric pressure is high (as on Venus), and less effective at trapping thermal radiation if the atmospheric pressure is low (as on Mars). == See also ==