Greenhouse effect
The greenhouse effect occurs when heat-trapping gases in a planet's atmosphere prevent the planet from losing heat to space, raising its surface temperature. Surface heating can happen from an internal heat source or come from an external source, such as a host star. In the case of Earth, the Sun emits shortwave radiation 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, reducing the rate at which the Earth can cool off.
Without the greenhouse effect, the Earth's average surface temperature would be as cold as. This is of course much less than the 20th century average of about. In addition to naturally present greenhouse gases, burning of fossil fuels has increased amounts of carbon dioxide and methane in the atmosphere. As a result, global warming of about has occurred since the Industrial Revolution, with the global average surface temperature increasing at a rate of per decade since 1981.
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, so it emits most of its energy as shortwave radiation in near-infrared and visible wavelengths. In contrast, Earth's surface has a much lower temperature, so it emits longwave radiation at mid- and far-infrared wavelengths. 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, thus accumulating energy and warming the Earth's surface.
The existence of the greenhouse effect was proposed as early as 1824 by Joseph Fourier. 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. The term greenhouse was first applied to this phenomenon by Nils Gustaf Ekholm in 1901.
Definition
The greenhouse effect on Earth is defined as: "The infrared radiative effect of all infrared absorbing constituents in the atmosphere. Greenhouse gases, 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, 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. 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 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. She concluded that "An atmosphere of that gas would give to our earth a high temperature..."John Tyndall was the first to measure the infrared absorption and emission of various gases and vapors. From 1859 onwards, he showed that the effect was due to a very small proportion of the atmosphere, with the main gases having no effect, and was largely due to water vapor, though small percentages of hydrocarbons and carbon dioxide had a significant effect. 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. 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 or percentage 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
Strengthening of the greenhouse effect through additional greenhouse gases from human activities is known as the enhanced greenhouse effect. As well as being inferred from measurements by ARGO, CERES and other instruments throughout the 21st century, 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 in 1960, passing the 400 ppm milestone in 2013. The current observed amount of exceeds the geological record maxima 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
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.Sunlight is reflected and absorbed by the Earth and its atmosphere. The atmosphere and clouds reflect about 23% and absorb 23%. The surface reflects 7% and absorbs 48%. Overall, Earth reflects about 30% of the incoming sunlight, and absorbs the rest.
Outgoing longwave radiation
The Earth and its atmosphere emit longwave radiation, also known as thermal infrared or terrestrial radiation. Informally, longwave radiation is sometimes called thermal radiation. Outgoing longwave radiation is the radiation from Earth and its atmosphere that passes through the atmosphere and into space.The greenhouse effect can be directly seen in graphs of Earth's outgoing longwave radiation as a function of frequency. The area between the curve for longwave radiation emitted by Earth's surface and the curve for outgoing longwave radiation indicates the size of the greenhouse effect.
Different substances are responsible for reducing the radiation energy reaching space at different frequencies; for some frequencies, multiple substances play a role. Carbon dioxide is understood to be responsible for the dip in outgoing radiation at around 667 cm−1.
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.
Effective temperature
The power of outgoing longwave radiation emitted by a planet corresponds to the effective temperature of the planet. The effective temperature is the temperature that a planet radiating with a uniform temperature would need to have in order to radiate the same amount of energy.This concept may be used to compare the amount of longwave radiation emitted to space and the amount of longwave radiation emitted by the surface:
- Emissions to space: Based on its emissions of longwave radiation to space, Earth's overall effective temperature is.
- Emissions from surface: Based on thermal emissions from the surface, Earth's effective surface temperature is about, which 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.