Heating of the earth" redirects here. For other uses, see Greenhouse (disambiguation). For the
general heating or cooling of Earth's surface, see Earth's energy budget. For the internal heating
of Earth, see Earth's internal heat budget. For the Supreme Court theory, see Greenhouse effect
(United States Supreme Court).
Greenhouse gases allow sunlight to pass through the atmosphere, heating the planet, but then absorb and
re-radiate the infrared radiation (heat) the planet emits
Quantitative analysis: Energy flows between space, the atmosphere, and Earth's surface, with
greenhouse gases in the atmosphere absorbing and emitting radiant heat, affecting Earth's energy
balance.
The greenhouse effect is a process that occurs after energy from a planet's host star goes
through the planet's atmosphere and heats the planet's surface. When the planet radiates the
heat back out as thermal infrared radiation, greenhouse gases and clouds in the atmosphere
absorb some of it. This traps the heat near the surface and reduces radiative cooling to space.
The Earth's average surface temperature would be about −18 °C (−0.4 °F) without the
greenhouse effect,[1][2] compared to Earth's 20th century average of about 14 °C (60 °F).[3] In
addition to naturally present greenhouse gases, burning of fossil fuels has increased amounts
of carbon dioxide and methane in the atmosphere.[4][5] As a result, global warming of about 1.2 °C
(2.2 °F) has occurred since the industrial revolution,[6] accelerating to a rate of 0.18 °C (0.32 °F)
per decade more recently.[7]
Greenhouse gases work by being transparent to wavelengths of radiation emitted by a star like
the sun, but absorb wavelengths of radiation emitted by planets like the Earth. The wavelengths
differ because matter radiates energy at a wavelength related to its temperature. The Sun is
about 5,500 °C (9,930 °F), so it emits most of its energy in near infrared and visible wavelengths
(as sunlight). The Earth's surface temperatures are much lower, so it emits longer-wavelength
thermal infrared radiation (radiated heat).[5]
A runaway greenhouse effect occurs when greenhouse gases accumulate in the atmosphere
through a positive feedback cycle to such an extent that they substantially block radiated heat
from escaping into space, thus preventing the planet from cooling.[8] A runaway greenhouse
effect involving carbon dioxide and water vapor appears to have occurred on Venus. However, it
is unlikely that human-caused greenhouse gas emissions alone could trigger a runaway effect on
Earth.
The term greenhouse effect comes from an analogy to greenhouses. Both greenhouses and the
greenhouse effect work by retaining heat from sunlight, but the mechanisms differ. Greenhouses
primarily retain heat by preventing the movement of air (blocking convection), although their
panels also limit heat radiation and conduction.[9][10] The greenhouse effect only limits heat loss
due to radiation; it has no impact on convection or conduction of heat.
History
Main article: History of climate change science
The greenhouse effect and its impact on climate were succinctly described in this 1912 Popular
Mechanics article meant for reading by the general public.
The existence of the greenhouse effect, while not named as such, was proposed by Joseph
Fourier in 1824.[11] 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..."[12][13]
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.[14] 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.[15] However, the
term "greenhouse" was not used to refer to this effect by any of these scientists; the term was
first used in this way by Nils Gustaf Ekholm in 1901.[16][17]
Definition
The greenhouse effect is defined by the Intergovernmental Panel on Climate Change as follows:
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. These substances emit infrared radiation in all
directions, but, everything else being equal, the net amount emitted to space is normally less
than would have been emitted in the absence of these absorbers because of the decline of
temperature with altitude in the troposphere and the consequent weakening of emission. An
increase in the concentration of GHGs increases the magnitude of this effect; the difference is
sometimes called the enhanced greenhouse effect. The change in a GHG concentration because
of anthropogenic emissions contributes to an instantaneous radiative forcing. Earth's surface
temperature and troposphere warm in response to this forcing, gradually restoring the radiative
balance at the top of the atmosphere.[18]: AVII-28
Principles
Incoming radiation
The solar radiation spectrum for direct light at both the top of Earth's atmosphere and at sea level
Earth receives energy from the Sun in the form of ultraviolet, visible, and near-infrared radiation.
About 26% of the incoming solar energy is reflected back to space by the atmosphere and
clouds, and 19% is absorbed by the atmosphere and clouds.[citation needed]
An ideal thermally conductive blackbody at the same distance from the Sun as Earth would have
a temperature of about 5.3 °C (41.5 °F). However, because Earth reflects about 30%[19][20] of the
incoming sunlight, this idealized planet's effective temperature (the temperature of a blackbody
that would emit the same amount of radiation) would be about −18 °C (0 °F).[21][22] The surface
temperature of this hypothetical planet is 33 °C (59 °F) below Earth's actual surface temperature
of approximately 14 °C (57 °F).[23] The greenhouse effect is the contribution of greenhouse gases
and aerosols to this difference, with imperfect modelling of clouds being the main uncertainty.[24]: 7–
61
Outgoing radiation
Atmospheric absorption and scattering at different wavelengths of electromagnetic waves. The largest
absorption band of carbon dioxide is not far from the maximum in the thermal emission from ground, and it
partly closes the window of transparency of water—explaining carbon dioxide's major heat-trapping effect.
The idealized greenhouse model is a simplification. In reality, the atmosphere near the Earth's
surface is largely opaque to thermal radiation and most heat loss from the surface is
by convection. However radiative energy losses become increasingly important higher in the
atmosphere, largely because of the decreasing concentration of water vapor, an important
greenhouse gas. Rather than the surface itself, it is more realistic to think of the greenhouse
effect as applying to a layer in the mid-troposphere, which is effectively coupled to the surface by
a lapse rate.[25] A simple picture also assumes a steady state, but in the real world, the diurnal
cycle, as well as the seasonal cycle and weather disturbances, complicate matters. Solar heating
applies only during daytime. At night the atmosphere cools somewhat, but not greatly because
the thermal inertia of the climate system resists changes both day and night, as well as for longer
periods.[26] Diurnal temperature changes decrease with height in the atmosphere.
Within the region where radiative effects are important, the description given by the idealized
greenhouse model becomes realistic. Earth's surface, warmed to an "effective temperature"
around −18 °C (0 °F), radiates long-wavelength, infrared heat in the range of 4–100 μm.[27] At
these wavelengths, greenhouse gases that were largely transparent to incoming solar radiation
are more absorbent.[27] Each layer of the atmosphere with greenhouse gases absorbs some of
the heat being radiated upwards from lower layers. It reradiates in all directions, both upwards
and downwards; in equilibrium (by definition) the same amount as it has absorbed. This results in
more warmth below. Increasing the concentration of the gases increases the amount of
absorption and re-radiation, and thereby further warms the layers and ultimately the surface
below.[22]
Greenhouse gases—including most diatomic gases with two different atoms (such as carbon
monoxide, CO) and all gases with three or more atoms—are able to absorb and emit infrared
radiation at specific wavelengths,[28] since their intramolecular vibrations produce a dipole
moment. Though more than 99% of the dry atmosphere is IR transparent (because the main
constituents—N
2, O
2, and Ar—have no dipole moment and are thus not able to independently absorb or emit infrared
radiation), intermolecular elastic collisions cause the energy absorbed and emitted by the
greenhouse gases to be shared with the other non-IR-active gases. Lastly as a weaker effect, all
gases can absorb and emit a relatively minor amount of broadband IR via inelastic collisions.[29]
Atmospheric components
How CO2 causes the greenhouse effect.
Greenhouse gases
Main article: Greenhouse gas
A greenhouse gas (GHG) is a gas capable of trapping solar radiation energy within a planet's
atmosphere. Greenhouse gases contribute most of the greenhouse effect in Earth's energy
budget.
Greenhouse gases can be divided into two types, direct and indirect. Gases that can directly
absorb solar energy are direct greenhouse gases, e.g., water vapor, carbon dioxide and ozone.
The molecules of these gases can directly absorb solar radiation at certain ranges of wavelength.
Some gases are indirect greenhouse gases, as they do not absorb solar energy directly or
significantly, but have capability of producing other greenhouse gases. For example, methane
plays an important role in producing tropospheric ozone and formation of more carbon
dioxide.[30] NOx[31] and CO[32] can also produce tropospheric ozone and carbon dioxide
through photochemical processes.
Atmospheric gases only absorb some wavelengths of energy but are transparent to others. The absorption
patterns of water vapor (blue peaks) and carbon dioxide (pink peaks) overlap in some wavelengths. Carbon
dioxide is not as strong a greenhouse gas as water vapor, but it absorbs energy in longer wavelengths (12–
15 micrometers) that water vapor does not, partially closing the "window" through which heat radiated by
the surface would normally escape to space. (Illustration NASA, Robert Rohde)[33]
By their percentage contribution to the overall greenhouse effect on Earth, the four major
greenhouse gases are:[34][35]




Water vapor (H2O), 36~72% (~75% including clouds);[36]
Carbon dioxide (CO2), 9~26%;
Methane (CH4), 4~9%;
Tropospheric ozone (O3), 3~7%.
It is not practical to assign a specific percentage to each gas because the absorption and
emission bands of the gases overlap (hence the ranges given above). A water molecule only
stays in the atmosphere for an average 8 to 10 days, which corresponds with high variability in
the contribution from clouds and humidity at any particular time and location.[24]: 1–41
There are other influential gases that contribute to the greenhouse effect, including nitrous oxide
(N2O), perfluorocarbons (PFCs), chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), and
sulfur hexafluoride (SF6).[24]: AVII-60 These gases are mostly produced through human activities, thus
they have played important parts in climate change.
Concentration change of greenhouse gases from 1750 to 2019[37] (ppm: parts per million; ppb:
parts per billion):



Carbon dioxide (CO2), 278.3 to 409.9 ppm, up 47%;
Methane (CH4), 729.2 to 1866.3 ppb, up 156%;
Nitrous oxide (N2O), 270.1 to 332.1 ppb, up 23%.
The global warming potential (GWP) of a greenhouse gas is calculated by quantifying
the lifetime and the efficiency of greenhouse effect of the gas. Typically, nitrous oxide has a
lifetime of about 121 years, and over 270 times higher GWP than carbon dioxide for 20-year time
span. Sulfur hexafluoride has a lifetime of over 3000 years and 25000 times higher GWP than
carbon dioxide.[37]