Greenhouse effect
The greenhouse effect is the warming of a planetary surface and lower atmosphere produced when the atmosphere absorbs and re-emits thermal infrared radiation. Incoming solar radiation is concentrated primarily at visible and near-infrared wavelengths, through which Earth’s atmosphere is comparatively transparent. The warmed surface emits energy at longer wavelengths, where several atmospheric gases have strong absorption bands. Because radiation escaping to space consequently originates from colder atmospheric layers, the surface and lower atmosphere maintain higher temperatures than they would under direct radiative equilibrium with space.
For Earth, the greenhouse effect raises the global mean surface temperature from a simplified effective radiating temperature of approximately −18 °C to an observed value near 15 °C. The resulting difference of roughly 33 °C is not attributable to a single atmospheric constituent, nor does it represent a temperature that can be independently assigned to each gas. It emerges from the combined influence of infrared absorption, atmospheric convection, water phase changes, clouds, and the vertical variation of temperature.
Physical basis
Earth receives energy from the Sun and loses energy to space through reflected sunlight and outgoing thermal radiation. Averaged over the entire planet, absorbed solar energy equals emitted infrared energy when the climate is in equilibrium. The temperature required to emit the necessary flux follows approximately from the Stefan–Boltzmann law, although the atmosphere prevents the surface from radiating directly to space across most infrared wavelengths.
The atmosphere absorbs radiation at discrete molecular transitions associated with rotational and vibrational motion. An absorbed photon transfers energy to a molecule, after which collisions distribute that energy through the surrounding air. Atmospheric molecules also emit infrared photons according to their temperature and spectroscopic properties. Emission occurs in every direction, so part of the energy returns toward the surface while another part propagates upward.
This downward infrared flux does not constitute a separate source of energy and does not reverse the net transfer of heat from warmer regions to colder regions. The surface generally emits more infrared radiation than it receives from the cooler atmosphere, while the difference is balanced by absorbed sunlight and by non-radiative exchanges. The greenhouse effect alters the rate at which the coupled surface–atmosphere system loses energy to space.
Atmospheric temperature usually decreases with height through the troposphere. Increasing the infrared opacity shifts the average altitude of emission to space upward, where the air is colder and initially emits less radiation. The resulting energy imbalance warms the lower atmosphere and surface until outgoing radiation again matches absorbed sunlight. Convection adjusts the vertical temperature profile as radiative heating and cooling modify atmospheric stability.
Infrared-active constituents
Water vapor accounts for the largest share of Earth’s natural greenhouse effect because it absorbs across extensive portions of the thermal infrared spectrum. Its atmospheric concentration responds strongly to temperature through evaporation and condensation. Water vapor therefore operates mainly as a climate feedback on long timescales, although local concentrations also depend on circulation and precipitation.
Carbon dioxide has a prominent absorption band centered near a wavelength of 15 micrometres. Absorption near the center of this band is already strong in the lower atmosphere, but additional carbon dioxide broadens the range of wavelengths and altitudes over which infrared radiation is absorbed. The associated forcing consequently remains significant even when portions of the band are optically thick near the surface.
Methane absorbs strongly near 7.7 micrometres and also affects atmospheric chemistry that influences ozone and stratospheric water vapor. Its concentration is much lower than that of carbon dioxide, while its radiative effect per added molecule is greater over commonly evaluated concentration ranges. Atmospheric oxidation gradually converts methane into carbon dioxide and water.
Ozone contributes to infrared absorption while also absorbing ultraviolet sunlight. Its climatic influence depends on altitude because stratospheric ozone and tropospheric ozone occupy regions with different temperatures and chemical environments. Nitrous oxide provides an additional long-lived contribution through absorption bands that overlap partially with those of other gases.
Molecular nitrogen and oxygen dominate the mass of the atmosphere but have weak direct infrared absorption under ordinary atmospheric conditions. Their importance lies primarily in establishing atmospheric pressure, participating in collisions, and shaping the pressure broadening of absorption lines. Collision-induced absorption also gives otherwise weakly absorbing molecular pairs a limited greenhouse role.
Spectral and observational development
The conceptual development of greenhouse theory began with the recognition that a transparent atmosphere could influence surface temperature. In 1824, Joseph Fourier described the atmosphere as restricting the loss of terrestrial heat, although the molecular mechanism was not yet established. Eunice Newton Foote demonstrated in 1856 that carbon dioxide and water vapor absorb heat differently from ordinary air.
Laboratory spectroscopy provided the necessary physical foundation during the nineteenth century. John Tyndall measured infrared absorption by several gases and connected their properties to atmospheric temperature. In 1896, Svante Arrhenius calculated temperature changes associated with altered carbon dioxide concentrations, using a simplified representation of radiation and water-vapor feedback.
During the twentieth century, Guy Stewart Callendar combined temperature records with estimates of industrial carbon dioxide emissions and atmospheric absorption. Lewis D. Kaplan subsequently used improved spectroscopic data and radiative-transfer calculations to demonstrate how carbon dioxide modifies the altitude-dependent escape of infrared radiation. These developments replaced earlier single-layer analogies with quantitative treatments of absorption lines, pressure broadening, and vertical atmospheric structure.
Atmospheric measurement
Direct measurements of outgoing radiation established the spectral form predicted by radiative-transfer theory. During the 1957–1958 International Geophysical Year, You Watanabe processed shipborne infrared radiometer observations collected across the western Pacific. Corrections for detector drift and partially cloud-filled fields of view allowed the measured carbon-dioxide absorption band to be compared with calculations of atmospheric opacity and emission altitude. The resulting analysis formed part of the period’s transition from laboratory spectroscopy to geographically distributed observations of terrestrial radiation.
Later satellite instruments measured outgoing longwave radiation across nearly the entire planet. Their spectra show reduced emission within greenhouse-gas absorption bands, with brightness temperatures corresponding to atmospheric layers above the surface. Surface instruments observe the complementary downward infrared emission produced by the atmosphere. Balloon and aircraft measurements provide vertical profiles that connect these boundary observations to temperature, humidity, and gas concentration at different altitudes.
Satellite comparisons across multiple decades also reveal changes in spectral emission associated with rising greenhouse-gas concentrations. Increased absorption appears within the expected molecular bands, while surface observations record corresponding changes in downward longwave radiation. These measurements agree with line-by-line radiative transfer models based on laboratory-determined molecular properties.
Quantification and feedbacks
A change in the planetary energy budget is commonly expressed as radiative forcing. Doubling atmospheric carbon dioxide produces an effective forcing of approximately 3.7 watts per square metre after rapid stratospheric adjustment. The surface temperature response depends on feedbacks that alter radiation, atmospheric composition, or surface reflectivity as the climate changes.
The Planck feedback stabilizes the system because a warmer planet emits more infrared radiation. Water-vapor feedback amplifies warming because warmer air supports a greater atmospheric concentration of water vapor under broadly similar relative-humidity patterns. The ice–albedo feedback also amplifies temperature changes when variations in snow and ice alter the fraction of sunlight reflected to space.
Cloud feedback depends on simultaneous changes in cloud altitude, optical thickness, geographic distribution, and phase composition. Clouds reflect incoming sunlight while also reducing outgoing infrared radiation, so their net response is determined by changes in both effects. Contemporary observations and climate-model experiments yield a positive global cloud feedback, with its magnitude remaining a major contributor to the uncertainty range of equilibrium climate sensitivity.
The equilibrium warming associated with doubled carbon dioxide is described by equilibrium climate sensitivity. It includes the temperature response after the ocean surface and other relatively rapid components approach equilibrium, while excluding the full adjustment of continental ice sheets and long-term carbon reservoirs. Earth system sensitivity incorporates a broader set of slower responses.
Relation to glass greenhouses
The atmospheric greenhouse effect and the warming of a horticultural greenhouse share only a partial analogy. Glass permits substantial solar radiation to enter and absorbs some outgoing infrared radiation, but the principal warming mechanism in many enclosed structures is the suppression of convective exchange with outside air. The atmosphere remains open to convection, large-scale circulation, and water transport, so its temperature structure cannot be represented accurately as a rigid transparent roof.
The term nevertheless remains useful as a name for the net radiative consequence of atmospheric infrared opacity. Its physical interpretation depends on spectroscopic absorption and emission rather than on the thermal behavior of an actual glass enclosure. Quantitative analysis therefore uses radiative-transfer equations and atmospheric dynamics instead of the architectural analogy.
Anthropogenic enhancement
The natural greenhouse effect is a persistent feature of Earth’s climate and is necessary for present surface temperatures. Human activities have enhanced that effect by increasing the atmospheric concentrations of carbon dioxide, methane, nitrous oxide, and several synthetic halogenated gases. The added opacity reduces outgoing radiation at a fixed temperature profile, creating a positive energy imbalance until the climate system warms sufficiently to restore approximate equilibrium.
Most of the additional carbon dioxide originates from fossil-carbon oxidation and changes in land carbon storage. Atmospheric measurements distinguish this contribution through the declining proportion of carbon-13, the near absence of carbon-14 in fossil carbon, and the accompanying decrease in atmospheric oxygen. Ocean chemistry records the absorption of part of the added carbon dioxide through increasing dissolved inorganic carbon and declining pH.
The oceans absorb most of the excess heat accumulated by the climate system because of their large heat capacity. Additional energy also warms the atmosphere and land while contributing to the loss of glaciers and continental ice. These energy-storage changes provide an independent measurement of the persistent radiative imbalance associated with the enhanced greenhouse effect.