The greenhouse effect is the warming of a planetary surface and lower atmosphere caused by atmospheric constituents that absorb and emit thermal radiation. On Earth, greenhouse gases and clouds reduce the escape of radiation to space at a given surface temperature, maintaining a warmer surface than would otherwise occur. This natural process is essential to Earth’s climate; its strengthening through human emissions is a principal cause of modern climate change. (ipcc.ch)
Physical mechanism
The Sun supplies energy mainly as shortwave electromagnetic radiation. Some is reflected by clouds, atmospheric particles, and the surface; the remainder is absorbed by the atmosphere and ground. The warmed surface emits longer-wavelength infrared radiation. Greenhouse gases absorb radiation within particular wavelength bands and emit thermal radiation both upward and downward. Downward atmospheric radiation reduces the surface’s net radiative energy loss, helping sustain its temperature. (science.nasa.gov)
Absorption depends on the structure of a molecule and its allowed changes in rotational and vibrational states. Carbon dioxide, methane, and water vapour interact strongly with portions of Earth’s thermal spectrum, whereas nitrogen and oxygen—the atmosphere’s dominant gases—are weak absorbers at those wavelengths. Consequently, atmospheric abundance alone does not determine a gas’s greenhouse importance. Cloud droplets and ice particles also absorb and emit infrared radiation. (giss.nasa.gov)
The effect involves radiative transfer through many atmospheric layers, not simply radiation reflected by an overhead barrier. At wavelengths strongly absorbed by the atmosphere, radiation reaching space generally originates above the surface. Increasing greenhouse-gas concentrations can raise the effective emitting altitude. Because temperature usually decreases with height in the lower atmosphere, these higher layers initially emit less energy, producing an imbalance that drives warming. (history.aip.org)
Energy balance and natural warming
Earth’s energy budget describes the balance between absorbed sunlight and outgoing thermal radiation. In an idealized equilibrium, the two are equal. Using Earth’s approximate present-day albedo, or reflectivity, a simple calculation gives an effective radiating temperature near 255 kelvin, about −18°C. The conventional approximate global mean surface temperature is 288 K, or 15°C. Their roughly 33°C difference illustrates the natural greenhouse effect. (science.nasa.gov)
This comparison is not an exact prediction of an Earth stripped of its atmosphere: removing atmospheric gases would also change clouds, ice cover, reflectivity, and heat transport. Effective radiating temperature is likewise not identical to the average temperature of a spatially varying surface. The comparison expresses a basic distinction between the temperature associated with planetary emission and the warmer conditions near the ground. (giss.nasa.gov)
The greenhouse effect does not create energy or violate thermodynamics. Energy is exchanged between the surface and atmosphere in both directions, while the planet ultimately loses energy to space. Convection and evaporation also transport energy upward and help determine the atmospheric temperature profile; they must be included alongside radiation in realistic calculations. (science.nasa.gov)
Gases, clouds, and feedbacks
Water vapour is a major contributor to the natural greenhouse effect. Its atmospheric concentration depends strongly on temperature and the water cycle. Warming generally increases atmospheric moisture, which strengthens infrared absorption and amplifies the original warming. This is a positive feedback, rather than an independent explanation for the sustained increase in temperature caused by long-lived greenhouse gases. (ipcc.ch)
Carbon dioxide behaves differently because its abundance is governed by emissions and exchanges within the carbon cycle, rather than rapid condensation and precipitation. Methane, nitrous oxide, ozone, and manufactured halogen-containing gases also contribute. Their effects differ because of their absorption spectra, concentrations, atmospheric persistence, and chemical interactions. Overlapping absorption bands mean that individual gases’ contributions cannot always be added or assigned unique percentages without specifying an accounting method. (ipcc.ch)
Clouds have competing effects: they reflect incoming sunlight, tending to cool Earth, and absorb outgoing infrared radiation, tending to warm it. Their overall influence depends on altitude, thickness, coverage, and particle properties. Changes in clouds, atmospheric moisture, and surface reflectivity are important components of climate feedbacks represented in climate models. (ipcc.ch)
Human enhancement and observations
Since industrialization, combustion of fossil fuels, land-use change including deforestation, and other activities have increased greenhouse-gas concentrations. The resulting change in the planetary energy balance is described as radiative forcing. Greenhouse warming is partly offset by cooling influences, notably some atmospheric aerosols; the observed climate response reflects these influences together with feedbacks and natural variability. (ipcc.ch)
Evidence comes from laboratory absorption measurements, atmospheric observations, satellite radiation measurements, and surface and ocean temperature records. These provide complementary tests of the mechanism and its climatic consequences. In its 2023 synthesis report, the Intergovernmental Panel on Climate Change concluded that human activities, principally greenhouse-gas emissions, had unequivocally caused global warming; global surface temperature in 2011–2020 was approximately 1.1°C above 1850–1900. (ipcc.ch)
Historical development and terminology
Joseph Fourier argued in the 1820s that the atmosphere helps explain Earth’s warmth. In 1856, Eunice Newton Foote reported experiments comparing the heating of gases in sunlight and suggested that a carbon-dioxide-rich atmosphere would be warmer. John Tyndall’s experiments in 1859 directly established infrared absorption by gases including water vapour and carbon dioxide. In 1896, Svante Arrhenius calculated how changing atmospheric carbon dioxide could alter surface temperature. (history.aip.org)
The name is an analogy, not an exact description of a horticultural greenhouse. A glass greenhouse warms substantially by restricting the exchange of heated interior air with cooler surroundings. The atmospheric greenhouse effect instead concerns infrared absorption and emission, coupled to atmospheric heat transport. (history.aip.org)