Greenhouse effect

[3][4] In addition to naturally present greenhouse gases, burning of fossil fuels has increased amounts of carbon dioxide and methane in the atmosphere.

The Sun has a surface temperature of 5,500 °C (9,900 °F), so it emits most of its energy as shortwave radiation in near-infrared and visible wavelengths (as sunlight).

In contrast, Earth's surface has a much lower temperature, so it emits longwave radiation at mid- and far-infrared wavelengths.

Greenhouse gases (GHGs), clouds, and some aerosols absorb terrestrial radiation emitted by the Earth's surface and elsewhere in the atmosphere.

She concluded that "An atmosphere of that gas would give to our earth a high temperature..."[11][12] John Tyndall was the first to measure the infrared absorption and emission of various gases and vapors.

[19] 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.

Scientists also measure the greenhouse effect based on how much more longwave thermal radiation leaves the Earth's surface than reaches space.

[22]: 968 [22]: 934  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.

[34] The current observed amount of CO2 exceeds the geological record maxima (≈300 ppm) from ice core data.

The greenhouse effect can be directly seen in graphs of Earth's outgoing longwave radiation as a function of frequency (or wavelength).

[24] 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).

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, 5.1×1014 m2 (5.1×108 km2; 2.0×108 sq mi).

[48] A UN presentation says "The EEI is the most critical number defining the prospects for continued global warming and climate change.

"[49] One study argues, "The absolute value of EEI represents the most fundamental metric defining the status of global climate change.

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.

[15] Greenhouse gases absorb and emit longwave radiation within specific ranges of wavelengths (organized as spectral lines or bands).

[62] In a separate process, greenhouse gases emit longwave radiation, at a rate determined by the air temperature.

Within the troposphere, greenhouse gases typically have a net cooling effect on air, emitting more thermal radiation than they absorb.

[46]: 139 [63] Effect on surface cooling: Longwave radiation flows both upward and downward due to absorption and emission in the atmosphere.

However, the mix of cooling and warming effects varies, depending on detailed characteristics of particular clouds (including their type, height, and optical properties).

[71] Thus, the overall effective temperature of a planet is given by where OLR is the average flux (power per unit area) of outgoing longwave radiation emitted to space and

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.

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).

[62] Simplified models are sometimes used to support understanding of how the greenhouse effect comes about and how this affects surface temperature.

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.

[82][81][60] Earth's overall equivalent emission altitude has been increasing with a trend of 23 m (75 ft)/decade, which is said to be consistent with a global mean surface warming of 0.12 °C (0.22 °F)/decade over the period 1979–2011.

[57] A runaway greenhouse effect involving carbon dioxide and water vapor has for many years been hypothesized to have occurred on Venus;[91] this idea is still largely accepted.

[97] The same radiative transfer calculations that predict warming on Earth accurately explain the temperature on Mars, given its atmospheric composition.

[57][86][100] The existence of a high-altitude haze, which absorbs wavelengths of solar radiation but is transparent to infrared, contribute to an anti-greenhouse effect of approximately 9 K (16 °F).

Collisions broaden the width of absorption lines, allowing a greenhouse gas to absorb thermal radiation over a broader range of wavelengths.

Energy flows down from the sun and up from the Earth and its atmosphere . When greenhouse gases absorb radiation emitted by Earth's surface , they prevent that radiation from escaping into space, causing surface temperatures to rise by about 33 °C (59 °F).
How CO 2 causes the greenhouse effect.
Earth's rate of heating (graph) is a result of factors which include the enhanced greenhouse effect. [ 27 ]
The Keeling Curve of atmospheric CO 2 abundance.
The solar radiation spectrum for direct light at both the top of Earth's atmosphere and at sea level
The greenhouse effect is a reduction in the flux of outgoing longwave radiation, which affects the planet's radiative balance. The spectrum of outgoing radiation shows the effects of different greenhouse gases.
Temperature needed to emit a given amount of thermal radiation.
Comparison of Earth's upward flow of longwave radiation in reality and in a hypothetical scenario in which greenhouse gases and clouds are removed or lose their ability to absorb longwave radiation—without changing Earth's albedo (i.e., reflection/absorption of sunlight). Top shows the balance between Earth's heating and cooling as measured at the top of the atmosphere (TOA). Panel (a) shows the real situation with an active greenhouse effect. [ 52 ] Panel (b) shows the situation immediately after absorption stops; all longwave radiation emitted by the surface would reach space; there would be more cooling (via longwave radiation emitted to space) than warming (from sunlight). This imbalance would lead to a rapid temperature drop. Panel (c) shows the final stable steady state, after the surface cools sufficiently to emit only enough longwave radiation to match the energy flow from absorbed sunlight. [ 52 ]
The temperature at which thermal radiation was emitted can be determined by comparing the intensity at a particular wavenumber to the intensity of a black-body emission curve . In the chart, emission temperatures range between T min and T s . "Wavenumber" is frequency divided by the speed of light).
Greenhouse gases (GHGs) in dense air near the surface absorb most of the longwave radiation emitted by the warm surface. GHGs in sparse air at higher altitudes—cooler because of the environmental lapse rate —emit longwave radiation to space at a lower rate than surface emissions.
Longwave absorption coefficients of water vapor and carbon dioxide. For wavelengths near 15 microns (15 μ m in top scale), where Earth's surface emits strongly, CO 2 is a much stronger absorber than water vapor.
Flow of heat in Earth's atmosphere, showing (a) upward radiation heat flow and up/down radiation fluxes, (b) upward non-radiative heat flow ( latent heat and thermals ), (c) the balance between atmospheric heating and cooling at each altitude, and (d) the atmosphere's temperature profile.
Increase in the Earth's greenhouse effect (2000–2022) based on NASA CERES satellite data.
The greenhouse effect can be understood as a decrease in the efficiency of planetary cooling. The greenhouse effect is quantified as the portion of the radiation flux emitted by the surface minus that doesn't reach space, i.e., 40% or 159 W/m 2 . Some emitted radiation is effectively cancelled out by downwelling radiation and so does not transfer heat . 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 overall heat flow. Heat (net energy) always flows from warmer to cooler , honoring the second law of thermodynamics . [ 75 ] (This heat flow diagram is equivalent to NASA's earth energy budget diagram. Data is from 2009.)
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 . Data as of 2007.