Outgoing longwave radiation

[2] The flux of energy transported by outgoing longwave radiation is typically measured in units of watts per metre squared (W⋅m−2).

[3] Emitting outgoing longwave radiation is the only way Earth loses energy to space, i.e., the only way the planet cools itself.

[6] Outgoing longwave radiation (OLR) constitutes a critical component of Earth's energy budget.

[10] In recent decades, energy has been measured to be arriving on Earth at a higher rate than it leaves, corresponding to planetary warming.

[7][8] It can take decades to centuries for oceans to warm and planetary temperature to shift sufficiently to compensate for an energy imbalance.

[11] Thermal radiation is emitted by nearly all matter, in proportion to the fourth power of its absolute temperature.

[13][14] The most common gases in air (i.e., nitrogen, oxygen, and argon) have a negligible ability to absorb or emit longwave thermal radiation.

Consequently, the ability of air to absorb and emit longwave radiation is determined by the concentration of trace gases like water vapor and carbon dioxide.

[15] According to Kirchhoff's law of thermal radiation, the emissivity of matter is always equal to its absorptivity, at a given wavelength.

This is because the atmosphere is generally much colder than the surface, and the rate at which longwave radiation is emitted scales as the fourth power of temperature.

[20][21]: 139 Assuming no cloud cover, most of the surface emissions that reach space do so through the atmospheric window.

[24] The interaction between emitted longwave radiation and the atmosphere is complicated due to the factors that affect absorption.

[27] More specifically, the greenhouse effect may be defined quantitatively as the amount of longwave radiation emitted by the surface that does not reach space.

On Earth as of 2015, about 398 W/m2 of longwave radiation was emitted by the surface, while OLR, the amount reaching space, was 239 W/m2.

[28]: 968, 934 [29][30] When the concentration of a greenhouse gas (such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor (H2O) and is increased, this has a number of effects.

Therefore, an increase in the concentrations of greenhouse gases causes energy to accumulate in Earth's climate system, contributing to global warming.

[18] This means there is enough gas present to completely absorb the radiated energy at that wavelength before the upper atmosphere is reached.

[citation needed] It is sometimes incorrectly argued that this means an increase in the concentration of this gas will have no additional effect on the planet's energy budget.

If the air at that higher altitude is colder (as is true throughout the troposphere), then thermal emissions to space will be reduced, decreasing OLR.

Outgoing long-wave radiation (OLR) has been monitored and reported since 1970 by a progression of satellite missions and instruments.

[citation needed] By using the radiance measured from a particular direction by an instrument, atmospheric properties (like temperature or humidity) can be inversely inferred.

Usually the solution is done numerically by atmospheric radiative transfer codes adapted to the specific problem.

[25] There are online interactive tools that allow one to see the spectrum of outgoing longwave radiation that is predicted to reach space under various atmospheric conditions.

Spectral intensity of sunlight (average at top of atmosphere) and thermal radiation emitted by Earth's surface.
The growth in Earth's energy imbalance from satellite and in situ measurements (2005–2019). A rate of +1.0 W/m 2 summed over the planet's surface equates to a continuous heat uptake of about 500 terawatts (~0.3% of the incident solar radiation). [ 7 ] [ 8 ]
Outgoing radiation and greenhouse effect as a function of frequency. The greenhouse effect is visible as the area of the upper red area, and the greenhouse effect associated with CO 2 is directly visible as the large dip near the center of the OLR spectrum. [ 26 ]
Example wavenumber spectrum of Earth's infrared emissions (400-1600 cm −1 ) measured by IRIS on Nimbus 4 in year 1970. [ 32 ]
Simulated wavenumber spectrum of the Earth's outgoing longwave radiation (OLR) using ARTS . In addition the black-body radiation for a body at surface temperature T s and at tropopause temperature T min is shown.
Simulated wavelength spectrum of Earth's OLR under clear-sky conditions using MODTRAN . [ 41 ]