Outgoing longwave radiation

In climate science, longwave radiation (LWR) is electromagnetic thermal radiation emitted by Earth's surface, atmosphere, and clouds. It is also referred to as terrestrial radiation. This radiation is in the infrared portion of the spectrum, but is distinct from the shortwave (SW) near-infrared radiation found in sunlight. Outgoing longwave radiation (OLR) is the longwave radiation emitted to space from the top of Earth's atmosphere. It may also be referred to as emitted terrestrial radiation. Outgoing longwave radiation plays an important role in planetary cooling. Longwave radiation generally spans wavelengths ranging from 3–100 micrometres (μm). A cutoff of 4 μm is sometimes used to differentiate sunlight from longwave radiation. Less than 1% of sunlight has wavelengths greater than 4 μm. Over 99% of outgoing longwave radiation has wavelengths between 4 μm and 100 μm. The flux of energy transported by outgoing longwave radiation is typically measured in units of watts per metre squared (W⋅m−2). In the case of global energy flux, the W/m2 value is obtained by dividing the total energy flow over the surface of the globe (measured in watts) by the surface area of the Earth, 5.1×1014 m2 (5.1×108 km2; 2.0×108 mi2). Emitting outgoing longwave radiation is the only way Earth loses energy to space, i.e., the only way the planet cools itself. Radiative heating from absorbed sunlight, and radiative cooling to space via OLR power the heat engine that drives atmospheric dynamics. The balance between OLR (energy lost) and incoming solar shortwave radiation (energy gained) determines whether the Earth is experiencing global heating or cooling (see Earth's energy budget).

Planetary energy balance

Outgoing longwave radiation (OLR) constitutes a critical component of Earth's energy budget. The principle of conservation of energy says that energy cannot appear or disappear. Thus, any energy that enters a system but does not leave must be retained within the system. So, the amount of energy retained on Earth (in Earth's climate system) is governed by an equation:

[change in Earth's energy] = [energy arriving] − [energy leaving]. Energy arrives in the form of absorbed solar radiation (ASR). Energy leaves as outgoing longwave radiation (OLR). Thus, the rate of change in the energy in Earth's climate system is given by Earth's energy imbalance (EEI):

E E I = A S R − O L R {\displaystyle \mathrm {EEI} =\mathrm {ASR} -\mathrm {OLR} } . When energy is arriving at a higher rate than it leaves (i.e., ASR > OLR, so that EEI is positive), the amount of energy in Earth's climate increases. Temperature is a measure of the amount of thermal energy in matter. So, under these circumstances, temperatures tend to increase overall (though temperatures might decrease in some places as the distribution of energy changes). As temperatures increase, the amount of thermal radiation emitted also increases, leading to more outgoing longwave radiation (OLR), and a smaller energy imbalance (EEI). Similarly, if energy arrives at a lower rate than it leaves (i.e., ASR < OLR, so than EEI is negative), the amount of energy in Earth's climate decreases, and temperatures tend to decrease overall. As temperatures decrease, OLR decreases, making the imbalance closer to zero. In this fashion, a planet naturally constantly adjusts its temperature so as to keep the energy imbalance small. If there is more solar radiation absorbed than OLR emitted, the planet will heat up. If there is more OLR than absorbed solar radiation the planet will cool. In both cases, the temperature change works to shift the energy imbalance towards zero. When the energy imbalance is zero, a planet is said to be in radiative equilibrium. Planets naturally tend to a state of approximate radiative equilibrium. In recent decades, energy has been measured to be arriving on Earth at a higher rate than it leaves, corresponding to planetary warming. The energy imbalance has been increasing. It can take decades to centuries for oceans to warm and planetary temperature to shift sufficiently to compensate for an energy imbalance.

Emission Thermal radiation is emitted by nearly all matter, in proportion to the fourth power of its absolute temperature. In particular, the emitted energy flux, M {\displaystyle M} (measured in W/m2) is given by the Stefan–Boltzmann law for non-blackbody matter:

M = ϵ σ T 4 {\displaystyle M=\epsilon \,\sigma \,T^{4}}

where T {\displaystyle T} is the absolute temperature, σ {\displaystyle \sigma } is the Stefan–Boltzmann constant, and ϵ {\displaystyle \epsilon } is the emissivity. The emissivity is a value between zero and one which indicates how much less radiation is emitted compared to what a perfect blackbody would emit.

Surface The emissivity of Earth's surface has been measured to be in the range 0.65 to 0.99 (based on observations in the 8-13 micron wavelength range) with the lowest values being for barren desert regions. The emissivity is mostly above 0.9, and the global average surface emissivity is estimated to be around 0.95.

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