Longwave radiation is the invisible infrared energy that Earth’s surface and atmosphere emit as heat. While the sun blasts the planet with high-energy, short-wavelength radiation, the Earth radiates energy back out at much longer wavelengths, peaking between 5 and 25 micrometers. This outgoing energy is central to how Earth regulates its temperature, and it’s the type of radiation that greenhouse gases intercept on its way to space.
Why Earth Emits Longwave Radiation
Every object with a temperature above absolute zero emits electromagnetic radiation. The wavelength of that radiation depends on how hot the object is: hotter objects emit shorter wavelengths, and cooler objects emit longer ones. This relationship, known as Wien’s displacement law, explains the fundamental difference between solar and terrestrial radiation.
The sun’s surface burns at roughly 5,500°C, so its radiation peaks at about 0.5 micrometers, right in the visible light range (blue-green, specifically). Earth’s surface averages only about 15°C (59°F), so its radiation peaks at wavelengths roughly 10 to 50 times longer, entirely in the infrared part of the spectrum. That’s why solar energy is called shortwave radiation and Earth’s emitted energy is called longwave radiation.
The intensity of this emission scales steeply with temperature. According to the Stefan-Boltzmann law, the total energy a surface radiates is proportional to the fourth power of its absolute temperature. A small increase in surface temperature produces a disproportionately large increase in emitted longwave radiation. This is one of the planet’s built-in cooling mechanisms: as the surface warms, it radiates more energy, which helps push the system back toward balance.
How Greenhouse Gases Trap It
Not all of Earth’s outgoing longwave radiation makes it to space. Greenhouse gases in the atmosphere, primarily water vapor, carbon dioxide, methane, nitrous oxide, and ozone, absorb a significant portion of it. The reason comes down to molecular structure. Nitrogen and oxygen, which make up 99% of the atmosphere, have simple two-atom structures that don’t interact much with infrared light. Greenhouse gas molecules are more complex, with three or more atoms, and they can vibrate in ways that allow them to capture infrared photons.
When a molecule of carbon dioxide absorbs an infrared photon, it begins to vibrate with the extra energy. A short time later, it re-emits another infrared photon in a random direction. That re-emitted photon might head upward toward space, or it might head back down toward the surface. This absorption and re-emission in all directions is the core mechanism of the greenhouse effect. It effectively slows the escape of heat energy, keeping the lower atmosphere warmer than it would be if longwave radiation passed straight through.
The Atmospheric Window
There are specific wavelength bands where the atmosphere is mostly transparent to longwave radiation, meaning infrared photons at those wavelengths pass through without being absorbed. These bands are called atmospheric windows. The primary window sits roughly between 8 and 13 micrometers (or about 795 to 1,230 inverse centimeters in spectroscopic terms). In these wavelength ranges, outgoing radiation reflects surface temperature changes fairly directly, because there’s little atmospheric interference on the way out.
The atmospheric window matters because it represents the planet’s most direct route for shedding heat to space. Some greenhouse gases, particularly certain industrial chemicals, absorb radiation right in this window, which is why even trace amounts of those gases can have an outsized warming effect.
Longwave Radiation in Earth’s Energy Budget
Earth’s climate is governed by a balance between incoming solar energy and outgoing longwave radiation. On a global annual average, the planet emits about 235 watts per square meter of longwave radiation from the top of the atmosphere. This roughly matches the 238 watts per square meter of solar energy the planet absorbs, keeping things close to equilibrium.
That balance has been shifting. Satellite observations between 2003 and 2018 show that Earth’s radiative forcing, the net energy imbalance driving warming, increased by about 0.53 watts per square meter. That may sound small against a backdrop of 235 watts, but spread across the entire planet’s surface, it represents an enormous amount of extra energy being retained. The increase is driven by rising concentrations of greenhouse gases and recent reductions in aerosol pollution (aerosols had been reflecting some sunlight back to space, partially masking the warming).
How Longwave Radiation Is Measured
On the ground and from aircraft, longwave radiation is measured with an instrument called a pyrgeometer. It uses a sensor called a thermopile to detect tiny changes in voltage caused by infrared radiation hitting it, covering wavelengths from about 4 to 50 micrometers. Pyrgeometers can be pointed upward to measure longwave radiation coming down from the atmosphere or downward to measure what Earth’s surface is emitting. From space, satellites measure outgoing longwave radiation across different wavelength bands, which is how scientists track changes in the energy budget over time and detect the spectral fingerprints of individual greenhouse gases.
Why It Matters for Climate
Longwave radiation is the only way Earth loses energy to space. Sunlight arrives, gets absorbed by the surface and atmosphere, and the only exit route for that energy is infrared emission. Anything that changes how efficiently longwave radiation escapes, whether it’s an increase in atmospheric carbon dioxide, a change in cloud cover, or a shift in water vapor concentrations, directly affects the planet’s temperature. The entire greenhouse effect is, at its core, a story about what happens to longwave radiation between the surface and the top of the atmosphere.