Climate Science - Longwave Radiation and the Atmoshpere

In the last article on climate science, we discussed the different interactions of longwave and shortwave radiation with the Earth's atmosphere. Most shortwave radiation is transmitted through the blanket of gas that surrounds the planet, while the atmosphere tends to absorb most of the long wave radiation it is exposed to.

These different types of radiation are absorbed to differing degrees because of the differences in their wavelength. The gases which comprise the Earth's atmosphere each have a unique signature of wavelengths that they will absorb and while few of these wavelengths are in the range of shortwave radiation, many more are in the range for longwave radiation, which is why far less longwave radiation is transmitted through the atmosphere.

The main atmospheric gases which absorb longwave radiation are methane, nitrous oxide, oxygen, ozone, carbon dioxide and water vapour. Tranmsissivity of longwave radiation through the atmosphere can happen in one of two ways; the radiation may not encounter any gas particles as it passes through the atmosphere, or the wavelength of the longwave radiation may not correspond to any of the wavelengths that can be absorbed by atmospheric gases.

Around the wavelength of 8 micrometers, there is a small window of longwave wavelengths that none of the atmospheric gases can absorb, and these wavelengths will therefore always be transmitted through the atmosphere, regardless of the concentration of atmospheric gases. The likelihood that other wavelengths of longwave radiation  will be absorbed by the atmosphere is however directly dependent on the concentration of atmospheric gases - the more gas particles in the atmospheric column, the greater the likelihood that longwave radiation will collide with these particles and be absorbed.

As more longwave radiation is absorbed by these atmospheric gases, there is more energy absorbed by the atmosphere, shifting the energy balance and resulting in more energy (also known as heat) in the entire earth-atmosphere system.