How does the earth radiate?

About one half of the solar energy that reaches the atmosphere's outer limits from space actually hits the surface of the earth.  The other half of solar insolation is already reverberated (reflected) or taken up (absorbed) earlier on its way through the atmosphere.  It it thus by the remaining half that reaches the ground that the surface of the earth is heated. Every heated body, though, radiates by itself, proportional to its temperature.  We are familiar with this experience in everyday life regarding, e.g., a stove plate.  The more energy (in the shape of electric power) we put in, the hotter it becomes and the more clearly we feel the heat radiated when we hold one hand above the plate.

According to the laws of physics, one is able to calculate the range of wavelengths in which radiation is emitted at a certain temperature of the stove plate or, more generally, a heated body.  The many-colored area in the ensuing illustration shows us how the radiation of heat is distributed if the temperature of a body is of the order of 280 Kelvin (+7° C).  This almost corresponds to the earth's mean global temperature at its surface.  The illustration shows a spectrum of the so called infrared radiation approximately between 400 and 1800 cm^-1 (the unit "cm^-1" simply represents a way to describe the energy of infrared radiation by means of so called "wave numbers" referring to the number of wave peaks that can be fitted into an interval of the width of one centimeter).

red+yellow+blue = total radiation of the earth at +7° C in the range between 400 and 1800 cm^-1.
blue = radiation that is absorbed by greenhouse gases.
yellow = radiation that is allowed to pass by greenhouse gases.
(red = absence of an absorption spectrum due to technical reasons concerning the measurements.)

But: in the range between approximately 500 and 1800 cm^-1, depicted in blue, the illustration shows the total amount of absorption caused by the most important atmospheric trace gases, namely water vapor (H₂O), carbon dioxide (CO₂), and ozone (O₃).

These so-called 'trace gases' - this term refers to their relatively insignificant abundance as compared to the influence these compounds exert to the atmosphere - constitute, so to say, the wool of the sweater in which the earth is wrapped up.  Here, the emitted heat catches on immediately.  Only at distinct energies (depicted in yellow color) the radiation is able to escape through the atmosphere into space without being moderated.

It can clearly be obtained from the illustration that water vapor absorbs over a wide range of the spectrum.

Water vapor is the most important greenhouse gas!

In a very rough approximation the following trace gases contribute to the greenhouse effect:
60% water vapor
20% carbon dioxide (CO₂)
The rest (~20%) is caused by ozone (O₃), nitrous oxide (N₂O), methane (CH₄), and several other species.

 

State of the Art The contribution of water vapor to the anthropogenic greenhouse effect (i.e., that portion of greenhouse warming caused exclusively by humans) is still controversial.  At numerous environmental conferences, greenhouse gases, such as CO2 and methane (CH4), are discussed primarily while many times the role of water vapor in both its natural and anthropogenic aspects remains unmentioned.  Yet water vapor not only holds the pole position concerning the natural greenhouse effect, but also participates in the additional absorption of heat in the atmosphere which is exclusively caused by human activities.   We're not speculating that we would blow enormous amounts of water vapor into the air and enhance the greenhouse effect.  On the contrary, the concerns are for so-called "secondary effects".  That is: if the average temperature of atmospheric layers near to the ground, as a consequence of anthropogenic CO2 and methane emissions, is rising, then the evaporation of water is increased.  Henceforth more water vapor will get into the air, and this additional abundance of water vapor will also absorb more heat.   It remains uncertain, though, which concentrations at which locations and at which altitudes in the troposphere will contribute the most to greenhouse warming.  In addition, it is unclear how this surplus of water vapor will alter the warming process of the earth. One of the major problems in climate research at present is the fact that we still cannot realistically reproduce the formation of clouds in currently available climate models.  Therefore an exact prediction of the influence exerted by water vapor and a prediction of the warming on the whole, remains very doubtful.  This area of research calls for an enormous amount of scientific work.   We conclude: Due to the so called "greenhouse effect" - caused by atmospheric trace gases such as carbon dioxide (CO2), ozone (O3), and water vapor (H2O) - infrared radiation from the earth is stored temporarily in the atmosphere.  Of all these trace gases, water vapor represents the most important constituent.  It contributes to the natural greenhouse warming process by approximately 60%.  Water vapor amplifies the anthropogenic contribution to greenhouse warming through a positive feedback.  This amplification is counteracted by the increased reflection off clouds.  How these two factors combine in the real atmosphere still remains an open question.