What is the "Greenhouse Effect"?*
The greenhouse effect is in no way an artificial result exclusively brought about by mankind. It is rather the atmosphere's natural ability to store the heat radiated from the earth on its way to space. Thus the earth is covered with a warmer atmospheric layer near the ground (+15° C on a global average), as compared to a situation without the greenhouse effect. In that case, the average temperature in the lower atmosphere would rather turn out to be approximately -18°C.
Hence the atmosphere surrounds "naked Mother Earth" like a natural "sweater" that retains the heat radiated from the ground. Therefore, "Mother" Earth, beneath her atmospheric sweater, experiences a soft springtime at +15° C rather than an icy winter with a mean temperature of -18°C. The difference between us and "Mother" Earth lies in the fact that the she obtains the heat she is re-radiating directly from the sun, i.e., through the solar insolation that hits the ground. In contrast, we of course obtain our body heat primarily by means of combustion of nutriments.
The boiling Earth -
too simple minded!
In principal, the greenhouse effect, of course, does not result in boiling the earth as it is shown in many illustrations but rather protects us from freezing to death.
Mankind has certainly contributed to the fact that the "natural sweater" has become "thicker" during the last 150 years. This indeed could possibly make us or our descendants sweat more than previous human generations have experienced.
*Here, we renounce the detailed description of the earth's radiation budget which would bring us too far away from the actual topic, i.e., water vapor. This subject is thoroughly discussed in many different sources (to be found also on the internet, see, e.g.: Earth Radiation Budget or The Earth's Radiation Energy Balance).
Absorption of Earth's Radiation
For a detailed explanation of absorption please read section 2 in advance!
We are going to examine earth's radiation once without its protecting atmosphere, and once taking into account the sheltering properties of the atmosphere.
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.
The status quo (IPCC-Report):
For the present, a consensus of selected high-level researchers at the international level has agreed that the contribution of water vapor to anthropogenic warming is assumed to be in the neighborhood of 50%. This assumption is based on model calculations. This means that global warming, caused by other greenhouse gases, is assumed to be 50% higher than would be the case if the distribution of water vapor in the air stayed the same. But the model calculations do not forecast a real 50% increase in warming because these simulations do not take into account the influence of clouds.
We state the fact that water vapor leads to a positive feedback in the process of greenhouse warming. If other greenhouse gases produce a warming process, this effect is amplified by the presence of water vapor and thus the temperature rises even more. But if the temperature rises, more and more water evaporates. The abundance of water vapor in the air again increases. Temperature rises ...
Until the oceans boil???
NO! NOT SO!
What about the clouds in our model?
More water vapor in the air also gives rise to an increase in the formation of clouds in the troposphere. Clouds do consist of small water droplets, though, and, hence, they do absorb radiation. But they also have a moderating effect on the process of earth's warming because clouds reflect a significant portion of solar insolation. Thus, this portion does not reach the surface of the earth and thus surface is less heated.
Through this process warming of the earth is favored by means of a positive feedback. But at the same time the overall influence of clouds (as a combination of absorption of earth's heat radiation and reflection of solar insolation) is negative. Clouds exert a cooling effect.
A better illustration of the scene, though still strongly simplified, would look like this:
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.