The greenhouse effect stems from the portion of solar radiation, that is not directly reflected or absorbed from gas molecules or clouds on its way to the earth's surface, but is reflected from the surface in the infrared range of the spectrum, that is at higher wavelengths. This thermal radiation does not escape into space, but is held back by gases, that absorb the infrared light of the atmosphere.

The most important of those gases are:

Carbon dioxide CO₂
Methane CH₄
Laughing gas N₂O
the Chlorofluorocarbons CFCs
and above all water vapour

Information about water vapour

The importance of these gases, the generation of their release, their sources and sinks are being discussed.

We distinguish between the natural greenhouse effect that is important for a moderate climate, and the anthropogenic (man made) one.

b) Solar cycle

The energy that determines the processes on our planet came and comes from the sun, in the end. If one thinks of the atmosphere of a cover wrapped around the earth, the sun radiates with a certain power on each square meter of this cover. This value is called the solar constant.

Indeed, the measurements of the last two decades have shown, that this value is not really constant, but varies in the order of some hundredth percent, and is subject to an 11 years cycle - the solar cycle. Besides this 11 years cycle, a long-time trend is assumed, that showed a slight intensification of the average solar radiation during the 20th century.

The current, even if uncertain knowledge of these phenomena shall be described in brief. Individual scientists attach a very high importance for the global warming to the solar intensification, whereas the majority assesses the anthropogenic greenhouse effect as dominating by far.

c) Aerosol effects

Direct and indirect aerosol effects shall also be mentioned as an important influence on the radiation budget. Due to their large impact we dedicated an own chapter to aerosols. The expulsion of big amounts of particles into the atmosphere leads to a degradation of the amount of radiation energy that arrives on earth. This degradation often, but not always, predominates the greenhouse effect of those particles reflecting the infrared radiation from the earth back to the surface. Depending on the composition of these aerosols, their direct influence can lead to a cooling or warming of the atmosphere.

A negative greenhouse potential is attributed to sulphate aerosols, for example. This effect has been globally measurable in a massive way during the outbreaks of big volcanoes (e.g. Mt. Pinatubo 1991), but also local cooling (e.g. above China), has been attributed to this. Soot, on the other hand, leads most likely to a positive net effect, whereas organic aerosol takes countermeasures again. To make it even more complex, most particles consist of a mixing of particles. One more important uncertainty factor is the indirect effect of such particle formation. For sure, they counteract the greenhouse effect, but the order of magnitude is in question and varies between hardly measurable and nearly compensating the effect of the greenhouse gases. This indirect effect has a simple cause: aerosols not only absorb and reflect radiation themselves, but also serve as cloud condensation nuclei. More clouds, again, enhance the direct reflection of solar radiation into space, and reduce the net radiation and thus also the warming of the earth's surface. Up to now, it is not possible to integrate such locally very variable effects into global climate predictions. Direct, and, even more, indirect aerosol effects thus lead to a high uncertainty in the prediction of global warming.

d) Feedback

Many climate systems can be influenced by positive and negative feedbacks. Global warming, for example, would lead to stronger evaporation and thus to a rise of the average humidity. As water vapour itself is a greenhouse gas, this effect would seemingly be self-increasing. On the other hand a higher average humidity would lead to stronger cloud condensation'see the role of water vapour.