Greenhouse gases absorb some of the short-wave radiation from the sun and also give off long-wave heat rays, in other words, infrared radiation. Like in a greenhouse, these heat rays can no longer escape the Earth's atmosphere completely. Some of the long-wave radiation from greenhouse gases is absorbed and and then reflected back towards the Earth. Through this process, low-lying levels of the Earth's atmosphere warm up. According to the current state of research, the surface of the Earth is heated up by 33°C as a result of this natural greenhouse effect, providing a life-sustaining temperature. The average temperature at ground level would be around -19°C without the natural greenhouse effect.
The part of the greenhouse effect that occurs because of the influence of human beings is known as the "anthropic" greenhouse effect. Since the beginning of industrialisation, the existence of long-lasting greenhouse gases has been increasing dramatically. The extent of the greenhouse effect depends negatively or positively on the concentration of greenhouse gases as well as on the reaction of the water cycle (vapour, clouds, precipitation, evaporation, snow cover, ocean expansion). Increased warming of the surface temperature can not only lead to changes in the water cycle and to extreme weather, but it can also result in global climate change.
Sources include: IPCC 2007 Glossar und Springer Gabler Verlag (Herausgeber), Gabler Wirtschaftslexikon