As possible explanations of the causes of climatic changes, a wide variety of theories have been advanced during the last century. There are theories which hold that the causes of the climatic variation are to be found outside the earth-atmosphere system.
In other words, there are extraterrestrial factors which bring about climatic changes. There are still other types of theories according to which the causes are to be sought in the earth itself. These are called the terrestrial causes.
There are also theories which are related primarily to man-made climatic changes. These theories examine the effect of increasing carbon dioxide content caused by combustion processes, and also the influence of the rising level of dust in the atmosphere because of human activities.