These theories are based on the premise that the output of solar energy is subject to a considerable variation. They state that terrestrial climatic fluctuations occur in accordance with variations in the absolute quantity of radiant energy emitted by the sun.

One of the most important theories in this group is what is called the variable sun theory. This theory of climatic change is based on the idea that the sun is a variable star and its radiant energy undergoes changes through time.

It holds that increased output of solar radiation leads to the warming of the atmosphere, whereas its decline would result in reduction of the atmospheric temperature.

Even though satellite technology has made it possible to measure the solar output of energy, no use of this sophisticated technology has yet been made. At present scientists have to place their reliance on ground-based estimates of the radiant energy from the sun.

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Sunspots:

Sunspots are the most conspicuous features on the solar surface which appear as dark spots. The sunspots are simply the huge magnetic storms that occur on the sun’s surface. There is a close relationship between the solar activity and the number of sunspots.

The number of sunspots increases and decreases in a cyclic manner. It may be pointed out that sunspots are a sign of increased solar activity. According to Huntington and Visher, the sunspot maxima coincide with the periods of increased storminess, greater precipitation, and hence ice accumulation.

From various lines of evidence it appears probable that historic pulsations and glacial fluctuations are nothing more than sunspot cycles on a large scale. The sunspot cycle averages 11 years in length, and has been called the 11-year cycle. There have been 23-year cycles or even cycles of greater lengths also.

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Since the climatic conditions are primarily controlled by the sun, any appreciable change in the radiant solar energy would cause climatic variations.

Since sunspots are a clear indication of increased solar activity, observations made during the last century show that the number of sunspots and the amount of rainfall and air temperature are correlated.

There is circumstantial evidence to prove that over the past 1000 years small advances of valley glaciers in the mountain regions are correlated with changes in the solar activity.

Scientists are of the opinion that unless we are able to calculate how the intensity of solar radiation varies with time, the idea that ice ages are caused by solar variations is neither proved nor disproved.

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Variations in solar constant:

A variation in the total output of thermal energy by the sun seems to be one of the most significant causes of the world-wide climatic change. Since the atmosphere contains numerous self-regulating mechanisms, it is not necessary that a minor increase or decrease in solar radiation would cause a general rise or fall in global temperatures.