The Sun's output varies. The Sun is obviously the source of most of the energy we perceive and it has been argued that the well known sunspot cycle (approximately 11 years) produces variations in the earth's climate. Another theory concerns the Milankovitch cycle which is linked to the earth's orbit around the sun and has a period measured in tens of thousands of years. There are differences of scientific opinion but it seems that neither of these effects can directly explain the known changes which have taken place. Rather, it may be that the fluctuations affect the earth's atmosphere which in its turn magnifies the importance of the changes. In particular a favoured theory is that the amount of CO2 in the atmosphere is a major feature which, inter alia, changes temperatures at the surface.

The chemical composition of the atmosphere is such that the phenomenon we call the "Greenhouse Effect" takes place, and it is generally accepted that CO2 is a major constituent in this process (although many other chemicals are involved). The mechanism traps a proportion of the heat and the temperature rises until the escaping energy equals the incoming energy. The result is that the earth is warmer than it would be without the effect. Now as we say in the details on global warming elsewhere, this phenomenon is not intrinsically bad, on the contrary life as we know it would not exist without it. However the balance which has been reached over countless ages is critical and must not be significantly upset if the beneficial conditions are to be sustained. As CO2 is produced naturally, an equal amount is absorbed; its part of nature's life cycle. Nowadays, over and above this, man is releasing more CO2 and all the evidence indicates that as a consequence the results of increasing temperatures globally are measurable. As the EPA states [see link] humanity are adding about 3% or 4% to the naturally generated CO2 and this is enough to knock the system out of balance.

Volcanic eruptions have an immediate impact on the temperatures at the earth's surface. The fine particles which remain suspended in the atmosphere after an eruption circulate the globe and have the opposite effect to CO2, producing significant cooling. Such consequences are very rapid but are short lived and not considered to be responsible for long term changes in the climate. Nevertheless, even in the short term, the impact can be devastating for a few years after a major eruption and we have current experience of minor instances, and records obtained from tree rings indicate the possibility that much more serious catastrophes have occurred. It can only be surmise on our part but it seems probable that if a sizeable asteroid struck the earth's surface the resulting pollution would trigger identical mechanisms with a severity that could be much worse (maybe we've seen too many Sci-Fi films).

El Niño and La Niña are well documented phenomena which occur in the Pacific ocean. The warming and cooling effects are influential at great distances away causing changes in the world's wind patterns and resulting in uncharacteristic droughts or heavy rains. These consequences are serious on a relatively local scale but are short lived (about a year) and are approximately periodic every few years and so are not major features in determining the major climatic conditions globally over the long term.

In summary, despite the enormous levels of energy fed to, and radiated from the earth's surface it seems that the beneficial state of affairs is sustainable providing the composition of the atmosphere remains sensibly constant. Some of the factors which maintain or disturb the critical balance have been identified above. It may appear to be beyond us to draw conclusions but we have to be concerned at the thought of a really major volcanic eruption or, dare we say it, an impact from a significant meteor? No point in worrying about those because there's little we can do anyway. On the other hand we can genuinely concern ourselves about excessive production of the greenhouse gases which is entirely our responsibility and totally under our control.