The Earth's weather cannot be considered in isolation. Thus far we have considered how the natural variability of the global climate could lead to quasi-cyclic changes over periods greater than a year. In so doing, it could be assumed that the Earth's tilt was constant and its orbit around the Sun was identical each year. Also it could be assumed that the energy output of the Sun is constant and that no other forces are present to perturb this orderly picture. None of this is correct. So we now have to consider how the natural autovariance of the climate may be affected by these external influences.

Three principal perturbations need to be addressed. First, there is the evidence that the Sun's output varies with time and that this is cyclic. Second, there are the tidal forces that act on the Earth due to the properties of the Moon's orbit and the more distant influence of the motion of the planets. Third, on the much longer timescale there are the periodic changes in the Earth's orbital parameters. The difference in timescale is important. Observations of periodicities in the Sun's behaviour only extend to a few hundred years, although they may well occur on longer timescales.