How stable is the period of the multi-annual seasons?
I know this is deliberately kept vague by the author, but there has been much speculation that the climate has some magical elements, and also that it has something to do with the doom of Valyria.
A simple explanation for multi-annual seasons would be simply a planet with a longer orbit than Earth. For simple orbits, the period is determined only by distance from the sun (for instance Jupiter has a 12 year cycle) and this could easily explain the longer seasons if we assume SOIAF's sun is stronger to compensate for being farther away.
However, if the reason was astronomical, the summer/winter cycle would have be very regular. I know that the books mention that some winters are longer than others, but this is separate from periodicity. For instance, on Earth, a given place may have a winter that lasts 3 months one year and 4 months another, but this doesn't change that the peaks of winter will on average be 12 months apart.
On the other hand, if the seasons occur because of a magic spell or something like that, or perhaps some kind of geologic (eg. fluctuating heat output of the planet's core) or atmospheric phenomena (for instance like our ice ages that happen once every couple dozen hundreds of years, but much faster), they need not follow a very strict schedule.
Which version does SOIAF's world most closely correspond to? Is the summer/winter cycle on a stable period? If not, is the period changing unpredictably, or has it steadily increased or decreased over the years?
