Brace Yourselves, Hyperactive Albedo Is Coming
A lot of worldbuilders who watch Game of Thrones or read the Song of Ice and Fire novels (and I fall into the first two pastimes) have pet theories about why the weather in Westeros, or moreover its climate, is so insanely erratic. The easiest reason is “because magic”, and that’s perfectly acceptable in its way – it is, after all, a work of fiction, written by an author whose worldbuilding is occasionally spotty insofar as, say, language or culture is concerned (“Crap, of the two songs in Westeros, that’s the more murderous one”).
Fantasy readers arguably shouldn’t hold George R.R. Martin up to the standards of Tolkien, whose interest in worldbuilding was more of a fixation – verging on the pathological, given his somewhat muted concern for characterization and plot. But I’m basically like Tolkien in this way, and I fixate on these things on Martin’s behalf, as do others. So, climate and geomorphology wank ahead.
Most non-magical theories about Westeros’ climate focus on astronomical factors. The most popular is probably orbital and axial eccentricity, which seems to make sense, but even most of its proponents admit this would probably stunt water and life from settling on Westeros at all. Also, this doesn’t normally account for why there seems to be an established period for a year in Westeros and Essos, which doesn’t correlate (at least not in Westeros) to the length of seasons, which so often last longer than a year. The axial theory gets a little closer, but that would seem to make seasons more or less extreme while not really adequately explaining why they might be longer without getting into tidal locking, which would make this place about as hospitable as the Moon.
But there’s some lower-hanging fruit that Occam would suggest we pick. Most fans of the series and show are familiar at least somewhat with the map of Westeros, trailing into the frigid, dangerous North:
Notice that we never actually see the North Pole on this map. Given the climate and various other threats, it’s probably unreasonable to assume that the humans of this universe that we know of, even
wildlings the Free First Men Beyond The Wall, would have the inclination or ability to reach it. We don’t even know how large the planet is on which Westeros is situated. But we do know of parts of the North are frozen and glaciated even in summer.
It’s possible that the continent of Westeros extends much further north than this. If it extended to and beyond the North Pole, these regions would be a permanent, landlocked ice cap. Landlocked ice caps tend to stay much colder than our present Arctic; we know on account of this that the Antarctic Peninsula would otherwise be a lot more like Norway or Iceland or Sweden than it currently is. Imagining this circumstance on Westeros would be as if Antarctica extended its peninsula halfway up what would otherwise be South America; assuming your normal west-coast climate regime, the Dornish mediterranean climate zone would be roughly where Santiago is and the maritime-oceanic Reach and (possibly) humid-subtropical Kings Landing would be situated roughly where we would expect Buenos Aires to be.
The bigger question, then, is not why Westeros’ winters are so long but why they ever end at all. This, I think, would need to have a partly astronomical explanation, but it’s not too difficult to imagine that a sun with only slightly more erratic cycles (in addition to volcanism, which we know is powerful enough on Essos to rip subcontinents apart) would cause enough insolation variance to send glaciation cycles all over the map regardless of actual seasonality. My theory is that their axial tilt is basically nil, and they don’t really experience seasons at all, but extremely short ice ages and warm periods, in much shorter bursts than their medieval equivalents.
To add a little grist to this conjecture, you often hear and read a lot about light and darkness, but not of actual days getting longer and shorter; but I’m more familiar with the TV show than the novels anyway, so take that with a grain of salt. This is all, of course, trivial bullshit anyway, but it’s fun for me to think about.