We real-world Earthlings take for granted that the seasons will change on schedule. Our planet's clockwork-like seasonality allows us to predict the passage of time with complete precision, and we can always be sure that spring is right around the corner. The same cannot be said, however, for the unlucky inhabitants of George R. R. Martin's Westeros.
Why is this? And what are the possible scientific explanations for Westeros' long, unpredictable seasons?
A unique feature of Martin's Song of Ice and Fire world is its extreme seasonal variability. Summers and winters have an indeterminate length, leaving its citizens wondering how long the current season will last — and how long they may have to endure the next one. At the opening of Game of Thrones Season Two, the good folk of the Seven Kingdoms learn that the summer, which has lasted seven years, is coming to a close, and with it, the onset of what could be a very long and bitter winter.
This makes for some pretty great fantasy, but is this actually possible? And is there any chance that variable-length seasons as portrayed in Game of Thrones could eventually happen on Earth?
The answer is yes. And in fact, there are at least five scientific explanations that can help explain what's going on in the Seven Kingdoms.
1. A wobbly planetary tilt
Earth's seasons are caused by the tilt of its axis of rotation - a 23.4° offset of the axis to be exact. The direction of the Earth's rotational axis stays nearly fixed in space despite the fact that we're also revolving around the Sun. As a result, depending on the Earth's location during its orbit, the northern hemisphere is tilted toward the sun, causing us to experience summer. Half a year later, when the Earth is on the opposite side of the Sun, the northern hemisphere is tilted away from the Sun, resulting in — yes, you guessed it — winter. The seasons are, of course, reversed for the southern hemisphere.
The seasons themselves are the result of shifting daylight exposures. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface. The less sunlight, the colder it is. Makes sense.
It's important to note that the Earth's axis of rotation is extremely stable. If it wasn't, the Earth's tilt would be very wobbly, resulting in inconsistent and unpredictable seasonal lengths like the ones portrayed in Game of Thrones.
But thankfully we have the Moon. Or more specifically, we have a very large moon. The Earth's moon is disproportionately large compared to other planetary satellites in the solar system. And without it, there might not be any seasons, or the seasons could be very different than what we're used to. The Moon has the effect of stabilizing the tilt of the Earth's rotational axis. Without it, Earth would be a wobbly mess.
Now, back to Game of Thrones — in the episode "The Kingsroad," we learn that Westeros has at least one moon. It's very possible, therefore, that they have a very small or distant moon, that is causing a variable tilt in their planet's rotational axis.
It's interesting to note that, according to legend, Westeros used to have two moons, but "one wandered too close to the sun and it cracked from the heat" pouring out a thousand thousand dragons. Well, dragons aside, it's conceivable that some kind of cataclysmic celestial event could have wiped out their second moon, which would have thrown their planet's rotational axis out of whack.
As for our situation here on Earth, we're not completely immune from this problem. If our moon got knocked out of its current orbit, say by a massive object or a nuclear explosion, we would be in quite a bit of trouble.
2. An extremely elongated orbit
It's a commonly held myth that the Earth's seasons are caused by its changing proximity to the Sun. This makes sense from an intuitive perspective; the Earth is in an elliptical orbit around the Sun, which would indicate that the further it is away from the sun, the colder it would be. Hence, Earth's location at the aphelion point (the farthest point from the Sun) would indicate winter.
But this is not the case. Earth may be in an elliptical orbit, but it's practically a circle. Our distance from the Sun at the aphelion point has virtually no impact on the Earth's climate, though some experts believe that it may account for the southern hemisphere's moderate winters.
Now, that said, not all planets have a near-circular orbit like the Earth's. Mercury, for example, has the largest orbital eccentricity of any planet in the solar system at 0.2056 (compared to the Earth's 0.0167). The closer to zero the orbit is, the closer it is to being circular.
In Game of Thrones, it's very possible that Westeros has a very eccentric or elongated orbit. Unlike the Earth, their world could be extremely far from its sun at the aphelion point, which would explain the long and severe winters. Conversely, during perihelion, the planet would have a prolonged summer. Our very own Mars experiences this kind of thing; it undergoes wide temperature variations and violent dust storms every year, when it reaches perihelion.
The problem with this theory, however, is it doesn't explain the unpredictability of the seasons. The citizens of the Seven Kingdoms would still experience consistent yearly cycles and fixed length seasons, even if they would be longer than what we're used to here on Earth. So this theory, at least on its own, is not a very good explanation.
3. A complex Milankovitch cycle
The Earth is subject to some significantly longer orbital and axial trends. Variations in orbital eccentricity, axial tilt, and precession of the Earth's orbit can determine climactic patterns that can take tens of thousands of years to play out. It takes about 26,000 years for Earth's axis to complete one full cycle of precession (the change in the orientation of the rotational axis of a rotating body), while at the same time the Earth is orbiting at a variable speed. The combined effect of these two phenomena creates a 21,000-year astronomical season. This is what is referred to as a "Milankovitch cycle."
This extreme long-term seasonality slowly changes the climate on Earth, typically resulting in colder winters in the Northern Hemisphere. It's thought that Milankovitch cycles are what's to blame for Earth's past ice ages. And anthropogenic global warming notwithstanding, current models suggest that the current warm climate may last another 50,000 years.
All planets have their own Milankovitch cycle which affect the weather and seasons in unique ways. Mars's polar caps vary in size on account of orbital instability related to a latent Milankovitch cycle. And Saturn's moon Titan has a 60,000 year cycle that changes the location of its methane lakes.
Looking at the situation in Game of Thrones, it's possible that Westeros's Milankovitch cycle is quick and complex. If this is the case, their seasons would be subject to variations in both length and severity — exactly the sort of thing that is seen in the series. Such long-term trends could be predicted when analyzing the physics of it, but it's nothing the planet's medieval-stage observers could measure or anticipate.
4. Oceans, currents, and winds
Any given region's climate is profoundly influenced by such factors as its latitude and proximity to large bodies of water. Take the South Pole, for example. It is in the middle of Antarctica, and a considerable distance from the moderating influence of the southern oceans. The North Pole, on the other hand, is in the Arctic Ocean, and its temperature extremes are buffered by the water. The result is that the South Pole is consistently colder during the Southern winter compared to the North Pole during the Northern winter.
Ocean currents and prevailing winds can also have an impact on climate, and they themselves are subject to cyclical variations. Currents like El Niño and La Niña impact on regional climates across timescales as long as five or more years. The power of Canada's warm Chinook winds are largely unpredictable, but their impact on the Prairies is significant.
The Seven Kingdoms may be subject to these sorts of long-term weather trends. The geography of their world may be considerably different than Earth's. Westeros may contain larger oceans, bigger mountains, stronger currents and more powerful prevailing winds — all of which would combine to create fairly unpredictable and long-term weather trends.
It's worth noting that global warming and rising ocean levels on Earth are stunting the ocean currents. Some experts believe that this could indeed result in a new ice age.
5. A combination of all factors
It's also possible, of course, that it's through a combination of some or all of these factors that Game of Thrones' seasonal variability can be explained. As shown, seasonality and climate are clearly the result of many factors.
Regardless, it's time to bundle up. Winter might be coming.