Many of you are probably familiar with the show Game of Thrones on HBO based on the A Song of Ice and Fire novels by George R. R. Martin. To me, one of the most interesting aspects of the world that Martin creates is the seasonal cycle. For those of you unfamiliar, basically seasons can last many years at a time before changing. I have not been able to find exact numbers referenced, but it is implied that a season typically lasts around ten years. However, in the mythology of the world, some winters last much longer than that, though that only happens every thousand years or so. I found these cycles interesting and wanted to know if such cycles might have a scientific basis. Read on to find out what kind of explanation I came up with.
When I was trying to think of physical mechanisms that may trigger such prolonged seasons, it is easy to rule out the same mechanisms that drive the seasons on Earth. Our seasons are driven by the Earth’s rotation around the sun combined with the tilt of the Earth’s axis. The Earth spins like a top around its axis, but that axis is tilted about 23 degrees. This means that different parts of the Earth are in direct sunlight during different parts of the year, creating our seasons. The fact that the world in A Song of Ice and Fire does not have a yearly seasonal cycle means that the axial tilt of the planet on which the story takes place must be practically zero. We can also infer that the orbit of the fictional planet must be very nearly circular, this would eliminate yearly climate variations due to the change in distance from the planet’s sun.
Ruling out these mechanisms for the prolonged seasons in the fictional world, we must come up with another climate driving force that operates on a longer time scale. One possible candidate that we hear about in North America quite often is the El Niño/La Niña-Southern Oscillation. This weather cycle operates on a roughly five year cycle and causes periodic warming and cooling around the Pacific Ocean. The effect does not follow a very fixed cycle and can vary in frequency quite a lot. The following plot shades years that experience El Niño red and the years that experience La Niña blue. Typically, El Niño causes higher temperatures for North America and La Niña causes cooler temperatures.
The wide variation in the frequency this cycle occurs could account for what seems to be a variation in the changing of the seasons in the fictional world. Characters do not seem to be able to predict when the seasons will change which would indicate it does not happen at regular intervals. However, the temperature differences caused by this cycle are quite small. The specific geography of the fictional planet would have to do two things: promote a longer time scale for the cycle and make the cycle a much more significant impact on the climate than it does on Earth. Speculating on what kind of geography would be necessary to bring about such an El Niño cycle is not very productive however because the causes of this cycle are currently unknown.
Another possible route through which such long term climate cycles may be obtained is the solar cycle. This cycle takes about 11 years to complete and causes an increase in sunspots and then a decrease. Rinse, repeat for billions of years and you have the solar cycle. However, there are other interesting features of the solar cycle including prolonged minima or maxima that occur on a much longer timescale than the 11 year regular cycle. The most famous example of this is the Maunder Minimum which was observed in most of the late 1600′s (the Sun has been continuously observed for hundreds of years because it does not take a very powerful telescope or lenses to do so).
While the actual amount of light that the sun produces during minima in the solar cycle is almost the same as during the maxima (a variation of only about 0.1%) there is a much bigger difference depending on the wavelength of light. Specifically, the ultraviolet radiation can increase during a maximum by as much as 4-5% and over longer time spans by even more. That may not sound like much, but when you also take into account fluctuations in the Ozone layer, the amount of ultraviolet radiation reaching the surface of the Earth can vary by as much as 400% (out of phase with the solar cycle). This huge variation in ultraviolet radiation does not make that much of a difference on Earth’s climate because the tilted axis dominates the seasonal cycle and tends to “wash out” longer term variations such as the solar cycle. So, in what ways could this variation lead to such dramatic climate change in our fictional world?
To get dramatic climate change, one would need a dramatic event of some kind. In Earth’s past that has included meteorites, volcanoes, and most recently, greenhouse gases. The first two methods change climate by blocking incoming solar radiation through physical layers of clouds, ash, and debris. This causes the average temperature of the globe to drop. Greenhouse gases work in the exact opposite way, they let solar radiation through to the Earth and then don’t let it back out into space. Trapping this solar radiation increases the temperature of the Earth. I think it is possible that the solar cycle on our fictional planet could drastically affect levels of greenhouse gases.
The way in which ultraviolet light and greenhouse gases may be connected is through vegetation. We are all familiar with the idea that plants consume carbon dioxide and release oxygen and that carbon dioxide is an effective greenhouse gas. What many people are not familiar with is how plants have evolved to utilize ultraviolet light. Flowering plants routinely use ultraviolet light to their advantage to attract pollinating insects (insects can see ultraviolet light).
If plants on Earth have evolved to utilize ultraviolet in a way to flourish given its environment, then why couldn’t plants utilize ultraviolet light on our fictional world. Specifically, what if some species of plant have evolved to use ultraviolet light for photosynthesis rather than the primarily blue light that is used by green plants on Earth. Specifically, what if a species of marine algae on our fictional planet has evolved to utilize ultraviolet light for photosynthesis? During the parts of the solar cycle where there are increased amounts of ultraviolet radiation, there would be massive algal blooms in the oceans of our fictional world. This happens all the time with different species of algae on Earth, but in order to make a difference on our fictional world, the bloom would need to be massive. This dramatic increase in photosynthetic activity would drastically reduce the amount of carbon dioxide in the atmosphere as it is converted into oxygen. This drop in greenhouse gases would correspond with a drop in global temperatures and thus, winter. These solar cycle driven seasons would provide seasons would provide the characteristics that are present in the fictional world. They would occur roughly every 11 years and on a much longer time scale, there would be great extended seasons. So, when the Stark family say that winter is coming, they might just mean that the solar maximum is coming.