The climate has been a persistent theme of Game of Thrones ever since Ned Stark (remember him?) told us "winter is coming" back at the start of season one. The Warden of the North was referring, of course, to the anticipated shift in Westerosi weather from a long summer to a brutal winter that can last for many years.
An unusual or changing climate is a big deal. George R R Martin's world bears many similarities to Medieval Europe, where changes to the climate influenced social and economic developments through impacts on water resources, crop development and the potential for famine.
We're interested in whether Westeros's climate science adds up, given what we've learned about how these things work here on Earth.
It's not easy to understand the mechanisms driving the climate system given we can't climb into the Game of Thrones universe and take measurements ourselves. It's hard enough to get an accurate picture of what's driving the world's climate even with many thousands of thermometers, buoys and satellite readings all plugging data into modern supercomputers -- a few old maesters communicating by raven are bound to struggle.
The fundamental difference between our world and that of Westeros is of course the presence of seasons. Here on Earth, seasons are caused by the planet orbiting around the sun, which constantly bombards us with sunlight. However the amount of sunlight received is not the same throughout the year.
If you imagine the Earth with a long pole through its centre (with the top and bottom of the pole essentially the North and South Pole) and then tilt that by 23.5 degrees, the amount of sunlight received in the Northern and Southern Hemispheres will change throughout the year as the Earth orbits the Sun.
Clearly the unnamed planet on which Game of Thrones is set is missing this axis tilt -- or some other crucial part of Earth's climate system.
How longer seasons might work
The simplest explanation could be linked to spatial fluctuations in solar radiation (sunlight) received at the surface. A reduction in incoming solar radiation would mean more snow and ice likely remaining on the ground during the summer in Westeros's far north. Compared to the more absorbent soil or rock, snow reflects more of the Sun's energy back out to space where in effect it cannot warm the Earth's surface. So more snow leads to a cooler planet, which means more snow cover on previously snow-free regions, and so on. This process is known as the snow albedo feedback.
The collapse of large ice sheets north of the Wall could also rapidly destabilise ocean circulation, reducing northward heat transport and leading to the encroachment of snow and ice southwards towards King's Landing.
To descend into glacial conditions would require a large decrease in solar radiation received at certain locations on the Earth's surface and likewise an increase would be needed to return to warmer conditions.
This is roughly what happened during the switches between "glacial" and "interglacial" (milder) conditions throughout the past million years on Earth. This is controlled primarily by different orbital configurations known as "Milankovitch cycles", which affect the seasonality and location of sunlight received on Earth.
However, these cycles are on the order of 23,000 to 100,000 years, whereas Game of Thrones seemingly has much shorter cycles of a decade or less.
When winter came back
Around 12,900 years ago there was a much more abrupt climate shift, known as the Younger Dryas, when a spell of near-glacial conditions interrupted a period of gradual rewarming after the last ice age peaked 21,000 years ago. The sudden thawing at the end of this cold spell happened in a matter of decades -- a blink of an eye in geological terms -- and led to the warm, interglacial conditions we still have today.
Various different theories have tried to explain why this spike occurred, including the sudden injection of freshwater into the North Atlantic from the outburst of North American glacial lakes, in response to the deglaciation, which destabilised ocean circulation by freshening the water and reducing ocean heat transport to the North Atlantic Ocean, cooling the regional climate.
Less likely explanations include shifts in the jet stream, volcanic eruptions blocking out the sun, or even an asteroid impact.
The shift from the Medieval Warm Period to the Little Ice Age that began around 1300 AD represents a more recent, and more subtle, example of a "quick" climate change. Although the overall temperature change wasn't too severe -- a Northern Hemisphere decrease of around 1˚C compared with today -- it was enough to cause much harsher winters in Northern Europe.
None of these events indicate the abrupt transitions from long summers to long winters as described in Game of Thrones -- and they still all happen on a much longer timescale than a Westeros winter. However they do demonstrate how extreme climate shifts are possible even on geologically short timescales.
Regardless of the causes of the long and erratic seasons, winter in Westeros won't be much fun. It may even make the struggle for the Iron Throne between the various factions seem irrelevant.
Indeed the House of Stark's motto: "winter is coming" may have a lesson for us here on Earth. Anthropogenic climate change is one of the biggest challenges facing humankind today and if left unmitigated the potential environmental impact on society may be far greater than any global recession. Stop worrying about the Iron Throne, everyone, winter is coming.