Every "Game of Thrones" fan knows by now that winter is coming, and it could be a long one. They also know that flying dragons are a thing, particularly if they've watched the latest episode on HBO's sex-swords-and-sorcery series. But there's no way that a chilling winter could last for years, right?
Scientists say it actually happened on Earth 65 million years ago — and it didn't turn out too well for the flying pterosaurs, the closest real-life analog to dragons. However, an earlier deep freeze may have been one of the factors behind the emergence of complex life as we know it.
Much has been made of the strangely erratic seasons in Westeros, the fictional setting for "Game of Thrones." Although the years proceed with Earthlike regularity, the meteorological seasons are unpredictable in length, conceivably lasting several years at a time.
A couple of years ago, researchers proposed an explanation that was totally appropriate for April Fool's Day: The saga takes place on a circumbinary planet where numerical three-body dynamics render it impossible "to predict either the length, or the severity of any coming winter."
That's one way to explain it. The fictional maesters quoted in author George R.R. Martin's latest tome, "The World of Ice and Fire," ascribe the phenomenon to magic — and pretty much leave it at that.
But based on the geological evidence, scientists suggest that there were episodes in Earth's history marked by an extended chill: One was a period known as the Huronian glaciation, extending from 2.4 billion to 2.1 billion years ago. That deep freeze has been traced to declining methane levels and rising oxygen levels in the early Earth's atmosphere — a shift in the greenhouse effect that cleared the evolutionary path for oxygen-breathing organisms.
Another was the "Snowball Earth" period, which is thought to have started 715 million years ago and lasted about 120 million years. "There are no other comparable glacial periods on Earth. This one was really quite catastrophic," Graham Shields-Zhou, a geologist at University College London, told the BBC.
Snowball Earth could have been triggered by a combination of factors, including volcanic eruptions that created a "global cooling" effect, atmospheric shifts, changes in the sun's energy output and variations in Earth's orbit. Whatever the cause, the evidence suggests that Earth virtually froze over, with ice forming even in tropical seas.
That sounds like a bad thing, but several years ago, researchers proposed that undersea life thrived beneath the ice — setting the stage for the emergence of complex animals in what's known as the Cambrian Explosion.
Hundreds of millions of years later, evolution ushered in the age of the reptiles — with dinosaurs ruling the land, plesiosaurs and their ilk holding sway in the sea, and pterosaurs patrolling the skies. Then, 65 million years ago, the asteroid hit.
The thermonuclear-scale blast was powerful, but it didn't kill off all the dinosaurs immediately: Instead, scientists suggest that the impact threw up mountains of debris and sparked global wildfires. All that dust and soot in the atmosphere reduced the amount of sunlight reaching Earth's surface by as much as 80 percent — leading to an extended deep freeze known as the "impact winter."
Earth's dragons doomed
The casualties included the pterosaurs — which were the most dragonlike creatures in Earth's history (even though there's no evidence that they ever breathed fire). The smaller ancestors of today's birds fared better, as did small burrowing mammals and freshwater species.
When scientists added up all the evidence, they concluded that the survivors were species that could take refuge from the initial blast and lie low for as much as a year or two — then quickly reproduce and take advantage of the evolutionary niche left open by the disappearance of bigger species.
As a general rule, scientists say big species are more vulnerable to extinction events, particularly if they have a slow reproduction cycle. That would be bad news for Queen Daenerys' dragons, which were difficult to hatch and grew to monstrous size over the course of just four HBO seasons.
The species most likely to weather an extinction-level winter? The dire wolves might do OK, but the big winners would be the rats of Westeros. And the same scenario might well play out during a future extinction event in real life.
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