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If you've ever harbored a distrust of comets, consider yourself vindicated. Nearly 1500 years ago, a piece of the Halley's comet is believed to have hit the Earth, setting off a decade-long chain of events that coincides with a plague, thought to be the first appearance of the bubonic plague, that rippled through the Byzantine Empire. It's a convoluted series of steps, but let's just go ahead and kick up the hyperbole a bit: A comet once tried to kill us all.
New research presented at the American Geophysical Union's annual meeting supports the hypothesis, and is the subject of a nice read from Mike Wall at LiveScience. To paraphrase, the specific chain of events looks something like this: Around 536 AD, a piece of a comet is presumed to have hit the Earth, covering the planet with atmospheric dust—a finding supported by new Greenland ice cores showing high levels of dust during that period, not all of which was from Earth—and causing global temperatures to drop. Drought and famine followed in the following decade, which is believed to have led to the outbreak of Justinian's plague that rocked the Byzantine Empire.
The cooling event that started in the year 536 is pretty well documented, and along with the comet scenario, it's been posited that such an explosion of atmospheric dust could have been produced by the eruption of a super-volcano. But a 2004 paper in Astronomy & Geophysics points out that, among other things, "no terrestrial volcano can be satisfactorily identified with this event." That paper argues instead that a comet exploding in Earth's atmosphere could have ignited massive forest fires; between the comet dust and fires, there could have been enough atmospheric dust emitted to lead to global cooling.
As the authors write, "The surprising result of these calculations is just how small a comet fragment we have estimated was needed to cause the observed effects. A comet less than a kilometre in diameter has not been previously considered to represent a global hazard (as opposed to a local hazard), let alone one half a kilometre across."
The presence of extraterrestrial dust in Greenland ice cores, analyzed in the recent study led by Dallas Abbott of Columbia University, supports the comet theory pretty handily. But how did such global cooling lead to the Justinian plague, which is thought to have killed about half of Europe in about a year's time?
Thanks to comet-caused drought and cold, Europe's agriculture collapsed, leaving the populace weakened. According Nicholas Wade's great history of the black plague in the New York Times, the Justinian plague was likely caused by an outbreak of the bubonic plague carried by fleas on ships of grain from Egypt.
Combine a weak populace with needed imports of grain supplies tainted by none other than Black Death, and you've got a recipe for widespread collapse. Thankfully, the plague killed so quickly that many people were spared, but losing half of Europe is no joke.
And to think, a piece of a comet likely started it all, which is a good reason why we should be watching for them (and asteroids too). As the Astronomy & Geophysics study authors write, "If such an event happened today, and crops failed over a significant part of the globe for several consecutive years, then once again a large percentage of the world's population would face starvation."