Once, Cold Weather Came And Stayed — For Years

It's been a bitterly cold winter in much of the country, but hey, it could be worse: You could have been alive in the 6th century. Starting in 536 A.D., scholars wrote of a cold snap that lasted not days, but years. Journalist Colin Barra speaks with NPR's Jacki Lyden about scientists' quest to determine what caused the epic cold spell.

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JACKI LYDEN, HOST:

All right. It's been a cold winter for much of this year - brutally cold, really - but think about it this way. It's only a few months and it could be worse - and has been. Take for example 536 A.D., when evidence suggests that the entire world entered a decade-long cold snap, replete with the bubonic plague and even temple burnings. With us now is Colin Barras. He wrote about that global chill for New Scientist magazine. And he joins us from WUOM in Ann Arbor, Michigan. Welcome, Colin.

COLIN BARRAS: Thanks. It's great to be here.

LYDEN: So, this is really an extraordinary piece. It's been known for some time that in the year A.D. 536 the sun had dimmed. Tell me about what scholars were describing then. Would you please?

BARRAS: To begin with, we heard accounts around the Mediterranean region of weather conditions going awry. But within a couple of years there were historical references to this major plague - an early form of bubonic plague - which was killing maybe 10,000 people a day, according to some of these accounts. We have accounts of empires in China losing maybe 75 percent of their people to famine, or something was killing these people anyway. Other accounts in Japan suggest things gotten very bad very suddenly. The mid-sixth century in general just really wasn't a great time to be alive.

LYDEN: So, these dire circumstances are happening in different parts of the world - Central America, China, Europe. And this had been known, as you say, from these records and scientists set up to look for these physical clues. How did they do that?

BARRAS: Yeah. Well, that's an interesting point, because with any historical account there's always the worry that perhaps people were maybe exaggerating things. So, what the scientists really needed was kind of an impartial witness just to double-check that things were as bad as they seemed. And they found that using a technique that involves looking at the rings in dead trees. And when you look at these tree rings from the mid-sixth century, you find that around this time the tree rings are really very narrowly spaced, which really does suggest that conditions were as bad as these early scholars were suggesting. And that's not just in one isolated area. Everywhere, they found this same pattern. So, this is worldwide prolonged winter period really when conditions were very cold and nothing was growing.

LYDEN: So, as they're carbon dating tree rings from around the world, they think, well, there has to be an explanation for this. And the likely candidate is what?

BARRAS: Well, there are two potential candidates really here. One obvious cause of environmental catastrophes of this kind of nature is major volcanic eruptions. We know that these eruptions can have that kind of effect. A very famous eruption in the early 19th century led to a year without a summer.

LYDEN: So, another possibility is Haley's Comet. Could you lay out the case for that explanation?

BARRAS: Right. Now, that's a really interesting one, 'cause obviously when you think of comet, the only comet that really everyone's heard of is Haley's, or Haley's ,Comet. And we know that the comet passed by Earth around the year 536 - just a few years previously. And luckily for us, the Chinese were making the very precise historical record. And they reported that Haley's Comet was quite a lot brighter than normal. And perhaps that reflects the fact that Haley's Comet may have been passing a bit closer to the sun during this passing through the inner solar system than normal. And it have done, it may have released more material. And if that material then rained down on Earth, then it's possible that that could be responsible for this cooling period. And as things stand right now, it looks suspiciously like both theories may actually be correct. Both of these camps have been pursuing their lines of inquiry now for a few years. And they seem to be coming up trumps in both spheres.

LYDEN: Colin Barras. His article about the decade-long cold spell following 536 A.D. is in New Scientist magazine. Thank you so much.

BARRAS: Thank you.

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