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The research team had been studying evidence from tree rings, which suggests that the Earth underwent a series of very cold summers around 536-540 AD, indicating an effect rather like a nuclear winter. Historical references from this period, known as the Dark Ages , are sparse, but what records there are, tell of crop failures and summer frosts.
The scientists in the School of Physics and Astronomy believe this was caused by a comet hitting the earth and exploding in the upper atmosphere. The debris from this giant explosion was such that it enveloped the earth in soot and ash, blocking out the sunlight and causing the very cold weather. This effect is known as a plume and is similar to that seen when comet Shoemaker-Levy-9 hit Jupiter in 1995.
Studies of tree rings, from preserved oaks retrieved from Irish bogs to ancient American pine trees, have shown that plant growth around the world almost stopped between about 536AD to 545AD. Chinese records from this time refer to a "dust veil" obscuring the skies. Mediterranean historians record a "dry fog" that blocked out much of the sun's heat for more than a year.
Scientists have suggested two causes, both involving the ejection of dust or debris into the atmosphere to block the sun and so prevent photosynthesis. One idea is that a super-volcano erupted, but neither the volcano nor its acidic deposits have been identified, Derek Ward-Thompson, who carried out the latest study at Cardiff University, said. The other proposal involved a collision with a big asteroid or comet, but there was no direct evidence such as a crater.
The scientists calculate that a comet not much more than half a kilometre across could cause a global nuclear winter effect. This is significantly smaller than was previously thought. Dr. Ward-Thompson said: "One of the exciting aspects of this work is that we have re-classified the size of comet that represents a global threat. This work shows that even a comet of only half a kilometre in size could have global consequences. Previously nothing less than a kilometre across was counted as a global threat. If such an event happened again today, then once again a large fraction of the earth's population could face starvation."
The timing coincides with the Justinian Plague, widely believed to be the first appearance of the Black Death in Europe. It is possible that the plague was so rampant and took hold so quickly because the population was already weakened by starvation. As a result, the plume would have spread around the world in a massive fountain of debris. "This period coincides with a mass population decrease in Europe, commonly known as the Justinian plague, and is believed to be the first appearance of the Black Death in Europe," the scientists say.
More info: Washington Times
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