Plague outbreak blamed on comet strike

By STEVE CONNOR IN LONDON

A collision between Earth and a passing comet in the 6th century AD may have caused the collapse of agriculture, mass famine and, indirectly, the emergence of bubonic plague in Europe.

Welsh scientists have calculated that a relatively small comet, or a fragment of one, could have caused huge amounts of dust and debris to be ejected into the atmosphere that would have blocked out the sun for months.

The effect would have been crop failures and mass famine that could have allowed bubonic plague to spread more easily among a physically weakened population.

Studies of tree rings - from preserved oaks retrieved from Irish bogs and ancient American pine trees - have shown that plant growth around the world almost stopped completely between about AD536 and AD545.

Chinese historical records refer to a "dust veil" obscuring the skies and Mediterranean historians record a "dry fog" that blocked out much of the sun's heat for more than a year.

Scientists have suggested two possible causes for this sudden shift to winter-like conditions, both involving the ejection of dust or debris into the atmosphere to block out the sun and so preventing plant photosynthesis.

One idea is that a super-volcano erupted somewhere in the world, but neither the volcano nor its acidic deposits have been identified, says Derek Ward-Thompson, who carried out the latest study at Cardiff University.

The other proposal involves a collision with a passing asteroid or comet big enough to cause the effect, but again Ward-Thompson said there was no direct evidence for this in terms of a suitable impact crater.

However, he and his colleagues Mel Symonds and Emma Rigby believe that a much smaller comet which exploded in the atmosphere above ground without leaving a crater could easily have generated the dust and debris that led to the 6th century catastrophe.

"The surprising result of these calculations is just how small a comet fragment we have estimated was needed to cause the observed effects," Ward-Thompson said.

"A comet less than a kilometre in diameter has not been previously considered to represent a global hazard ... let alone one half a kilometre across."

Using information gathered from the impact of Comet Shoemaker-Levy with the planet Jupiter in 1994, the scientists have produced a new model of how comet fragments would behave if they collided with Earth.

"The comet plunges into the upper atmosphere leaving an effectively hollow tube behind it, where it has been, and into which the surrounding air has not yet had time to diffuse,"the scientists write in the jour-nal Astronomy and Physics.

"This tube then acts rather like a gun barrel, focusing much of the energy of the airburst explosion along the tube and carrying with it much of the comet debris."

As a result, the plume would have escaped the atmosphere completely and spread around the world in a massive fountain of debris that would have blocked out the sun for many months, causing widespread crop failures and famine around AD536.

"This period coincides with a mass population decrease in Europe. This is commonly known as the Justinian plague, and is believed to be the first appearance of the Black Death in Europe."

- INDEPENDENT