"The team has 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.
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. vThis effect is known as a plume and is similar to that which was seen when comet Shoemaker-Levy-9 hit Jupiter in 1995.
Historical references from this period - known as the Dark Ages - are sparse, but what records there are, tell of crop failures and summer frosts."