1,500 years ago, something happened to the world. Crops failed everywhere, there was massive starvation, and the spotty historical record shows frost conditions in the middle of the summer. Now two undergraduate students at Cardiff University, in the UK, think they've figured out why.
A half-kilometer-wide comet may have hit the Earth, exploding in the atmosphere, spreading soot and ash. This would have partially blocked the sun, reducing the amount of light and heat hitting the surface. Tree ring data shows a global reduction in temperatures for the years 536-540; the amount of material in the atmosphere required to cause that degree of temperature drop is nicely explained by a moderate-sized comet, big enough to cause problems but small enough to explode in the air rather than actually impact the ground.
If this theory is supported by subsequent research, it will be further evidence that disastrous comet/asteroid strikes are relatively commonplace in our history. A similar-sized hit would exacerbate global climate instability, and the loss of crops resulting from the temperature drop would leave millions starving. It's a wildcard, but nowhere near as unlikely as we'd like to believe. It wouldn't take a technological breakthrough to be able to watch out for dangerous spaceborne objects, just a willingness to do so.