Continuing the ancient climate change theme, scientists think multiple comet impacts 1500 years ago plunged the world into darkness and caused worldwide famines.
Historical records tell of a "global dimming" in 536CE where a fog of dust blanketed the Earth cutting daylight, dropping temperatures and causing crops to fail. Until now the cause of this dimming was unknown though some speculated it could have been due to volcanic activity. A team from Columbia University's Lamont-Doherty Earth Observatory now think it was due to multiple comet impacts. Tiny balls of condensed rock vapour found in ice cores taken at Greenland date back to 536, this is the kind of debris thrown up by a major impact. The length of time the debris was laid down indicates there was more than one impact. Two possible craters have also been identified by the team led by Dallas Abbott.
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The alternative explanation is summarised at http://booty.org.uk/booty.weather/climate/500_750.htm and detailed in The Royal Meteorological Society Weather Log.
Work from Los Alamos (http://www.ees1.lanl.gov/Wohletz/Krakatau.htm) cites Krakatoa as a possible culprit citing ice core sulfuric acid spikes in 535CE. They do admit that impacts are also potential causes. As I understand it sulfuric acid production is not an impact event phenomenon, but volcanic.
There is, of course, nothing to stop there being closely spaced impact and volcanic events, but mutual causality is not implied.
yes some of the comments in the NS article pointed that out, of course comet hits are "sexier" and more likely to gain the headlines!
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