ScienceDaily (Apr. 1, 2010) — Some 13,000 years ago the Earth was struck by thousands of Tunguska-sized cometary fragments over the course of an hour, leading to a dramatic cooling of the planet, according to astronomer Professor Bill Napier of the Cardiff University Astrobiology Centre.
http://www.sciencedaily.com/releases/2010/04/100401101527.htm