Did large comet explosion trigger earth's cooling?

Some 13,000 years ago, thousands of Tunguska-sized cometary fragments struck the earth for over an hour, leading to a dramatic cooling of the planet, says a new study.
The cooling, by as much as eight degrees Celsius, interrupted the warming which was occurring at the end of the last ice age and caused glaciers to readvance.

The Tunguska explosion, with the destructive power of 1,000 Hiroshima type atomic bombs, knocked over an estimated 80 million trees over 2,150 square km, not far from the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai in Russia, on June 30, 1908.

'A large comet has been disintegrating in the near-Earth environment for the past 20,000 to 30,000 years, and running into thousands of fragments from this comet is a much more likely event than a single large collision,' said Bill Napier, professor at the Cardiff University Astrobiology Centre, who conducted the study.

Evidence has been found that this catastrophic change was associated with some extraordinary extraterrestrial event.

These findings suggest that the catastrophic changes were caused by the impact of an asteroid or comet four km across on the Laurentide ice sheet, which at that time covered what would become Canada and the northern part of the US.

The cooling lasted over a thousand years, and its onset coincides with the rapid extinction of 35 genera of North American mammals, as well as the disruption of the Palaeoindian culture.

The chief objection to the idea of a big impact is that the odds against the earth being struck by an asteroid this large only 13,000 years ago are a thousand to one.

Napier has now come up with an astronomical model which accounts for the major features of the catastrophe without involving such an improbable event.

The new model indicates that such an encounter would last for about an hour during which thousands of impacts would take place over continental dimensions, said a Cardiff release.

Each of these impacts released the energy of a megaton-class nuclear bomb, generating the extensive wildfires which took place at that time.

The new model has been presented in Notices of the Royal Astronomical Society.