The documentary, "Journey to 10,000 BC", aired again this week. It presents a theory that as the Earth began to come out of the last Ice Age it was hit by a massive meteorite strike in a heavily glaciated region in North America. This impact event blanketed contemporary Clovis (proto-Indian) sites with galactic carbon spherules encapsulating a fine nano-diamond dust, wiped out 80% of the large mammals of North America, and caused the entire Earth to become much colder for the next 1,000 years (a period known as the Younger Dryas).
After the Younger Dryas, the Earth began a fitful warming trend, punctuated by a continued bombardment by meteorite strikes (perhaps occurring on average about every 600 years) on both water and land for the following 10 millennium, and which respectively cooled and warmed the Earth's climate. It sounds like the career of a Navy carrier-based pilot - hours of boredom interspersed by moments of sheer terror.
Here's a related post I made at The Daily Grail this week on the subject of Global Warming:
© Charles N. Pope, US Library of Congress. All rights reserved.