Catastrophe!

First part:
...Annual tree ring growth was discovered by the ancient Greeks, and as far back as Leanardo da Vinci, the connection between climate and ring growth has been known. Trees have always had the potential to become silent witnesses to thousands of years of climatic change....
...David Keys's years of detective work suggest to him that there is overwhelming evidence of a massive volcanic eruption around 535 AD. While some scientists believe there is not enough evidence to point the finger at Krakatoa - others feel Keys' hypothesis is not unreasonable.... (site)

Second part:
...535AD has come and gone- the world has been hit by a catastrophe.Now comes bizarre weather - the sun is darkened, skies are turbulent, rain is red and snow falls yellow.There is frost and famine.Seasons are blurred... (site)

Stories in the Ice (by Peter Tyson)
Nature's Time MachineHow would you like to have a time machine that could take you back anywhere over the past 300,000 years? You could see what the world was like when ice sheets a thousand feet thick blanketed Canada and northern Europe, or when the Indonesian volcano Toba blew its top in the largest volcanic eruption of the last half million years... (site)

timeline: http://www.pbs.org/wnet/secrets/flash/flash.html