The sun not only varies in brightness, it also has a major impact on how much cosmic radiation reaches the earth. Between the two, it is the primary cause of change in our climate.1 This article focuses on a series of mud cores from Canada, but also has a number of references to scholarly journals.
Mr. R. Timothy Patterson. "Read the sunspots" National Post. 2007-06-21. http://www.canada.com/nationalpost/financialpost/comment/story.html?id=597d0677-2a05-47b4-b34f-b84068db11f4&p=4 ↩