The Center for Sun-Climate Research at the DNSC (Danish National Space Center) investigates the connection between variations in the intensity of cosmic rays and climatic changes on Earth. This field of research has been given the name 'cosmoclimatology'"..."Cosmic ray intensities – and therefore cloudiness – keep changing because the Sun's magnetic field varies in its ability to repel cosmic rays coming from the Galaxy, before they can reach the Earth." :
http://www.spacecenter.dk/research/sun-climate
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Here's an excellent ~new book out on the subject titled "The Chilling Stars, 2nd Edition: A Cosmic View of Climate Change". It's written by one of the top scientists advancing the theory (Henrik Svensmark, of the above mentioned Danish National Space Center/DNSC).
"The authors explain their theory that sub-atomic particles from exploded stars have more effect on the climate than manmade CO2."
http://www.sciencedaily.com/books/t/1840468157-the_chilling_stars_the_new_theory_of_climate_change.htm
Amazon Books: "The Chilling Stars, 2nd Edition: A Cosmic View of Climate Change"
http://www.amazon.com/Chilling-Stars-2nd-Cosmic-Climate/dp/1840468661 zs90]
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Here's a sunspot chart showing the various peaks and dips throughout the past centuries. Note the "Medieval Max". It corresponds to the "Medieval Warm Period". The "Maunder Minimum" corresponds to the "Little Ice Age", and the "Modern Max" to the recent warming trend we had been experiencing, at least until the past 15 years or so.
"Changes in carbon-14 concentration in the Earth's atmosphere, which serves as a long term proxy of solar activity. Note the present day is on the left-hand side of this figure."