Have you ever wondered how long the Sun has been giving us unfailing energy including light and heat? Well, it has been an energy soured for about five billion years and driving the weather system of the earth.
So the scientists have thought whether the Sun has an impact on these climatic changes and the results were that the Sun has been stable since the human evolution. However, increased emissions of Carbon Dioxide lead the Sun to have a powerful impact on climate change. Further, the radiation of ultraviolet affects the ozone layer. Ozone layer is the most important part in the atmosphere which works as a cover to our planet and Ozone layer is the place where numerous chemical reactions happen. Moreover, we all know that the functions of earth are controlled by the magnetic field of the Sun. Thus, protons and electron comes towards the earth through solar wind, which protect the whole solar system by standing like a protection from cosmic rays, the most energetic rays in the universe. In addition, the cosmic rays have an important part in cloudy atmosphere. 
Due to the arrival of cosmic rays in different amounts, the overall cloudiness of the earth differs. Consequently, the total radiation from the Sun to the surface of the earth is affected which causes global warming. Though these factors are long term alterations and are omitted from scientists’ consideration they have important influences in climate changes. Compared with green house gases, the Sun is more responsible for global warming which is more than 50%.
References: http://www.gcrio.org/CONSEQUENCES/winter96/sunclimate.html, http://www.aip.org/history/climate/solar.htm, http://pubs.usgs.gov/fs/fs-0095-00/fs-0095-00.pdf, http://www.windows.ucar.edu/tour/link=/earth/climate/cli_sun.html
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