Open access e-Journal Earth Science India, eISSN: 0974 – 8350 Vol. 7 (I), January, 2014, pp. 15-36 http://www.earthscienceindia.info/ 15 Solar Variability, Galactic Cosmic Rays and Climate: A Review A. K. Singh 1 , R. P. Singh 2 and Devendraa Siingh 3 1 Physics Department, University of Lucknow, Lucknow – 226007, India 2 Physics Department, Banaras Hindu University, Varanasi – 221005, India 3 Indian Institute of Tropical Meteorology, Pune-411008, India Email: aksphys@gmail.com Abstract Climate is among the most important natural resources for all living beings. The main external and internal driver for climate change is variability of the Sun. The Sun is the ultimate source of the energy on the Earth and play a key role in its climate change. What is still not known and needs to be found is the extent to which these variations in solar behavior affect the total energy that the Sun emits. The small variation in the solar irradiance and changes in its spectral distribution leads to a large variation in climate. An attempt has been made to report the present understanding of the solar variability and its consequences on the variability of solar irradiance which exhibits 11-year cyclic variation and modulation on higher time scale. Galactic Cosmic Rays (GCR) reaching the Earth’s atmosphere is modulated by the solar wind and geomagnetic field, which also exhibit 11-year cyclic variation. In the present paper, we have reviewed our current understanding on physics of solar variability, galactic cosmic rays and their effects on the Earth’s climate. Keywords: Solar variability; Solar activity; Total solar irradiance; Galactic cosmic rays; Earth’s climate Introduction Solar variability, cosmic rays and their effects on the Earth’s climate is very common issue to talk these days but their understanding of quantitative and qualitative impacts is still very complex and unexplained phenomena (Reid, 2000; Gray et al., 2010; Usoskin, 2012). The solar variability is caused by solar evolution drive by conditions within the Sun and the magnetic field evolution in the solar convection zone. Various phenomena observed at and above the solar surface such as small flux tube, sunspots, active region, coronal mass ejections and flares, manifest themselves in violent fluctuations in the radiation of ultraviolet (UV) emission lines, the spectral distribution, the total power of broadband electromagnetic emissions as well as in the solar wind and energetic particle fluxes (Frohlich, 2006). The electromagnetic emission of the Sun, therefore, also increases and decreases in a cycle that matches the 11-year cycle of the sunspot count. Surprisingly, visible light emissions from the Sun hardly vary over the course of this cycle; the Sun brightens and dims in the visible light region of the spectrum by only about 0.1% (Wilson and Hudson, 1991; Weart, 2006). This corresponds to a change in solar insolation at Earth of 1 or 2 W/m 2 out of the 1,368 W/m 2 that reach the top of Earth's atmosphere (Willson and Mordinov, 2003).