The Earth’s climate isn’t just a product of what’s going on within its own atmosphere. Fluctuations in climate are also determined by forcing factors (causes) external to the climate system itself. One such cause is the variation in energy from throughout the galaxy that reaches the Earth.
Relative cosmic ray flux and tropical temperature changes over time in millions of years ago (Ma). Reconstruction of cosmic ray flux is based on iron meteorite record, and that of temperature changes is based on oxygen isotope ratios d18O. Blue band indicates the error range of the cosmic ray flux. After Shaviv and Veizer 2003.
Our sun lies in the Milky Way, a swirling spiral galaxy of 200 billion to 400 billion stars. Every 150 million to 350 million years, our solar system rotates around the Milky Way. The quantity and quality of energy reaching Earth from nearby star systems and from the gases and dust that pervade interstellar space undoubtedly varies during this rotation. The cyclical fluctuations in this energy are so long and so uncertain that they obscure the influence of galactic rotation on Earth’s climate. Nonetheless, several major climatic events, such as tropical temperature changes, the K-T boundary, the Great Dying, and the Ordovician-Silurian Extinction are separated by about 150 million to 190 million years and might be coincidental with the period of galactic rotation. 
 Scherer, K., H. Fichtner, T. Borrmann, J. Beer, L. Desorgher, E. Flukiger, H. J. Fahr, S. E. S. Ferreira, U. W. Langner, M. S. Potgieter, B. Heber, J. Masarik, N. J. Shaviv, and J. Veizer (2006) Interstellar-terrestrial relations: Variable cosmic environments, the dynamic heliosphere, and their imprints on terrestrial archives and climate. Space Science Reviews 127:327-+ doi:10.1007/s11214-006-9126-6.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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