Ice Ages and Spiral Arms

New research suggests that ice age epochs on the earth may result from our solar system's trek through the spiral arms of the Milky Way. Nir Shaviv (shaviv@phys.huji.ac.il, +972-54-738555), of the University of Toronto and Jerusalem's Hebrew University bases this hypothesis on correlations he has found between apparent changes in the flux of cosmic rays reaching the earth and geological evidence for major ice ages in the past billion years. Galactic spiral arms are not permanent, rigid fixtures; rather they are transient and result from density ripples traveling around the galaxy. Many massive stars form in the wake of the density waves and later explode as supernovae, which are a primary source of cosmic rays. It seems reasonable to expect our planet to receive more cosmic rays when it is near the supernovae in a major spiral arm. If there is a connection between cosmic ray flux and climate (see Update 401), past ice ages should correlate with the solar system's location relative to the traveling spiral arms.

One of the challenges in making the climatic connection is finding records of cosmic ray flux over past eons. Shaviv deduced the earth's exposure to cosmic rays by considering the cosmic ray exposure of 42 iron meteorites. The meteorite record seems to indicate that the cosmic ray flux varies with a period of about 143 million years, which correlates well with both the geological records of ice age epochs and the solar system's location relative to the spiral arms. Our current position in the minor Orion spiral arm should lead to cosmic ray fluxes about half of what we would receive in a major spiral arm. Shaviv's model places us in the wake of a major ice age epoch, and is consistent with the global temperatures that we are now experiencing. Shaviv points out that the weakest link in his proposal is uncertainties in the extent and timing of glacial periods indicative of ice age epochs, and that further geological research is necessary to confirm that galactic spiral arms affect our climate. (Nir J. Shaviv, Physical Review Letters, 29 July 2002.)