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Bill  Bryson - A Short History of Nearly Everything

Croll was the first to suggest that cyclical changes in the shape of the Earth's orbit, from elliptical to nearly circular to elliptical again might explain the onset and retreat of ice ages.  No one had ever thought before to consider an astronomical explanation for variations in the Earth's weather ..........  Milankovitch went on to prove the relationship and Vladimir Koppen decided that it was the cool summers which started the process as lasting snow reflects sunlight exacerbating the cooling effect....................

.. a fall in solar radiation by about 6% and a drop off in the production and retention of greenhouse gases is said to have caused one of our most complete ice ages, where temperatures plunged by as much as 45 degrees centigrade.  The entire surface of the planet may have been frozen solid...

..............one key purpose of the magnetic field is to divert cosmic rays away from the surface of the earth into 2 zones called the Van Allen belts.  Without this protection, the cosmic rays would tear into life forms on earth and destroy many of them.....

Did a gamma-ray burst initiate the late Ordovician mass extinction? - Brian C. Thomas, A.L. Melott, B.S. Lieberman, C.M. Laird, L.D. Martin, M.V. Medvedev, J.K. Cannizzo, N. Gehrels, & C.H. Jackman

1. University of Kansas, Lawrence, KS 66045 USA

2. NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA

At least five times in the history of life, the Earth has experienced mass extinctions that eliminated a large percentage of the biota. Many possible causes have been documented, but gamma-ray bursts (GRB) could have been a contributory factor.  ……………………..

The late Ordovician is one of the largest mass extinctions in terms of its scale and scope. It appears to comprise two large, abrupt extinction events, separated by 0.5-2 million years, and all major marine invertebrate groups show high rates of extinction during this interval. The late Ordovician is unusual in that many groups like the trilobites, important Ordovician animal groups in terms of their relative abundance, diversity, and geographic range, go extinct while the more restricted taxa persist.  This is counterintuitive because one might predict that (due to stochastic factors) widespread, more abundant groups should be more extinction resistant

Long-term effects of GRB would spread around the Earth and include ozone layer depletion, global cooling, acid rain, and radionuclide production. …………  Nitric acid exceeding anthropogenic levels is a probable product. Global cooling is expected from the absorption of visible light by NO2. Substantial ozone depletion will result from NOx constituents, which catalyse conversion of ozone to oxygen molecules. Ozone absorbs biologically damaging solar UV radiation before it reaches the surface, so its depletion may have strong consequences.  A GRB may have paradoxically produced darkened skies and heightened UV radiation.

Modest increases in UV flux around 300 nm, can be lethal to a variety of organisms including the phytoplankton which are the basis for the marine food chain as well as oxygen production.  UV is attenuated by water, though the precise absorption is heavily dependent upon particulates and dissolved organics. Penetration depths vary from meters to tens of meters.  As would be expected, UV effects on microorganisms have been found to decrease with water depth.  ………During the late Ordovician, species dwelling in shallow water were more likely to go extinct than species dwelling in deeper water

This extinction has been related to alternating global cooling and warming correlated with the two pulses of the late Ordovician mass extinction.  We do not dispute the role global cooling may have played in mediating this extinction. Instead, we emphasize that there may be a link between GRB and global cooling. GRB produce atmospheric nitrogen dioxide.  Its opacity provides a natural mechanism to initiate global cooling. Climate models of the Ordovician show that it is difficult to initiate glaciation without a forcing impulse, such as a period of reduced sunlight.