What is nuclear winter? 
   Carl Sagan, and other authors introduced the idea of "Nuclear Winter" in a 1983 scientific journal article (Science, Dec. 1983, pg. 1283). In the theory of Nuclear Winter, after the explosions of a nuclear exchange have died down - the spread of smoke, soot and debris in the atmosphere from nuclear-started fires, could absorb sunlight, darken the sky and ultimately lower the temperature of the Earth from 1 to 5 degrees Centigrade within a few months, which could unbalance the ecosystem and affect the survival of many species on Earth, including the extinction of mankind.

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   The idea of the effects of too much smoke in the sky are not just an idle theory either - something similar may have already happened in the Earth's past, at the end of the Cretaceous period some 65 million years ago.
   In 1979, a researcher named Walter Alvarez was sifting through sediments from Gubbio, Italy when he discovered a surplus of a radioactive element - known as iridium - in sediments dating back to the boundary between the Cretaceous and Tertiary periods (called the K-T boundary). This iridium did not have an easy terrestrial explanation.
   Alvarez's research gave support to an already proposed asteroid theory of dinosaur extinction that figures in many modern sci-fi films. This theory suggests that a large meteorite hit the Earth around 65 million years ago, causing fires. This meteor would explain the iridium deposits and the fires would explain a surplus of carbon that has also been discovered at the K-T boundary layer. Scientists know that iridium is much more common in asteroids and meteors than it is on Earth.
   Other researchers studying carbon deposits in sedimentary layers have documented a period in Earths past when ancient wildfires were widespread. Did these ancient wildfires lead to the extinction of the Dinosaurs? Fossils in the sediments in the K-T boundary also show a strange disappearance of about 50 percent of the genera in this period of time.
   These ancient fires may provide evidence from Earth's past that give us an idea of how a nuclear war climate might affect the climate. It would be hard to prepare for the striking of an asteroid, though in Hollywood films mankind always gets together to defeat even such a random event.
   And no experimentation on Nuclear Winter has been done, and hopefully won't be done.
         
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