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John Hollenhorst ReportingImagine a world so cold the oceans freeze and the entire Earth becomes a giant snowball. As astounding as it may sound, many scientists now believe that really happened, at least twice.
Tonight the leading advocate of the "Snowball Earth" theory is in town to deliver a lecture at the University of Utah.
Stephen Spielberg imagined a Manhattan of the future in his film "A.I". The entire Earth frozen, with ice hundreds of yards deep. Yes, it may happen because it did happen far in the past, according to a controversial theory being explored by geologist Paul Hoffman of Harvard.
Dr. Paul Hoffman, Professor of Geology, Harvard: "Yes, the oceans would have been completely covered in ice. And the thicknesses would have been about a kilometer in thickness, three quarters of a mile or so in the polar regions and down to a couple hundred yards thick at the equator."
Geologists once found such an idea almost impossible to swallow. But Hoffman says the evidence is accumulating, especially sedimentary deposits made by glaciers long ago.
Dr. Paul Hoffman, Professor of Geology, Harvard: "Six or seven hundred million years ago, you find it everywhere, widespread on every continent."
For some unknown reason the Earth's climate gradually changed. The global average temperature plunged to about 50 below zero, a runaway deep freeze as huge sheets of sea-ice spread from the North and South Poles, eventually covering the equator.
Hoffman believes it happened at least twice because of some unknown stress on the planet.
So now you have an 8,000 mile wide snowball. And it's so white it reflects away most of the sun's energy -- a permanent deep-freeze with no escape. Except that, according to the theory, something equally astounding happens.
Volcanoes under the ice and under the oceans begin spewing vast amounts of carbon dioxide -- a greenhouse gas.
Dr. Paul Hoffman, Professor of Geology, Harvard: "So it builds up, you get more and more of a greenhouse warming, eventually you start to melt the ice."
The runaway deep freeze becomes a runaway furnace. Hoffman says the possibility that the Earth's climate can swing wildly should give us pause as we spew greenhouse gases into the atmosphere.
Dr. Paul Hoffman, Professor of Geology, Harvard: "We're doing an experiment and we don't know what the implications are."
The snowball Earth theory is under attack by many geologists, making it one of the liveliest debates in modern science.
Doctor Hoffman's lecture on "Snowball Earth" is tonight at 7:30 at the University of Utah. It's in the Skaggs Biology Auditorium, just west of the University Bookstore.