Over 60 theories have been proposed to explain the Ice Age, all with serious difficulties. The most popular ice age theory maintains the earth has gone through 15-30 ice ages, driven by Milankovitch cycles (major peaks in the high-latitude summer insolation). However, such cycles have more "misses" than "hits," with the "hits" showing stimulus to warming 4,000 to 7,000 years after other records show results of warming in sea level variations. This leads to the conclusion "Pleistocene climate phenomena are aperiodic and therefore that their timing is probably unpredictable."
Not surprisingly, estimates of the number of ice ages the earth has experienced vary with the choice of theory. The Alpine Model, based upon research done in the Swiss Alps, claiming four ice ages, was the predominant model for over 60 years, during which time all data throughout the world supported it. Then the Milankovitch model, originally displaced by the Alpine Theory, came back in, and again, all data throughout the world supported it! This tendency of data to adapt to the latest theory is called the Reinforcement Syndrome, and is frequently observed.
Most of the evidence on dry land points to a single ice age. Multi-Ice-Age proponents forgive the data by claiming that the last ice age acted as an eraser to remove previous evidence. So, they resorted to oxygen isotope evidence on the sea floor to validate the Milankovitch Model. Problems with this are the requirements that the sediment be undisturbed by erosion or sea-bottom life, and to know the surface temperature within a few degrees, and the residence time for sea-bottom waters.
Further complicating the issue is the evidence for a single ice age. A spot of land in southwest Wisconsin, and another in northeast Montana, have never been glaciated, though surrounded by ice sheet in the ice age of 10,000 years ago. How did all the other ice ages miss these spots as well? The probability of this oddity occurring once is far greater than it repeatedly occurring 15 to 30 times. The giant woolly mammoths and 33 other large mammals became extinct in the ice age of 10,000 years ago (two to five times the number of extinctions in all previous "ice ages"). How did they survive all the other ice ages? No consensus has been reached on this question.
OVERCONFIDENCE
Overconfidence can destroy the credibility of any scientific tool. This was demonstrated in the PRA industry when a 10-6/year cutoff was selected as a design goal for nuclear reactors. For years PRA analysts tried to stretch their art to meet this incredible goal, and the Advisory Committee on Reactor Safeguards exposed their weaknesses. Eventually, approved reactor PRAs admitted to accident frequencies as high as 10-3/year.
When the National Academy of Sciences (NAS) recommended extension of the previous 10,000-year repository mission time to one million years, part of their justification for going so far into the future was "reasonable geologic stability."
Other experts do not share the NAS's confidence:
"For high level waste, the shortest timescales specified are 10,000 years, and modeling performance over such timescales causes skepticism among both lay and technical people."
"...geological conditions are not easily determined, and even if they were known, we could not predict future geological events with any certainty."
The geologists are confident that the earth will have "reasonable geologic stability" for the next 1 million years, but they can't speak for uncertainties in other scientific fields for that length of time. In addition, they are running the risk that any expert is subject to:
"It is difficult for experts to put uncertainties in their specific fields in perspective."
"It is therefore most important to be wary of our over-confidence, for this over-confidence is at its greatest in our own area of expertise -- in short, just where it can do the most damage."
Excerpted from "Uncertainties in Repository Modeling," presented at the Probabilistic Safety Assessment '97 Conference in Park City, Utah by James R. Wilson.
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Probabilistic Risk Assessment, a method used for predicting the future.