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By the late 19th century, after passionate debate, most scientists accepted the incredible.
Long ago (although not very long as geological time went, for Stone Age humans had lived through it), northern regions had been buried kilometers deep in continental sheets of ice.
The timing of the cycles was apparently set by minor changes in sunlight caused by slow variations of the Earth's orbit.
Just how that could regulate the ice ages remained uncertain, for the climate system turned out to be dauntingly complex.
Many things in the natural world come and go in cycles, so it was natural for people to suppose that there was a regular pattern to the ebb and flow of ice sheets.
After all, there was evidence convincing to many meteorologists, although doubted by as many more that temperature and rainfall varied in regular cycles on human timescales of decades or centuries.
A solution to the puzzle would bring deep satisfaction and eternal fame to whoever solved it.
In particular, it turned out that"greenhouse" gases like carbon dioxide played a surprisingly powerful role in governing global climate.
One lesson was clear: the system is delicately poised, so that a little stimulus might drive a great change.
These paleoclimatologists succeeded brilliantly, discovering a strangely regular pattern of glacial cycles.
The pattern pointed to a surprising answer, so precise that some ventured to predict future changes.
Much farther back there had been a few other relatively brief epochs of glaciation, revealed by very ancient ice-scraped rocks and gravel deposits.