A short history of nearly everything - Bill Bryson [212]
Croll was the first to suggest that cyclical changes in the shape of Earth's orbit, from elliptical (which is to say slightly oval) 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 Earth's weather. Thanks almost entirely to Croll's persuasive theory, people in Britain began to become more responsive to the notion that at some former time parts of the Earth had been in the grip of ice. When his ingenuity and aptitude were recognized, Croll was given a job at the Geological Survey of Scotland and widely honored: he was made a fellow of the Royal Society in London and of the New York Academy of Science and given an honorary degree from the University of St. Andrews, among much else.
Unfortunately, just as Agassiz's theory was at last beginning to find converts in Europe, he was busy taking it into ever more exotic territory in America. He began to find evidence for glaciers practically everywhere he looked, including near the equator. Eventually he became convinced that ice had once covered the whole Earth, extinguishing all life, which God had then re-created. None of the evidence Agassiz cited supported such a view. Nonetheless, in his adopted country his stature grew and grew until he was regarded as only slightly below a deity. When he died in 1873 Harvard felt it necessary to appoint three professors to take his place.
Yet, as sometimes happens, his theories fell swiftly out of fashion. Less than a decade after his death his successor in the chair of geology at Harvard wrote that the “so-called glacial epoch . . . so popular a few years ago among glacial geologists may now be rejected without hesitation.”
Part of the problem was that Croll's computations suggested that the most recent ice age occurred eighty thousand years ago, whereas the geological evidence increasingly indicated that Earth had undergone some sort of dramatic perturbation much more recently than that. Without a plausible explanation for what might have provoked an ice age, the whole theory fell into abeyance. There it might have remained for some time except that in the early 1900s a Serbian academic named Milutin Milankovitch, who had no background in celestial motions at all—he was a mechanical engineer by training—developed an unexpected interest in the matter. Milankovitch realized that the problem with Croll's theory was not that it was incorrect but that it was too simple.
As Earth moves through space, it is subject not just to variations in the length and shape of its orbit, but also to rhythmic shifts in its angle of orientation to the Sun—its tilt and pitch and wobble—all affecting the length and intensity of sunlight falling on any patch of land. In particular it is subject to three changes in position, known formally as its obliquity, precession, and eccentricity, over long periods of time. Milankovitch wondered if there might be a relationship between these complex cycles and the comings and goings of ice ages. The difficulty was that the cycles were of widely different lengths—of approximately 20,000, 40,000, and 100,000 years, but varying in each case by up to a few thousand years—which meant that determining their points of intersection over long spans of time involved a nearly endless amount of devoted computation. Essentially Milankovitch had to work out the angle and duration of incoming solar radiation at every latitude on Earth, in every season, for a million years, adjusted for three ever-changing variables.
Happily this was precisely the sort of repetitive toil that suited Milankovitch's temperament. For the next twenty years, even while on vacation, he worked ceaselessly with pencil and slide rule computing the tables of his cycles—work that now could be completed in a day or two with a computer.