Saturday, November 19, 2011

Ice Ages: Tiny Variations, Huge Effects

They say that people old enough then to remember, know exactly what they were doing when President Kennedy was assassinated. I was at the office and reading an article in Look magazine about global cooling. Yes, cooling, believe it or not. And why? I had become very interested in ice ages and keen to discover what had caused them. That was November 22, 1963. Many things interested me at every point in my life. The assassination pinned that interest precisely on my personal time map.

The prevailing explanation in those days, at least those easily accessible without gargantuan studies (no Internet then, remember)—the broad explanation, still repeated today, was that the cause of ice ages was not known precisely, but the suspects were (1) volcanic dust and carbon dioxide in the air, (2) shifts in the earth’s crust, and (3) continental uplifts. The Encyclopedia Britannica (“last word”) available then, lead with these causes and had them numbered, as above; the discussion pointed out the problems with each. Then followed a discussion of two other theories; let’s call them Tilt-and-Orbit and Solar Radiation. Tilt-and-Orbit is based on known variations in the earth’s orbital eccentricity, its axial tilt, and shifts in the earth’s perihelion, thus the point where it approaches the sun most closely. In the discussion of this theory, the EB mentions the name of M. Milankovitch, a Serbian geophysicist. EB then proceeds to produce a longish paragraph in which it presents four numbered objections to this theory. Solar Radiation says that the sun’s radiating heat-output increases and declines over long periods of time. Now that was the view in 1953, the date of the EB’s edition. Needless to say popular media in the 1960s, like Look magazine, never got past Volcanoes-Crusts-and-Continents.

The work of Milutin Milanković (1879-1958) was first published in 1912 under the title of Contribution to the mathematical theory of climate. In 1914 he wrote a monograph called About the issue of the astronomical theory of ice ages. Interned in Budapest during World War I, he perfected his thought and produced a theory known as Milankovich cycles. His theory of cycles, based on the Tilt-and-Orbit approach (as I’ve dubbed it), was up there as a theoretical offer until scientific exploration, namely deep-ocean core sampling, finally showed that Milankovich was right on target. A paper published in Science by James Hayes, John Imbrie, and Nicholas Shackleton, “Variations in the Earth’s Orbit: Pacemaker of the Ice Ages,” is viewed as the defining link between the math-predictions and physical observation. And that paper appeared in 1976.

Well, yesterday, November 18, 2011, “Science Friday” on NPR, I was trying to find a tire and hence listening to the car radio—and now I learned that perhaps, just perhaps, we do have a straightforward, but very complicated, theory explaining the ice ages. It is complicated because changes in the tilt of the earth’s axis (40,000 years from min to max), changes in the earth’s orbital eccentricity (91,800 years), and in the earth perihelion (21,000 years) each has its own timing. And sometimes they counteract each other’s effects. Furthermore, the differences between minima and maxima are rather minute. Let’s look at eccentricity. If the earth’s orbit were perfectly circular, its deviation would be 0.0. But actual values range from 0.0034 to 0.058, presently at 0.0167. Now that is a tiny deviation from circularity, so much so that for all practical purposes our orbit is a circle. Similarly with the axial tilt. At minimum it is 22.1 degrees, at maximum 24.5 degrees. Our current tilt is 23.44 and decreasing. The inset illustrates just how small a change that is. Yet when at last the time cycles coincide appropriately—and there is no interfering human meddling, like burning up all of the oil and coal stored over millions in just two hundred years, thus loading the air with carbon dioxide, ice ages are in the cards. Tiny deviations, huge consequences, but our lives are too short to notice much of a change. And experts on long-term trends are not in agreement. Some see the earth cooling for another 23,000 years. Others see it warming for another 50,000. Take your pick. What I’d like to know is how on earth anyone could precisely calculate even such a thing as the earth’s tilt now, never mind what it was 30-, 40,000 years ago.

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