Isolated in Budapest under house arrest during WW 1 a Serbian astronomer and mathematician named Milutin Milankovitch wondered if there might be some relationship between the complex cycles of the Earth with the Sun as it moves and wobbles through space and the comings and goings of ice ages.
The difficulty was that these cycles were of widely different lengths -20,000, 40,000, and 100,000 years- but each varying by up to a few thousand years, which meant that Milankovitch had to compute the angle and duration of incoming solar radiation at every latitude on earth, in every season, for a million years, adjusted for three constantly changing variables.
The result of his house arrest was the 1930 book Mathematical Climatology and the Astronomical Theory of Climate Change, which proved that he was right about the relationship between ice ages and planetary wobble.
We are all familiar with the notion that the Earth rotates on its axis and revolves in orbit around the sun. There are at least five other motions which complicate this simple picture: precession of the equinoxes, precession of the Earth's orbit, axial tilt, eccentricity, and obliquity of the Earth's orbit.
Only three variables, the precession of the equinoxes, axial tilt, and eccentricity are thought to play a role in the Milankovitch cycle.
Precession of the equinoxes is the apparent change in stellar background from one equinox to another -In popular terms, every 2000 years or so, the sun "moves" from one house of the zodiac to the next--"This is the dawning of the age of Aquarius." The widely accepted model for this precession states that it is caused by the wobbling of the Earth's rotation around its axis, in a period that is variously estimated as from 26,000 to 21,000 years.
Axial tilt is the inclination of the Earth's axis of rotation to the plane of its orbit, an inclination which varies from approximately 21.5 degrees to 24.5 degrees from the vertical, and which is responsible for seasons. As the Earth moves about its orbit, first one pole and then the other is tilted toward the sun (at the equinoxes, the tilt is parallel to the sun's axis). It has been discovered that axial tilt varies with a period of about 41,000 years. We are currently approximately in the middle of this period, with an axial tilt of 23.44 degrees.
Eccentricity refers to the shape of the Earth's orbit. Imagine the orbit as showing a regular pulse from a shape more like a circle to a shape more like a squashed circle, or ellipse, and you will have the idea. The greater the eccentricity, the more like an ellipse. The eccentricity of our orbit is primarily caused, according to our calculations, by the gravity of Jupiter and Saturn. When the orbit is more elliptical, the point of closest approach to the sun is closer than when the orbit is more circular, and the point of farthest distance from the sun is farther away.
According to the inverse square law, as the Earth approaches the sun more closely, it receives more radiation, and as it moves farther away it receives less.
Currently the eccentricity of the orbit is rather low, and at perihelion when the Earth is closer to the Sun, the planet receives only 6 or 7% more radiation than it does at aphelion.
In times of greater eccentricity, the variation may be as much as 20 or 30%. The major period of eccentricity is approximately 400,000 years in length, with minor peaks occurring at intervals ranging from 95,000 to 136,000 years.
For the last million years, ice ages have occurred at intervals of approximately 100,000 years, with lesser variations occurring at intervals of 21,000 and 41,000 years. (Some scientist recently report that landmasses which straddle the equator apparently experience cycles of half the length of the 21,000-year cycle.)
For many years, it was unclear that there was a definable cycle in climatological variations, but evidence from ice cores, cores from the bottom of the ocean and ratios of carbon isotopes in organic strata indicate that clear-cut cycles do exist, and many scientists take this as confirmation of the Milankovitch cycle.
A question that has not been answered is the mechanism by means of which the astronomical periods are reflected in the climate cycles.
The leading candidate for an explanation is the theory that since most of the landmass is in the northern hemisphere, this landmass acts as an accumulator for snowfall in periods of glaciation, and that ice ages occur when warmer winters produce more water vapor in the atmosphere (hence more snow) and cooler summers fail to melt the accumulations as completely. (This theory would not account for climatic variations when all the continents were bonded into the supercontinent we call Gondwanaland).
Do the Milankovitch cycles mean that you might as well stop worrying about global warming, since climate variation occurs in response to astronomical forces and there is nothing we can do about it?
Not exactly. For one thing, no scientific consensus has been reached on the validity of the Milankovitch cycles. While many scientists consider the cycles factual, many others raise questions. The "Transition Problem," for example, asks why, prior to a million years ago, cycles occurred at 41,000-year intervals, but after a million years ago, they occurred only at 100,000-year intervals.
It seems likely that climate is a complex system, and while astronomical variations are likely to influence it, according to the law of sensitive dependence on initial conditions it is very difficult to isolate those influences and show a direct cause-and-effect correlation.
For another thing, a significant body of scientists cautions that, even if the Milankovitch cycles are accurate descriptions of the past, current human activities may have an overwhelming affect on the delicately balanced systems of the Earth's climate.
Posted by Jack Butler
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