Posted on Tuesday 20 July 2004

Periods of glaciation and thawing have been so regular during the course of the earth's history that it is possible to unambiguously identify at least two major cycles, one with a period of about 41,000 years and another of about 100,000 years. In the early twentieth century mathematician Milutin Milankovitch was able to build a theory that argued that these cycles were directly caused by the mechanics of planetary motion.

The earth's orbit is characterised by three major cycles:

• Eccentricity: over a period of 100,000 years the shape of the earth's orbit varies from an almost perfect circle to a mildly elliptical one causing a 6% variation in radiation from the sun.
• Obliquity: over a period of 40,000 years, the tilt of the earth's axis varies between 22.1 and 24.5 degrees. The greater the tilt to more marked are the seasons: colder winters and warmer summers
• Precession: over a period of about 20,000 years, the elliptical orbit itself rotates around the sun (the earth actually traces out something a bit like a spirograph pattern around the sun). This has the effect of changing the timings of when the earth is at its closest and furthest points from the sun and biasing the seasons differently in the different hemispheres (i.e. making one more or less severe than the other).
  

Linking climate change to planetary mechanics has been so successful in explaining the cyclical nature of the earth's glaciation periods that since the 1970s the Milankovitch theory has become the standard model for climatologists. A closer reading of the historical record, however, has cast some doubts over whether these variations could be solely due to changes in solar heating alone.

Apart from light and heat the sun also beams to earth colossal quantities of changed particles known as solar winds. These winds would strip the earth of its atmosphere in a comparatively short time if they weren't being constantly deflected by the earth's magnetic field. New research by Jasper Kirkby of CERN, Augusto Mangini of the University of Heidelberg and Richard Muller of the University of California at Berkeley is suggestive of the possibility that while these cycles are still almost certainly orbital in their nature, the main mechanism driving glaciation is not the regular fluctuation in sunlight but rather changes in the flux of cosmic radiation reaching the atmosphere.


The earth's magnetic field also fluctuates in a cyclical manner and Kirky et al argue that this also correlates with the orbital precession cycle. They also argue that the earth's atmosphere which is prone to make more cloud cover when exposed to more solar wind and thus cooling the climate is extraordinarily sensitive to variations in flux. This phenomena may even go as far as having the earth's climate affected by interstellar winds due solar system's position relative to the nearest spiral arm of the Milky Way(!)

That kind of sensitivity may be a demonstration of just how connected events on earth really are with the rest of the universe. Enough to give pause when thinking about the Heliopause.