What causes ice ages?
Posted on Tuesday 20 July 2004 to Quintessence
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.
