This section discusses some of the various external forcing mechanisms operating over time scales of 10 years to 109 years.
2.5.1. Galactic Variations
The orbit of the Solar System about the centre of the Galaxy has been considered as a possible external climate forcing mechanism (Huggett, 1991). During the course of a galactic year (now estimated at 303 million years), variations in the interstellar medium (Williams, 1975a) may influence the amount of solar radiation incident at the Earth’s surface, thus acting as a radiative forcing mechanism to induce climate change. Williams (1975a) also suggests that variations in gravitational torque induced by our Galaxy’s near neighbors, the Small and Large Magellanic Clouds, could have far-reaching consequences for the Earth’s climate.
Unfortunately, the enormous time scale associated with this forcing (and any hypothesised global climatic change) makes empirical confirmation of this premise exceedingly imprecise. Nevertheless, it is indeed possible that the ice age supercycles during the last 700 million years (Fischer, 1984) (see section 5.2.2) could be the result of such galactic forcing mechanisms.