The graph above illustrates how strongly the major ice ages are influenced by the orbital eccentricity of the Earth about the Sun.
Today the seasons are dominated by the tilt of the Earth’s axis–the Northern Hemisphere pointing away from the sun when the Earth is closest to the sun–and the Northern Hemisphere pointing toward the sun when the Earth is farthest from the sun.
However, today, the Earth is about 20,000 years from the last major ice age. The eccentricity, meant to tell us how perfect Earth’s orbit is (a perfect circle is an eccentricity of zero) is how the Earth orbits today, so there is little difference in distance during the seasons of summer and winter.
Notice in the graph above that the last nine major eccentricities correlate to the last nine ice ages. How this happens is that the Earth tilts based on its precession (as if it were a top, spinning but winding down in potential energy). When the Northern Hemisphere (unlike today) tilts away from the sun when the Earth is farthest away from the sun, there is less insolation (incoming heat) and so the Earth begins to cool down.
We know there are other factors in play, because there are cycles on the Earth itself: cycles in the atmosphere, hydrosphere, and lithosphere. Our climatological computer models encompass all but the biological component. And why might that be important? What effect can microorganisms have on such huge shifts in temperature?
Notice how regular the connection of ice age fluctuations to eccentricity for the last million years. Before that, there might have been less or little correlation. Why? Because as the sun cools, as anything cools, it may show regular, lower-energy fluctuations. In the last nine or ten cycles, microorganisms, or smaller snippets of evolutionary life, might have adapted to the cycling and played into it.
Next, we will confirm that more than 50% of the biomass of the Earth exists (perhaps in the form of latent spores) in the crust (lithosphere). We believe carbon dioxide to be a trigger along with the sun’s incoming heat for part of the cycling, but have we considered the succession of microorganism bloom associated with these factors?
For a more complete overview of the effects of carbon dioxide on Earth’s climate, please refer to the following link: Scientific American Article on Carbon Dioxide and Climate