What is the mechanism that triggers an ice age, and what is the mechanism that brings them to an end?

This, interestingly, is a consequence of the energy of Gaia itself. It is a kind of long-term energetic rebalancing that takes place as a consequence of all that happens on the surface and within the planet and its conjunction with other astral bodies. The Earth cannot be considered in isolation as it is a member of the solar system and the larger galaxy of the universe as a whole in which it resides. The interplay of energies requires a releasing of certain kinds of energy that works in a negative fashion and this requires periodic discharges. This depletes energy available for maintaining things such as the atmospheric temperature. That is the reason for the comings and goings with the temperature fluctuation driving glaciation. It is not in response to the atmospherics and composition per se, it is a larger issue governed and mandated by the consciousness of Gaia itself. It is somewhat akin to human mood states which are a shorter-term phenomenon by far but will color the experience of a person during the changes across time from highs to lows in enthusiasm, happiness, and having less energy and even lethargy reining for a period of time. This is not abnormal, it is simply part of the characteristic makeup of a large body of this kind possessing a quite active consciousness, because many of the interactions it has with other forms of consciousness present upon it in living form, as well as the energetics from other bodies within the universe, have a greater impact because consciousness is attuned to other consciousness and will react to and absorb what it sends by way of information, and then must take some steps to balance this in order to maintain its own integrity. It is not so different than the human who is bombarded continually by psychic attacks on the part of the thoughts of others in the human family. There are many dark energies and forces launched by humans themselves that remain in the environment and can be encountered and then build up within a person who may take on a reaction to those energies and it becomes a personal imprint and deviation from their prior energetic makeup as a kind of defect, and ideally a person will rebalance this at some point to regain a more even tone and equilibrium—that is what the Earth does on its own to cope with this energetic flow.