QUOTE : 15 million years ago ice sheets began forming over Greenland, and by the beginning of the Quaternary the cooling had passed the threshold for the North Pole’s ice cap to begin expanding. We entered the current phase of pulsing ice ages.23 It seems the Earth has been committed to a concerted effort towards cooling down. What grand-scale planetary processes have been conspiring to drive this global chilling? Gases like carbon dioxide and methane, as well as water vapour, in the atmosphere act like the panes of glass in a greenhouse: they allow the short-wavelength visible sunlight to shine right through and heat the Earth, but block the longer-wavelength infrared light given off by the warm planet surface. The effect of these greenhouse gases is to trap heat energy from escaping back into space, and so insulate the planet, leading to higher temperatures. Any mechanism that reduces the amount of these greenhouse gases in the air will therefore drive a global cooling.