The phenomenon is obviously rare. A pulsating series of ice sheets seems to have been set up in the discernible history of the world roughly once every three hundred million years. It happens so infrequently that it must be the result of coinciding circumstances that could not stand alone as explanations. There are components fast and slow. The atmosphere has been gradually cooling for sixty million years. Possibly this is explained by the great orogenies that have occurred during that time-the creation of the Rockies, the Andes, the Alps, the Himalaya-and the volcanism that is associated with mountain building. Volcanic ash in the stratosphere reflects kantoor huren per uur amsterdam sunlight back into space. Also, the weathering of mountains, particularly their granites, brings on a chemical reaction that removes carbon dioxide from the atmosphere, diminishing the greenhouse effect and chilling the earth. In any case, the essential requirement is a cool summer. A little snow from one winter must last into the next. Eve:ry forty thousand years, the earth’s axis swings back and forth through three degrees. Summers are cooler when the earth is less tilted toward the sun. The sun, for that matter, is not consistent in the energy it produces. Moreover, the relative positions of the sun and the earth, in their lariat voyage through time, va:ry, too-enough for subtle influence on climate. Carbon dioxide also affects climate, and the amount of carbon dioxide in the atmosphere is not constant. Somewhere in such a list, which runs to many items, lie the simultaneous events that set the ice to growing. The change they bring is not at first dramatic. So critical is the earth’s temperature that a drop of just a few degrees will cause ice to form and spread. A cool summer. Unmelted snow. An early fall in some penarctic valley. An overlap of snow. A long winter. A new cool summer. An enlarged kantoor huren per uur eindhoven residue of snow. It compacts and recrystallizes into granules, into ice. Because it is white, it repels the sun’s heat and helps cool the air on its own. The process is self-enlarging, unstoppable, and once the ice is really growing it moves. Clear bands form near the base, along which the ice shears and slides upon itself in horizontal layers like the overthrust Appalachians.