Natural cycles of the earth,moon and sun. Earth is very much like a living organism, it needs sleep or rest periods which lowers it temperature, and warm robust periods which raises its temperature. These cycles come approximately every 72 years, 200 years, 1500 years, 110k years and 450k years. The earth's temperature and carbon dioxide varies during these cycles, it always has and always will. And earth needs carbon dioxide to thrive, higher levels translates to robust plant life and more oxygen. Lower levels during ice ages translates to less carbon dioxide in the atmosphere, and less plant life, and a lowering of oxygen. The earth moon and sun has hundreds of cycles which produce more radiance, more gravitation, all of which cause cycles.