levels of carbon dioxide. Almost every time the chill of an ice age descended on the
planet, carbon dioxide levels dropped. When the global temperature dropped 9°F (5 °C), carbon dioxide levels dropped to 190 parts per million or so. Generally, as each
ice age ended and the Earth basked in a warm inter glacial period, carbon dioxide
levels were around 280 parts per million. Through the 160,000 years of that ice
record, the level of carbon dioxide in the atmosphere fluctuated between 190 and
280 parts per million, but never rose much higher-until the Industrial Revolution
beginning in the eighteenth century and continuing today.