Studying ice cores at the National Ice Core Lab, Dr. Richard Alley shows how ancient ice contains records of Earth's past climate. Over 400,000 years, and even longer, levels of carbon dioxide (CO2) have risen and fallen from about 180 parts per million to 280, varying from colder in the Ice Ages to warmer in interglacial periods. Today, however, for the first time in more than 400,000 years, CO2 is at 390 parts per million and continuing to rise at 2 parts per million (ppm) per year. Alley relates the onset of this change to the Industrial Revolution.

Credit: EARTH: The Operators' Manual (Segment 5)