> "Since the industrial revolution in the 18th century, the concentration of carbon dioxide in the atmosphere (an important greenhouse gas) has risen significantly above the level it would be naturally. It now stands at over 400 parts per million (ppm) in the atmosphere. Evidence from ice cores (see Lesson four) tells us that the normal level for CO2 in the atmosphere during ‘interglacial’ times (such as the Holocene in which we now live) is 270 to 290 ppm, and that at no time over the last 800,000 (the time covered by ice cores) has the CO2 level been as high as it is now. (CO2 is thought to be at its highest level for three to five million years.)"