Historical CO2 record over 400, 000 years. Animated graph showing the large variation in carbon dioxide (CO2) concentration in the Earth's atmosphere over a period of around 400, 0000 years before the present. At the far right-hand side of the graph is the dramatic increase seen in modern times (since the Industrial Revolution). The CO2 concentration (measured in parts per million, ppm) has increased from below 300 ppm to over 400 ppm. This increase in the greenhouse gas CO2, due to human activities such as fossil fuel consumption, is a key cause of global warming. The Earth's temperature and climate has varied in the past, but the changes due to human activities can cause dramatic changes. For two versions of the graphs showing the increase in modern times, see clips K005/6411 and K005/6413.