Global carbon dioxide concentration. Map of the concentration of carbon dioxide (CO2) in the mid-troposphere (about 8 kilometres above the Earth's surface) as a monthly average for July 2003. The concentration is in parts-per-million (ppm) by volume ranging from low concentrations (blue) to high concentrations (red). The burning of fossil fuels has increased the levels of CO2 from 280 ppm in pre-industrial times to over 380 ppm today. It is estimated that it would take several hundred years for the CO2 to return to previous levels if all non-natural emissions stopped today. Data obtained using the Atmospheric Infrared Sounder (AIRS) instrument on the NASA Aqua Spacecraft.