Globe, centred on Africa, showing atmospheric carbon dioxide (CO2) levels. CO2 is a greenhouse gas that contributes to global warming. CO2 levels vary from low (blue, 365 parts per million) through medium (yellow and orange) to high (red, 380 parts per million). The CO2 levels were measured in the mid-troposphere, around eight kilometres above the Earth's surface. The levels are highest above industrialised areas (South Africa, Europe and China) that produce CO2, and lowest over uninhabited areas (central Africa). Data acquired in July 2003 by the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite.