Map 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 in the industrialised Northern Hemisphere and in a belt around the Southern Hemisphere, particularly over South Africa, Australia and Argentina. Levels are lowest over uninhabited and forested areas (the Amazon and central Africa). Data acquired in July 2003 by the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite.