Atmospheric carbon dioxide levels centred on the eastern Pacific Rim, satellite data. Carbon dioxide (CO2) is a greenhouse gas that contributes to global warming. CO2 levels vary from low (blue, 360 parts per million) through medium (yellow and orange) to high (red, 385 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 (the USA, upper right) that produce CO2, and lowest over uninhabited (the Pacific Ocean) or forested areas. This image is based on data acquired in July 2003 by the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite.