Atmospheric carbon dioxide levels centred on the Pacific Ocean, 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). Wind directions (arrows) are also shown. The CO2 levels were measured in the mid-troposphere, around eight kilometres above the Earth's surface. The levels are generally low over uninhabited areas such as this, but winds still ensure CO2 is spread here from elsewhere in the world. This image is based on data acquired in July 2003 by the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite.