Model of a molecule of carbon dioxide (CO2) rotating. Carbon dioxide comprises two oxygen atoms (red) joined by double bonds (grey) to a central carbon atom (black). Carbon dioxide is a gas that makes up some 0.4% of the atmosphere. It is used by plants during photosynthesis as the building block for carbohydrates (sugars), and is released by human activities such as the burning of fossil fuels. It is a greenhouse gas, helping the atmosphere retain heat, and most scientists agree that human activities releasing the gas are affecting the global climate. Master has a white background. For the same clip on a transparent alpha background, see K004 3339.