This amount depends on climate. Global warming means less ice, and that extra water has to go somewhere. It will go into the oceans, resulting in a rise in sea level. The movement of water on land, and through the vegetation, is critical to the way other nutrient cylces work, because most of the other critical elements have a stage in which they occur in compounds that dissolve in water. This is by far where most of the carbon is, in the rocks.
Carbon occurs in the atmosphere and in water in the form of carbon dioxide.
In living organisms in various organic compounds. Plants take Carbon Dioxide and build sugars through photosynthesis. It takes energy to drive these chemical reactions. Oxygen is a product of photosynthesis. Although the amount of carbon dioxide in the atmosphere is small, it is important because carbon dioxide is a greenhouse gas.