Water evaporates from the surface. Eventually this water condenses and returns to the surface as precipitation. Once global warming begins to occur the amount of water vapor becomes subject to a positive feedback effect. The warmer the air gets the more water vapor it can hold and the more evaporation will occur. A small amount of carbon dioxide exists naturally in the atmosphere.
Humans also produce a lot of carbon dioxide.
Visit the American Forests web site if you want to know how much carbon dioxide you produce. To some extent the amount of carbon dioxide in the atmosphere is determined by the carbon cycle . The oceans and plants absorb some carbon dioxide, but human activity produces it faster than it can be absorbed. Carbon dioxide may last 50-200 years in the atmosphere. Coal produces the most carbon dioxide of all the fossil fuels.