A fixed amount of carbon exists on earth and is constantly recycled through a number of carbon sinks, where carbon is stored for a period of time. For example, photosynthesis allows plants to absorb carbon out of the atmosphere in the form of carbon dioxide; then animals consume this carbon when they eat plants or other animals. As plants and animals die and decay, the carbon is slowly released back into the atmosphere from which it came. The carbon cycle maintains a healthy and balanced ecosystem.
The use of fossil fuels disturbs the natural carbon cycle, because more carbon is released into the atmosphere than what is absorbed. This increase of carbon in the atmosphere (mostly in the form of carbon dioxide and methane) traps heat and causes global warming.
On the other hand, the use of biomass from waste resources as a fuel is carbon neutral, because no additional carbon is released into the atmosphere compared to disposing of the biomass waste on landfills or through open field burning. Therefore, biomass from waste resources does not contribute to global warming. In fact, burning the biomass only releases carbon dioxide, while leaving the biomass to decay anaerobically produces methane gas as well, and methane gas is about 30 times more potent as a greenhouse gas than carbon dioxide.