Being one of the essential building blocks of life, carbon can be found almost anywhere.  People use carbon through respiration, and plants use carbon through photosynthesis.  The carbon cycle is the term used to describe the complex processes through which carbon is interchanged through humans, animals, plants, bodies of water, and the atmosphere.

Carbon is also the primary component of most fuels, including both fossil fuel – coal, oil, and natural gas – and biomass fuels (wood and animal waste).  Coal, oil, and natural gas are named fossil fuels because they are made from the buried remains of plants and animals that lived millions of years ago.  Globally, the majority of electricity comes from fossil fuels.

When fossil fuels are burned, some of the carbon is unused.  A portion of the unused carbon is then released into the atmosphere as carbon dioxide (CO₂ ), a greenhouse gas. CO₂ is made of one molecule of carbon and two molecules of oxygen.

Because 85% of the U.S. greenhouse gas emissions are energy related, and 95% of energy-related emissions are CO₂, researchers are looking for ways to capture and store CO₂ emissions to prevent their release into the atmosphere.

The figure above presents a simplified version of the global carbon cycle. The large arrows represent natural paths of carbon exchange, and the small arrows represent the human or anthropogenic contributions to the carbon cycle. The flow of carbon is measured in billions of metric tons (gigatons).