Carbon Cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere. Along with nitrogen and water cycle, the carbon cycle compromises a sequence of events that are key to making the Earth sustain life. Carbon is the main component of all biological making it crucial for life on Earth. In the past two centuries, human activities have seriously altered the global carbon cycle, most seriously in the atmosphere. Even though carbon dioxide levels have changed over thousands of years, human emissions have exceeded natural flucuations. The largest and most direct form of human influence on the carbon cycle comes from the burning of fossil fuels. Carbon has to main forms in the atmosphere, carbon dioxide and methane. Both of the gases help retain heat and are partially responsible for the green house effect.