Carbon in atomosphere and hydrosphere is available to the living world as free or dissolved CO₂. Most of CO₂ enters the living world through photosynthesis. The amount of carbon fixed by photosynthesis is 7 × 10¹³ kg/yr. Oxygen is released as a bye-product. Carbon so fixed enters the food chain and making its way through herbivores, carnivores and decomposers reaches back to its original level. The carbon content of environment reduced during photosynthesis is replenished by five methods:

These form coal and oil. Major exchange in carbon cycle is between organisms and environment (atmosphere, hydrosphere). The cycling is a self-regulated feed back system but now-a-days it has been upset by excessive burning of fossil fuels. As a result CO₂ content of atmosphere is increasing which as a result is liable to disturb the climatic conditions of the world, melting away polar as well as alpine ice resulting in 18-20m rise in sea level.