Human activity affecting carbon cycle is

Carbon constitutes 49% of dry weight of an organism. Out of total global carbon, 71% carbon is found dissolved in the ocean. A large amount of carbon is returned to the atmosphere as CO${}_2$ through respiration of producers and consumers. Decomposers also return CO${}_2$ to the reservoir during the decomposition process. Some amount of carbon is lost to sediments and removed from circulation. Burning wood, forest fire, combustion of organic matter, fossil fuel, volcanic activities are additional sources for releasing CO${}_2$ to the atmosphere.