Carbon dioxide is generated through the oxidation of carbon or carbon compounds, including combustion and cellular respiration. Carbon dioxide is also released into the atmosphere when water thaws, as well as during volcanic eruptions. While many fear its theorized effects on global temperatures, carbon dioxide is absolutely essential to the vast majority of life on Earth.
The vast majority of carbon dioxide released into the atmosphere each year comes from natural sources. Animals and microbes together release about the same amount of carbon dioxide as plants, while the oceans are the greatest single source. Only about 4 percent of carbon dioxide emissions come from artificial human sources. Carbon dioxide currently constitutes about 0.03 percent of Earth's atmosphere.
Carbon dioxide is one of the crucial inputs for producers at the base of almost every food chain. The basic mechanism of photosynthesis is the use of solar energy to bond water and carbon dioxide into carbohydrates. Plants grow faster at higher carbon dioxide levels, since most of their structures are built from carbohydrates. Carbon dioxide is toxic to humans at higher concentrations,but normal atmospheric concentrations of carbon dioxide, even with any conceivable increase due to human activity, are far too low to risk toxicity.