<P> Natural sources of atmospheric carbon dioxide include volcanic outgassing, the combustion of organic matter, wildfires and the respiration processes of living aerobic organisms . Man - made sources of carbon dioxide include the burning of fossil fuels for heating, power generation and transport, as well as some industrial processes such as cement making . It is also produced by various microorganisms from fermentation and cellular respiration . Plants, algae and cyanobacteria convert carbon dioxide to carbohydrates by a process called photosynthesis . They gain the energy needed for this reaction from absorption of sunlight by chlorophyll and other pigments . Oxygen, produced as a by - product of photosynthesis, is released into the atmosphere and subsequently used for respiration by heterotrophic organisms and other plants, forming a cycle . </P> <P> Most sources of CO emissions are natural, and are balanced to various degrees by natural CO sinks . For example, the natural decay of organic material in forests and grasslands and the action of forest fires results in the release of about 439 gigatonnes of carbon dioxide every year, while new growth entirely counteracts this effect, absorbing 450 gigatonnes per year . Although the initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity, modern volcanic activity releases only 130 to 230 megatonnes of carbon dioxide each year . These natural sources are nearly balanced by natural sinks, physical and biological processes which remove carbon dioxide from the atmosphere . For example, some is directly removed from the atmosphere by land plants for photosynthesis and it is soluble in water forming carbonic acid . There is a large natural flux of CO into and out of the biosphere and oceans . In the pre-industrial era these fluxes were largely in balance . Currently about 57% of human - emitted CO is removed by the biosphere and oceans . From pre-industrial era to 2010, the terrestrial biosphere represented a net source of atmospheric CO prior to 1940, switching subsequently to a net sink . The ratio of the increase in atmospheric CO to emitted CO is known as the airborne fraction (Keeling et al., 1995); this varies for short - term averages and is typically about 45% over longer (5 year) periods . Estimated carbon in global terrestrial vegetation increased from approximately 740 billion tons in 1910 to 780 billion tons in 1990 . </P> <P> Carbon dioxide in the Earth's atmosphere is essential to life and to the present planetary biosphere . Over the course of Earth's geologic history CO concentrations have played a role in biological evolution . The first photosynthetic organisms probably evolved early in the evolutionary history of life and most likely used reducing agents such as hydrogen or hydrogen sulfide as sources of electrons, rather than water . Cyanobacteria appeared later, and the excess oxygen they produced contributed to the oxygen catastrophe, which rendered the evolution of complex life possible . In recent geologic times, low CO concentrations below 600 parts per million might have been the stimulus that favored the evolution of C4 plants which increased greatly in abundance between 7 and 5 million years ago over plants that use the less efficient C3 metabolic pathway . At current atmospheric pressures photosynthesis shuts down when atmospheric CO concentrations fall below 150 ppm and 200 ppm although some microbes can extract carbon from the air at much lower concentrations . Today, the average rate of energy capture by photosynthesis globally is approximately 130 terawatts, which is about six times larger than the current power consumption of human civilization . Photosynthetic organisms also convert around 100--115 thousand million metric tonnes of carbon into biomass per year . </P> <P> Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from CO and water using energy from light . However, not all organisms that use light as a source of energy carry out photosynthesis, since photoheterotrophs use organic compounds, rather than CO, as a source of carbon . In plants, algae and cyanobacteria, photosynthesis releases oxygen . This is called oxygenic photosynthesis . Although there are some differences between oxygenic photosynthesis in plants, algae, and cyanobacteria, the overall process is quite similar in these organisms . However, there are some types of bacteria that carry out anoxygenic photosynthesis, which consumes CO but does not release oxygen . </P>

Which is considered the world's most harmful fuel to the atmosphere