Carbon is one of the fundamental elements in living organisms, comprising about 18% of the human body. Carbon is essential for the existence of cytoplasmic membranes and cells, and it is also the basis of DNA. However, the carbon cycle in nature involves more than just humans. Carbon occurs in the air mainly in the form of carbon dioxide (CO2), a gas that dissolves in water and reacts with water molecules to form bicarbonate—HCO3. Through photosynthesis, terrestrial plants, bacteria, and algae convert carbon dioxide or bicarbonate into organic molecules, which then migrate through food chains. Ultimately, carbon atoms are released into the atmosphere during respiration.
In addition to the exchange of carbon between living organisms in what is known as the short cycle (period), there are also long-term geological processes (long cycle/period). This primarily involves the formation of sedimentary rocks and fossil fuels. Although the short and long cycles are analyzed separately, they are interconnected. For example, the same sources of atmospheric and oceanic carbon dioxide (CO2) used by organisms are also involved in geological processes. The geological carbon cycle lasts much longer than the biological cycle, measured in millions of years. Carbon is stored for long periods in the atmosphere, water reservoirs—mainly in oceans—in oceanic sediments, soil, rocks, fossil fuels, and within the Earth's interior. Sinking remains of organisms create sediments on the ocean floor. Over time, this sediment turns into limestone, the largest carbon reservoir on Earth. On land, carbon from the decomposition of living organisms or as inorganic from the weathering of rocks and minerals is stored in the soil. At greater depths below the Earth's surface, fossil fuels such as oil, coal, and natural gas form as a result of the decomposition of plant remains in anaerobic conditions.
The Industrial Revolution and the general use of fossil fuels to meet basic needs such as heating and transportation involve their combustion, which releases very large amounts of carbon dioxide into the atmosphere. A similar process occurs during volcanic eruptions. Carbon sediments from the ocean floor enter deep into the Earth through subduction (movement of tectonic plates). In this process, carbon dioxide is released into the atmosphere as a result of volcanic eruptions or hydrothermal vents.
Fossil fuels are considered non-renewable energy sources because they are consumed much faster than they are formed through geological processes. It is also significant that humans have contributed to the disruption of the carbon cycle in nature, which has had a negative impact on climate change—primarily warming. It is also important that the release of CO2 into the atmosphere due to human activity is not only from the burning of fossil fuels but also from urbanization processes, especially the massive deforestation. The forest ecosystem stores carbon, and any reduction in it causes most of the carbon to be released as carbon dioxide directly into the atmosphere.