The carbon cycle is a process in which carbon is continually circulating through the biosphere. Carbon is one of the fundamental building blocks of life on Earth. Carbon is contained in the molecules of dead and living organisms. It exists in the atmosphere as a gas molecule, and as dissolved carbon dioxide in the oceans. In the lithosphere, it's stored as fossil fuels and sedimentary rock deposits. The normal flow of carbon between the atmosphere, biosphere, lithosphere and ocean is quite balanced; however, human activity has seriously altered the carbon cycle by disturbing the natural exchange between reservoirs.
Carbon is exchanged between reservoirs (land, atmosphere and oceans) in an ongoing process known as the carbon cycle. Carbon exchanges occur due to a number of chemical, geological, physical and biological processes. The oceans represent the largest concentration of carbon with more than 60% as much carbon as it is in the atmosphere. Any modification of the quantities of carbon in one reservoir, places more carbon in the remaining reservoirs. Changes that put extra carbon in the atmosphere lead to global warming. The carbon cycle occurs in four main reservoirs on Earth.
The biosphere includes all living organisms thriving in parts of the lithosphere, hydrosphere and atmosphere. Most organisms that rely on the Sun as a source of energy live in the biosphere. During photosynthesis, plants and microscopic organisms in the ocean (phytoplankton) use sunlight and carbon dioxide (CO2) to convert light energy into chemical energy. Animals, including people eat plants to obtain the energy they need. In the process, carbon is exchanged from plants to animals and humans. Through respiration animals and humans add carbon into the atmosphere. Additionally, carbon is added to the atmosphere through volcanic eruptions, fossil fuel burning and deforestation.
The movement of carbon from the atmosphere to the lithosphere usually begins with the formation of clouds of rain. Carbon in the atmosphere combines with rain water to form carbonic acid (H2CO3). Carbonic acid reaches the surface in rain water. Through a process known as chemical weathering, carbonic acid dissolves rocks, releasing magnesium, calcium, potassium and sodium ions. Rivers transport these ions into the ocean, where they combine with bicarbonate ions dissolved in sea water, resulting in calcium carbonate (CaCO3). Calcium carbonate is deposited into the ocean floor as limestone.
At the surface of the ocean, CO2 dissolves in and out of the ocean in a steady exchange with the atmosphere. CO2 reacts with water molecules to release hydrogen, causing acidification. Like the plants on land, plants (algae, kelp) in the ocean use photosynthesis to capture sunlight, and in the process, they absorb CO2 dissolved in the water, which is later released through respiration. Fish that feed on plankton, kelp or algae absorb CO2. Like other animals on the Earth´s biosphere, fish release CO2 through their gills. When a fish dies, it decays and much of its carbon is released back into the ocean. The weathering of silicate rocks produce bicarbonate ions, which are carried to the ocean, and used by organisms, such as shells to build their structures. When sea shells die, their structures sink to the ocean floor, forming limestone deposits.
Carbon in the atmosphere exists as CO2 and methane (CH4). Before the industrial age, the exchange of carbon between the ocean and the atmosphere remained balanced, along with the carbon received through rock weathering; however, increases in carbon concentration in the atmosphere, contribute more carbon into the waters of the ocean than the ocean releases. Carbon dioxide is a greenhouse gas that helps control global temperature in the planet; however, humans have contributed more greenhouse gas into the atmosphere by burning fossil fuel, deforestation, manufacture of concrete, among other processes.
Greenhouse gases (carbon dioxide, methane and halocarbons) absorb a wide range of energy, included heat re-emitted by the Earth. Greenhouse gases act as a shield that keeps the Earth sufficiently warm to sustain life. Without greenhouse gases, the Earth would freeze; however, with big quantities of gases, the Earth would become an uninhabitable place to live. Rising concentrations of CO2 warm the atmosphere. Rising temperatures cause evaporation (water vapor accounts to about 50% of global warming). CO2 concentrations provide the heat needed for increased water vapor concentrations.
Rising CO2 concentrations have already caused global warming. According to nasa.org, global temperatures have increased by 0.8° C (1.4° F) since 1880. It is estimated that the concentrations of CO2 already in the atmosphere will increase temperature by another 0.6° C (1° F). The rate at which temperature increases will depend, in part, to the amount of carbon dioxide humans release into the atmosphere in the future. Since the industrial revolution, CO2 concentrations have risen from 280 parts per million to 387 parts per million. For every million molecules in the atmosphere, 387 are CO2, which is the highest concentration in two million years.
In the past, the carbon cycle has altered due to variation in Earth's orbit and climate change; however, in modern times, changes in the carbon cycle have occurred because people is perturbing the natural carbon cycle by adding extra carbon through deforestation, fossil fuel burning, manufacturing of cement, etc. It is estimated that the burning of fossil fuel contributes with the majority of CO2 present in the atmosphere today. According to eaarthobservatorynasa.org, the burning of fossil fuel accounted to approximately 8.4 billion tons of carbon into the atmosphere in 2009.