What is The Carbon Cycle?
Carbon is present throughout the natural environment in a fixed amount. It takes many forms and moves through the environment via the carbon cycle.
The carbon cycle is the circulation and transformation of carbon back and forth between living things and the environment. Carbon is an element, something that cannot be broken down into a simpler substance. Other examples of elements are oxygen, nitrogen, calcium, iron, and hydrogen. Carbon compounds are present in living things like plants and animals and in nonliving things like rocks and soil. Carbon compounds can exist as solids (such as diamonds or coal), liquids (such as crude oil), or gases (such as carbon dioxide). Carbon is often referred to as the "building block of life" because living things are based on carbon and carbon compounds.
The amount of carbon on the earth and in Earth's atmosphere is fixed, but that fixed amount of carbon is dynamic, always changing into different carbon compounds and moving between living and nonliving things. Carbon is released to the atmosphere from what are called "carbon sources" and stored in plants, animals, rocks, and water in what are called "carbon sinks." This process occurs in a number of steps. In the first step, through photosynthesis (the process by which plants capture the sun's energy and use it to grow), plants take carbon dioxide out of the atmosphere and release oxygen. The carbon dioxide is converted into carbon compounds that make up the body of the plant, which are stored in both the aboveground parts of the plants (shoots and leaves), and the belowground parts (roots). In the next step, animals eat the plants, breath in the oxygen, and exhale carbon dioxide. The carbon dioxide created by animals is then available for plants to use in photosynthesis. Carbon stored in plants that are not eaten by animals eventually decomposes after the plants die, and is either released into the atmosphere or stored in the soil.
Large quantities of carbon can be released to the atmosphere through geologic processes like volcanic eruptions and other natural changes that destabilize carbon sinks. For example, increasing temperatures can cause carbon dioxide to be released from the ocean.
Carbon Moves between Sources and Sinks
One important aspect of the carbon cycle is the speed with which carbon moves from a carbon source to a carbon sink and then back again. Some living things grow and decompose more quickly than others. For example, living things with shells, like oysters or snails, take a longer time to decompose than "squishy" living things like slugs or tomatoes. The rate of decomposition, and the resulting release of carbon, can be hastened by the actions of specialized microscopic and macroscopic plants and animals, called "decomposers," that break down plant and animal matter. The decomposition process ends up creating carbon dioxide and other gases, such as methane. Plant and animal growth and decomposition occur simultaneously, all the time. We see live trees growing, and leaf litter and downed trees rotting in the same forest, for example.
While a portion of the total amount of carbon present on the earth runs through the carbon cycle relatively quickly, transitioning from atmospheric carbon dioxide to plant and animal matter, and back into atmospheric carbon dioxide within hundreds of years, another portion of the carbon is caught up in long-lived and stable carbon sinks. Examples of these stable sinks include subsurface hydrocarbon reservoirs from which oil and gas are produced, and coal formations.
Human Activity Releases Carbon to the Atmosphere
For most of the history of the earth, significant amounts of carbon remained locked up in the subsurface in coal and oil and gas deposits. Since the start of the industrial age, however, humans have harvested and burned these deposits for energy, releasing the carbon compounds that were stored in the coal, oil, and gas deposits back into the atmosphere as carbon dioxide and other gases. Significant amounts of carbon compounds are being released into the atmosphere as a result of human activity, much faster than they would have been released naturally, and this rapid release is the primary cause of currently observed anthropogenic (human-influenced) global warming. The figure to the right depicts carbon dioxide emissions from energy generation sources in the US in 2003.