The atmospheric carbon cycle accounts for the exchange of gaseous carbon compounds, primarily carbon dioxide (), between Earth's atmosphere, the oceans, and the terrestrial biosphere. It is one of the faster components of the planet's overall carbon cycle, supporting the exchange of more than 200 billion tons of carbon (i.e. gigatons carbon or GtC) in and out of the atmosphere throughout the course of each year. Atmospheric concentrations of remain stable over longer timescales only when there exists a balance between these two flows. Methane (), Carbon monoxide (CO), and other man-made compounds are present in smaller concentrations and are also part of the atmospheric carbon cycle.
Human activities, primarily the extraction and burning of fossil carbon from Earth's lithosphere starting with the industrial revolution, have disturbed the previous balance of the atmospheric carbon cycle and have been mostly responsible for the ongoing rapid growth in and concentrations. As of year 2019, annual emissions grew to 10 GtC/year, with a cumulative total of about 450 GtC injected into the cycle. The terrestrial and ocean sinks have thus far absorbed half of the added carbon, and half has remained in the atmosphere primarily as . Assuming the growth trend in emissions continues, the concentration is on a path to at least double by the latter half of this century.
The atmospheric carbon cycle also strongly influences Earth's energy balance through the greenhouse effect, and affects the acidity or alkalinity of the planet's surface waters and soils. Despite comprising less than 0.05% of all atmospheric gases by mole fraction, the recent rise in carbon concentrations has caused substantial global heating and ocean acidification. Such effects are generally projected to accelerate further until net emissions are stabilized and reduced.
Relevant gases
The atmosphere is one of the Earth's major carbon reservoirs and holds approximately 720 gigatons of carbon as of year 2000. The concentration of mostly carbon-based greenhouse gases has increased dramatically since the onset of the industrial era. This makes an understanding of the carbon component of the atmosphere highly important. The two main carbon greenhouse gases are methane and carbon dioxide.
Methane
Methane (CH4) is one of the more potent greenhouse gases and is mainly produced by the digestion or decay of biological organisms. It is considered the second most important greenhouse gas, yet the methane cycle in the atmosphere is currently only poorly understood. The amount of methane produced and absorbed yearly varies widely.
Large stores of methane can be found in the form of methane ice under permafrost and on continental shelves. Additional methane is produced by the anaerobic decay of organic material and is produced in organisms' digestive tracts, soil, etc. Natural methane production accounts 10-30% of global methane sources.
Anthropogenic methane is produced in various ways, e.g. by raising cattle or through the decay of trash in landfills. It is also produced by several industrial sources, including the mining and distribution of fossil fuels. More than 70% of atmospheric methane comes from biogenic sources. Methane levels have risen gradually since the onset of the industrial era, from ~700 ppb in 1750 to ~1775 ppb in 2005.
Methane can be removed from the atmosphere through a reaction of the photochemically produced hydroxyl free radical (OH). It can also leave the atmosphere by entering the stratosphere, where it is destroyed, or by being absorbed into soil sinks. Because methane reacts fairly quickly with other compounds, it does not stay in the atmosphere as long as many other greenhouse gases, e.g. carbon dioxide. It has an atmospheric lifetime of about eight years. This keeps the concentration of methane in the atmosphere relatively low and is the reason that it currently plays a secondary role in the greenhouse effect to carbon dioxide, despite the fact that it produces a much more powerful greenhouse effect per volume.