Atmospheric carbon cycle

The atmospheric carbon cycle accounts for the exchange of gaseous carbon compounds, primarily carbon dioxide (CO₂), 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.^[2] Atmospheric concentrations of CO₂ remain stable over longer timescales only when there exists a balance between these two flows. Methane (CH₄), Carbon monoxide (CO), and other human-made compounds are present in smaller concentrations and are also part of the atmospheric carbon cycle.^[3]

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 CO₂ and CH₄ concentrations.^[4] As of year 2019, annual emissions grew to 10 GtC/year, with a cumulative total of about 450 GtC injected into the cycle.^[5] The terrestrial and ocean sinks have thus far absorbed half of the added carbon, and half has remained in the atmosphere primarily as CO₂. Assuming the growth trend in emissions continues, the CO₂ concentration is on a path to at least double by the latter half of this century.^[6]

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,^[7] the recent rise in carbon concentrations has caused substantial global heating and ocean acidification.^[8] Such effects are generally projected to accelerate further until net emissions are stabilized and reduced.^[6]