Carbon Cycle Explained

The Fast Carbon Cycle

The fast carbon cycle operates over timescales of days to centuries and involves exchanges between the atmosphere, ocean surface, and terrestrial ecosystems. On land, photosynthesis removes approximately 120 billion tonnes of carbon from the atmosphere each year as plants convert CO2 and water into organic compounds using solar energy. This carbon enters the terrestrial food web, passing through herbivores, predators, and decomposers before eventually returning to the atmosphere through respiration and decay.

Soil represents the largest terrestrial carbon reservoir, holding roughly 1,500 billion tonnes in the top meter alone. Dead plant material that resists rapid decomposition accumulates as soil organic matter, where it can persist for decades to millennia depending on temperature, moisture, and soil chemistry. Warming temperatures accelerate microbial decomposition, potentially releasing stored soil carbon as CO2, a positive feedback that could amplify warming.

The ocean surface exchanges roughly 90 billion tonnes of carbon with the atmosphere annually. CO2 dissolves in seawater through gas exchange, a process enhanced by cold temperatures and wind mixing. Cold, CO2-rich surface water at high latitudes sinks to the deep ocean through thermohaline circulation, transporting dissolved carbon away from atmospheric contact for centuries. This solubility pump removes about 2 billion tonnes of anthropogenic carbon per year.

Marine biology drives the biological pump, in which phytoplankton fix dissolved CO2 through photosynthesis, forming organic matter that sinks as dead organisms and fecal pellets. Some of this organic carbon reaches the deep ocean floor, where it can be buried in sediments and removed from the active carbon cycle for millions of years.

The Slow Carbon Cycle

The slow geological carbon cycle operates over millions of years. Chemical weathering of silicate minerals consumes CO2 dissolved in rainwater as carbonic acid. The resulting dissolved bicarbonate ions are carried by rivers to the ocean, where marine organisms use them to build calcium carbonate shells. When these organisms die, their shells accumulate on the seafloor, eventually forming limestone and other carbonate rocks.

Subduction of ocean crust at tectonic boundaries carries carbonate rock into the mantle, where heat and pressure release CO2 that returns to the atmosphere through volcanic eruptions. Volcanoes emit roughly 0.15 to 0.26 billion tonnes of CO2 per year, a tiny fraction of the 36+ billion tonnes from human activities. The slow carbon cycle acts as a geological thermostat because weathering rates increase with temperature, removing more CO2 when climate is warm.

Over hundreds of millions of years, some organic carbon is buried in sediments before decomposing, forming coal, oil, and natural gas. These fossil fuels represent solar energy captured by photosynthesis long ago and locked in geological storage.

Human Disruption

Fossil fuel combustion reverses millions of years of geological carbon storage in a geological instant. Humans transfer approximately 9.5 billion tonnes of carbon per year from geological reservoirs to the atmosphere. Deforestation adds another 1.2 billion tonnes annually. Total emissions exceed the capacity of natural sinks, with the ocean absorbing about 2.5 billion tonnes per year and the terrestrial biosphere about 3.1 billion tonnes, leaving roughly 5.1 billion tonnes to accumulate in the atmosphere.

The airborne fraction has remained at about 44 percent over 60 years, though natural sinks may weaken as warming progresses. A weakening of sinks would accelerate atmospheric CO2 increase for any given emission rate.

Ocean Carbon Chemistry

When CO2 dissolves in seawater, it forms carbonic acid that dissociates into bicarbonate and hydrogen ions. The increased hydrogen ion concentration lowers ocean pH, a process called ocean acidification. Since pre-industrial times, surface ocean pH has dropped from 8.21 to 8.10, a 30 percent increase in acidity. This threatens calcium carbonate-building organisms as lower pH reduces carbonate mineral saturation.

The ocean's CO2 absorption capacity decreases as it acidifies because buffer chemistry becomes less effective. Under high-emission scenarios, the ocean could lose 15 to 20 percent of its buffering capacity by 2100, leaving more emissions in the atmosphere.