How the Carbon Cycle Works: Reservoirs, Fluxes and Turnover Times

Carbon is the backbone of every living molecule, from sugars and fats to proteins and DNA. It is also present in the air as carbon dioxide, dissolved in the oceans, locked in rock, and stored in soil. The carbon cycle describes how it moves among these places, and how fast each move happens.

Understanding the cycle matters for ecology and for climate. Living things pull carbon out of the air and return it, while geology stores it for millions of years. When people burn fossil fuels, they move carbon from the slow geological store into the fast cycle, and the effects last far beyond a single season.

Carbon moves in loops

The carbon cycle has two speeds. The fast cycle runs through living things, the air and the surface ocean, often within years to decades. The slow cycle runs through rock, sediments and the deep ocean, and it can take millions of years to complete.

Each movement of carbon between places is called a flux. A reservoir is the place where carbon is stored. The cycle is defined by the sizes of the reservoirs and the speeds of the fluxes that connect them.

The main reservoirs and their sizes

The table below gives approximate amounts of carbon in the major reservoirs. The figures are rounded and vary between published estimates, but the order of magnitude is what matters.

Reservoir	Approximate carbon	Typical turnover
Sedimentary rock (carbonates, kerogen)	Tens of millions of gigatonnes	Millions of years
Deep ocean	Tens of thousands of gigatonnes	Centuries to millennia
Soil and detritus	Roughly 1,500 to 2,500 gigatonnes	Decades to centuries
Atmosphere (as CO2)	Roughly 900 gigatonnes at present	About a decade
Surface ocean	Roughly 900 gigatonnes	Years to decades
Land vegetation	Roughly 450 to 550 gigatonnes	Years to decades

The atmosphere is one of the smallest reservoirs, yet it has a large effect because it exchanges carbon quickly with the land and ocean. A small change in its size changes the balance of the whole system.

Photosynthesis pulls carbon in

Photosynthesis is the main entry point of carbon into the living cycle. Plants, algae and some bacteria take carbon dioxide from the air or water and combine it with water, using energy from sunlight, to make sugars. The reaction releases oxygen as a by-product.

Globally, photosynthesis fixes a very large amount of carbon each year. Roughly half of that fixed carbon is respired back to the atmosphere by the plants themselves, and the rest enters the food webs of consumers and decomposers. The balance between fixation and release decides whether a forest or ocean is a net sink or a net source of carbon.

Respiration sends carbon back

Respiration returns carbon to the atmosphere. Plants, animals, fungi and bacteria break down sugars and other organic molecules to release energy, and they exhale carbon dioxide or release it into the water. This happens every second, in every living cell.

Decomposition is respiration by decomposers. When leaves, wood or animal bodies fall to the ground, fungi and bacteria break them down. Some of the carbon goes back to the air, and some is stored in soil as humus, a slowly decaying material that can stay in the ground for centuries.

The ocean as a carbon sink

The ocean is one of the largest carbon reservoirs on the surface. Carbon dioxide dissolves into seawater at the surface, where it reacts to form carbonic acid, bicarbonate and carbonate ions. Phytoplankton take up carbon through photosynthesis, and when they die, some of the carbon sinks into deeper water.

This biological pump moves carbon from the surface to the deep ocean, where it can stay for centuries before mixing back up. The ocean also absorbs carbon dioxide physically, through the difference in partial pressure between air and water. The rate of absorption depends on temperature, salinity and the chemistry of the water. Warmer water holds less dissolved gas, so the same rise in air carbon dioxide produces a smaller uptake in a warmer ocean.

Slow carbon: rock, weathering and volcanoes

The slow part of the cycle begins when carbon dioxide reacts with silicate rocks in a process called weathering. Rain carries dissolved carbon dioxide into the ground, where it reacts with minerals and produces bicarbonate. Rivers carry the bicarbonate to the sea, where marine organisms use it to build shells and skeletons made of calcium carbonate.

When these organisms die, their shells settle and become limestone over long periods. This carbon can remain locked in rock for millions of years. Volcanic eruptions and metamorphism release some of it back to the air. Because volcanic release is slow, the long-term balance works as a thermostat: when the climate warms, weathering tends to speed up and draw more carbon out of the air, and when it cools, weathering slows.

Keeling's record: measuring the rise

Since 1958, scientists at Mauna Loa Observatory in Hawaii have measured atmospheric carbon dioxide almost continuously. This record, called the Keeling Curve, shows a steady rise from about 315 parts per million at the start to above 420 parts per million by the mid-2020s. It also shows a seasonal wave, because Northern Hemisphere forests draw down carbon in summer and release it in winter.

The pre-industrial level, before large-scale fossil fuel burning, was about 280 parts per million. The rise matches the carbon released by burning coal, oil and gas, and by clearing forests.

Tracing carbon by its isotopes

Carbon has isotopes, and scientists use their ratios to identify where carbon came from. Carbon-12 is the common form, and carbon-13 is a rarer stable form. Carbon-14 is radioactive and decays with a half-life of about 5,700 years, so it is almost absent from fossil fuels, which are millions of years old.

Burning fossil fuels therefore dilutes the atmosphere's carbon-14 share, a shift known as the Suess effect. Measuring this dilution, along with the carbon-13 ratio, lets researchers estimate how much of the rise in air carbon dioxide comes from fossil sources rather than from natural exchange between the land, ocean and air.

Human changes to the cycle

People affect the carbon cycle in two main ways. Burning fossil fuels moves carbon that was locked in rock for millions of years into the atmosphere within decades. Land-use change, such as clearing forests for farming, reduces the amount of carbon stored in vegetation and soil and often releases what was stored.

The two effects add together. Each year, the atmosphere gains more carbon than it would under natural conditions, and the ocean and land absorb only part of the extra. The remainder stays in the air and affects climate for a long time.

Key takeaways

Carbon cycles through a fast loop of living things, air and surface ocean, and a slow loop through rock and the deep ocean.

Reservoir sizes and turnover times matter as much as the fluxes between them. The atmosphere is small but quickly exchanged, while rock holds most of the carbon but changes very slowly.

Photosynthesis and respiration are the fast engines of the cycle, and the ocean's biological and physical pumps move carbon into deep storage.

Human fossil fuel burning and land-use change move carbon from slow storage into the fast cycle, which is why the atmospheric record has risen steadily since the 1950s.