The carbon cycle moves carbon atoms between the atmosphere, living organisms, oceans, soil, and rock through five main processes: photosynthesis, respiration, decomposition, combustion, and ocean absorption. These steps work together to keep carbon flowing through Earth’s systems, though human activity has thrown the balance off. Atmospheric CO2 now sits at about 426 parts per million, up from roughly 280 ppm before the Industrial Revolution.
Step 1: Photosynthesis Pulls Carbon From the Air
The cycle begins when plants, algae, and phytoplankton absorb carbon dioxide from the atmosphere and use sunlight to convert it into sugars. This process, photosynthesis, is the primary way carbon leaves the atmosphere and enters the living world. Plants on land and microscopic phytoplankton in the ocean split CO2 apart, keep the carbon to build their cells, and release oxygen as a byproduct.
Phytoplankton alone are responsible for roughly half the world’s oxygen production, which gives some sense of how much carbon they pull from the air. On land, forests are the heaviest lifters. Every leaf on every tree is essentially a small carbon-capturing factory, using energy from the sun to stitch carbon atoms into glucose molecules that fuel growth, build wood, and produce fruit and seeds. This stored carbon stays locked in living tissue until one of the next steps in the cycle releases it.
Step 2: Respiration Returns Carbon to the Atmosphere
Every organism that breathes reverses a small piece of what photosynthesis accomplished. When animals, plants, fungi, and bacteria break down sugars for energy, they strip away the carbon atoms and combine them with oxygen, producing CO2 as waste. In animals, this happens inside cells through a chain of chemical reactions that gradually liberates the energy stored in the bonds of fats, sugars, and proteins. The final product of that process is a fully oxidized carbon atom bound to two oxygen atoms: carbon dioxide.
In your body, that CO2 travels through the bloodstream to the lungs, where it diffuses across thin capillary walls into the air sacs and gets exhaled. Plants respire too, burning some of the sugars they made during the day to power their own metabolism at night. Respiration is continuous and global. Billions of organisms exhale CO2 around the clock, making it the single largest natural pathway for returning carbon to the atmosphere.
Step 3: Decomposition Recycles Carbon Through Soil
When organisms die, their carbon doesn’t vanish. Bacteria, fungi, and other decomposers break down dead leaves, fallen trees, animal carcasses, and waste products, consuming the organic carbon and releasing CO2 and methane in the process. This is essentially respiration performed by microbes on someone else’s carbon.
Not all of that carbon escapes into the air, though. A significant fraction gets transformed into stable organic compounds that bind to soil particles and stay underground for decades, centuries, or longer. Soils store more carbon than all other terrestrial ecosystems, making them one of the planet’s most important carbon reservoirs. How much carbon stays locked in soil versus how much gets breathed out by microbes depends on temperature, moisture, and the efficiency of the microbial community. Researchers describe this balance using a concept called carbon use efficiency: the share of carbon that microbes channel into building their own bodies (which stays in the soil) versus the share they burn off as CO2. Higher efficiency means more soil carbon storage.
In cold or waterlogged environments like peatlands and permafrost, decomposition slows dramatically, and carbon can accumulate for thousands of years. In warm, moist tropical soils, decomposition runs fast and carbon cycles through more quickly.
Step 4: Combustion Releases Stored Carbon Rapidly
Combustion is the fast track. Whether it’s a wildfire burning through a forest or a power plant burning coal, the result is the same: carbon that was locked away in organic material gets oxidized and released as CO2 in hours or minutes instead of years.
Natural fires have always been part of the carbon cycle, returning carbon from vegetation to the atmosphere and clearing space for new growth. But human combustion has changed the math dramatically. In 2024, fossil fuel burning released approximately 10.3 billion tons of carbon into the atmosphere. Land use changes like deforestation added another 1.3 billion tons, bringing total human-caused emissions to about 11.6 billion tons of carbon per year. That’s carbon that was buried underground as coal, oil, and gas for millions of years, now re-entering the atmosphere on a timeline of decades. Deforestation contributes by both burning trees directly and removing the photosynthetic capacity that would have pulled carbon back out of the air.
This is the step that has pushed the carbon cycle out of balance. The other four steps evolved together over billions of years into a rough equilibrium. Fossil fuel combustion adds a massive new source of atmospheric carbon that the natural sinks can’t fully absorb.
Step 5: Ocean Absorption Buffers the Atmosphere
The ocean acts as a carbon sponge, capturing and storing about 25% of the CO2 humans emit each year. It does this through two linked mechanisms. First, CO2 from the atmosphere dissolves directly into surface seawater, much like carbonation dissolving into a glass of soda. Cold water absorbs more CO2 than warm water, so polar oceans are particularly effective sinks. As surface currents carry this carbon-rich water toward the deep ocean, it can stay sequestered for centuries.
Second, the biological pump moves carbon downward through living organisms. Phytoplankton near the surface photosynthesize just like land plants, pulling dissolved CO2 into their cells. When they die or get eaten, their carbon-rich remains sink toward the ocean floor. Some of this material gets consumed and respired at intermediate depths, but a portion reaches the deep ocean and seafloor sediments, where it can remain locked away for thousands of years. The ocean’s total carbon absorption slows the rate of climate change significantly, though it comes at a cost: dissolved CO2 makes seawater more acidic, which threatens coral reefs, shellfish, and other marine life.
The Geological Cycle Runs on a Longer Clock
Beyond these five main steps, a much slower geological cycle moves carbon over millions of years. When marine organisms with calcium carbonate shells die and settle on the ocean floor, their remains eventually compress into limestone. Tectonic forces can push these rocks deep underground, where heat and pressure release the carbon as CO2 through volcanic eruptions. On the surface, rainwater slowly dissolves silicate rocks in a process called chemical weathering, which pulls CO2 out of the atmosphere and washes it into rivers and eventually the ocean.
Recent research has complicated the old assumption that rock weathering only removes CO2. When ancient organic carbon trapped in rocks gets exposed at the surface through erosion, it oxidizes and releases about 68 megatons of carbon per year, a flux that may rival or even exceed the CO2 that silicate weathering draws down. Carbon’s residence time in these geological reservoirs exceeds a million years, compared to roughly three years for a given CO2 molecule in the atmosphere and less than a year for carbon in ocean plankton.
Why the Cycle Is Currently Out of Balance
For most of Earth’s history, the carbon entering the atmosphere through respiration, decomposition, volcanic activity, and natural fires was roughly matched by the carbon leaving it through photosynthesis, ocean absorption, and rock weathering. That balance no longer holds. In 2024, of the 11.6 billion tons of carbon humans emitted, about 7.9 billion tons accumulated in the atmosphere, meaning natural sinks absorbed the rest but couldn’t keep pace with the total output. Atmospheric CO2 grew by 3.73 parts per million that year alone.
The five steps of the cycle haven’t changed. Carbon still moves through the same pathways it always has. What’s changed is the volume and speed. Combustion of fossil fuels has taken carbon that was safely stored underground for geological timescales and dumped it into an atmosphere-ocean system that cycles carbon on timescales of years to centuries. The sinks are working harder than ever, but the source has grown faster.