Carbon Cycle
The carbon cycle is the continuous movement of carbon between the atmosphere, oceans, soil, rock, and living things. Burning fossil fuels releases carbon that was locked underground for millions of years, and it enters the atmosphere far faster than natural processes remove it.
Carbon moves through the cycle on two timescales. The fast cycle runs over years to centuries: plants take up CO₂ through photosynthesis, animals and microbes return it through respiration and decay, and the ocean surface exchanges it with the air. The slow cycle runs over millions of years, as carbon is buried in sediment, turns into rock or fossil fuel, and returns through volcanic activity and weathering.
Fossil fuel combustion, cement production, and land clearing add roughly 40 billion tonnes of CO₂ to the atmosphere each year. Land and ocean sinks absorb a little more than half of that. The remainder stays in the air, which is why atmospheric concentrations keep rising.
For a company that has to report, the cycle explains two rules that otherwise look arbitrary. First, fossil and biogenic carbon are counted separately, because carbon released from recently grown biomass was drawn out of the air recently and fossil carbon was not. Second, a removal only counts as a removal if the carbon stays out of the atmosphere, which is why storage duration and reversal risk sit at the center of every credible removal claim. Gravity applies both rules when it calculates an inventory, so biogenic and fossil quantities stay on separate lines.
Frequently asked questions
What is the carbon cycle? +
The carbon cycle is the movement of carbon between the atmosphere, oceans, soil, rock, and living things. Photosynthesis, respiration, decay, ocean exchange, and geological burial all move carbon between these stores.
How do human activities change the carbon cycle? +
Burning fossil fuels, producing cement, and clearing land move carbon into the atmosphere faster than land and ocean sinks remove it. Around half of annual emissions stay in the atmosphere, raising CO₂ concentrations.
Related terms
Greenhouse Gas (GHG)
Greenhouse gases are atmospheric gases that trap infrared radiation and warm the Earth's surface. The six main GHGs covered by the Kyoto Protocol are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆). The Kigali Amendment added nitrogen trifluoride (NF₃).
Greenhouse Effect
The greenhouse effect is the warming that happens when gases in the atmosphere absorb infrared radiation leaving the Earth's surface and re-emit part of it back downward. Without it the planet's average surface temperature would be about 33°C colder.
Carbon Sink
A carbon sink is any reservoir that absorbs more carbon than it releases. Oceans, forests, and soils are the main natural sinks, and together they remove a little over half of the CO₂ that human activity emits each year.
Carbon Sequestration
Carbon sequestration is the long-term storage of carbon dioxide so that it stays out of the atmosphere. It happens biologically, in trees and soils, and geologically, by injecting captured CO₂ into deep rock formations.
FLAG (Forest, Land, and Agriculture)
FLAG is the SBTi sector guidance for companies with significant emissions from forestry, land use, and agriculture. It provides target-setting methods for land-related emissions and removals.
Carbon Accounting
Carbon accounting is the systematic process of measuring, recording, and reporting the greenhouse gas (GHG) emissions produced by an organization, product, or activity. It follows standardized methodologies — most commonly the GHG Protocol — to quantify emissions across Scope 1 (direct), Scope 2 (purchased energy), and Scope 3 (value chain) categories, producing an auditable inventory that underpins disclosure, reduction planning, and regulatory compliance.