Carbon cycle
The continuous exchange of carbon between atmosphere, vegetation, soil, ocean and geological reservoirs.
Carbon moves between reservoirs through photosynthesis, respiration, decomposition, combustion and dissolution. Fluxes are large and mostly balanced, so the net imbalance created by human activity is what changes the atmospheric stock.
This framing is why a tonne stored is not the same as a tonne not emitted. A removal must be held long enough to matter against the residence time of the avoided emission, which is the reasoning behind durability requirements.
Frequently asked questions
Why does residence time matter?
Because carbon that returns to the atmosphere within years provides little climate benefit compared with lasting storage.
Which reservoir is largest?
Geological and oceanic reservoirs dwarf the atmosphere, but their exchange rates are slow.