Burning fossil carbon and weakening sinks add carbon faster than land and oceans remove it, increasing the atmospheric store.
The global carbon cycle links the atmosphere, oceans, living organisms, soils and rocks. A store is an amount held in a component; a flux is a transfer per unit time. A sink has net carbon uptake over the stated period, while a source has net release. A large store is not automatically a current sink.
Photosynthesis transfers atmospheric carbon into biomass. Respiration, decomposition and fire return it. Carbon dioxide dissolves in the ocean and can also return to the air. Marine organisms move carbon through food webs, and some is buried in sediment. These exchanges operate on different timescales.
Fossil-fuel combustion rapidly transfers carbon from long-term geological stores to the atmosphere. Cement manufacture also releases carbon dioxide through the chemical breakdown of limestone, in addition to fuel use. Livestock, rice cultivation, waste and fossil-fuel systems contribute methane; fertiliser use contributes nitrous oxide.
Deforestation releases carbon as biomass burns or decomposes and reduces future uptake by vegetation. Draining peat exposes stored organic matter to oxygen, accelerating decomposition and carbon release. Land management can therefore increase sources and weaken sinks at the same time.
Land and oceans absorb part of human carbon dioxide emissions, but not all of them. Atmospheric concentration rises while inputs exceed removals. Natural gross exchanges can be much larger than the human addition yet remain closer to balance; comparing gross natural flows with net human emissions is misleading.
Step by step
Locate the store
Identify fossil carbon, biomass, soil, ocean or atmosphere.
Change a transfer
Explain the combustion, decomposition or uptake affected by people.
Assess the net balance
Compare total additions to removals from the atmosphere.
Worked example: A simple store-and-flow calculation
In an illustrative system, natural inputs and removals are each 100 units per year. People add 6 units while extra uptake removes 3. The atmospheric store grows by 3 units per year, even though the natural gross flows are much larger. These numbers demonstrate accounting, not measured global fluxes.
Watch out for this
Forests must always be carbon sinks because they contain much carbon.
A forest is a carbon store. Whether it is a net sink or source depends on growth, respiration, decomposition and disturbance during the period.
Check your understanding
A drained peatland releases more carbon dioxide while vegetation uptake falls. What changes?
- Only the colour of the land surface.
- The atmospheric store must shrink because peat contains carbon.
- A source strengthens and a sink weakens, increasing net atmospheric input.