Carbon continuously cycles through Earth's atmosphere, biosphere, oceans, and geosphere. The main steps include photosynthesis, in which plants and algae absorb CO₂; respiration, in which organisms release CO₂ back into the air; decomposition of organic matter; absorption of CO₂ by the oceans; and the formation and combustion of fossil fuels over geologic timescales. A carbon sink is a natural or artificial reservoir that absorbs more carbon than it releases — forests, oceans, and soils are the major natural sinks. A carbon source, by contrast, releases more carbon than it absorbs; burning fossil fuels is the dominant human-driven source. The difference between sources and sinks determines whether atmospheric CO₂ rises or falls over time.
Oceans play a particularly important role. They act as a major carbon sink, absorbing roughly 25–30% of human-emitted CO₂. The biological carbon pump contributes to this storage: marine organisms fix CO₂ via photosynthesis and, when they die and sink, transport carbon to the deep ocean, sequestering it for long periods. However, the CO₂ that oceans absorb comes at a cost. Dissolving in seawater forms carbonic acid, lowering the pH of the ocean in a process known as ocean acidification. This harms coral reefs, shellfish, and the broader marine food web because many organisms struggle to build calcium carbonate shells in more acidic water.
On land, deforestation disrupts the carbon cycle in two ways. Cutting and burning forests releases stored carbon as CO₂ while simultaneously removing the trees that would otherwise continue absorbing CO₂ through photosynthesis. Protecting and expanding forests is therefore central to maintaining the balance between carbon sources and sinks, alongside reducing emissions at their source.