Carbon Cycle

The carbon cycle moves carbon among the atmosphere, oceans, land biosphere, and geological reservoirs, and its perturbation by fossil-fuel use drives climate change.

Definition

The global cycling of carbon among atmospheric, oceanic, terrestrial, and geological reservoirs through chemical and biological processes.

Scope

This topic covers the chemical and biological exchanges of carbon, including photosynthesis and respiration, ocean uptake and the carbonate system, soil and vegetation storage, and the geological reservoir, together with the anthropogenic perturbation that has raised atmospheric carbon dioxide.

Core questions

• What are the main carbon reservoirs and the fluxes between them?
• How do oceans and land take up anthropogenic carbon dioxide?
• Why is there no natural sink large enough to absorb all emissions?
• How does the carbon cycle couple to climate?

Key theories

Earth-system carbon budget

Carbon moves among well-defined reservoirs through photosynthesis, respiration, ocean exchange, and weathering; analysis of the budget shows that natural sinks slow but cannot fully offset anthropogenic emissions.

Mechanisms

Photosynthesis fixes atmospheric carbon dioxide into organic carbon, returned by respiration and decomposition; the ocean absorbs carbon dioxide through the carbonate system and the biological pump; and over long timescales silicate weathering and sediment burial regulate the atmosphere. Human emissions exceed the rate at which these sinks can respond, so carbon accumulates in the air.

Clinical relevance

The carbon cycle is the central framework for understanding rising atmospheric carbon dioxide, climate change, and ocean acidification, and for evaluating carbon sinks and mitigation.

History

Keeling's continuous carbon dioxide measurements from 1958 revealed the relentless atmospheric rise, and Earth-system synthesis at the turn of the century quantified the global carbon budget and its human perturbation.

Key figures

• Charles David Keeling
• Paul Falkowski

Seminal works

• falkowski2000
• schlesinger2013

Frequently asked questions

Where does most anthropogenic carbon dioxide end up?

Roughly half remains in the atmosphere, with the rest taken up by the oceans and the land biosphere; none of these sinks can absorb it all.