Chapter 01 · Climate & transitionClimate & Greenhouse Gas Emissions
Carbon cycle
Definition
The set of processes by which carbon atoms move between Earth's reservoirs — the atmosphere, oceans, terrestrial biosphere, soils and rocks — through fluxes such as photosynthesis, respiration, decomposition, ocean exchange, weathering and volcanism. Fast (biological) cycling operates over hours to centuries; the slow (geological) cycle moves carbon between rocks, sediments, ocean and atmosphere over millions of years.
References
Reservoirs and fluxes; sources vs sinks; seasonal and geological cycling.
Reservoir sizes; human disruption via fossil-fuel combustion; sinks definition.
Overview
What it means
The carbon cycle is the reference frame for climate change: natural fluxes are large but roughly balanced, while fossil-fuel burning and land-use change add carbon one-way, hundreds of times faster than the slow cycle can remove it, so CO₂ accumulates in the atmosphere and ocean (driving warming and ocean acidification).
Natural sinks — oceans and land ecosystems — currently absorb roughly half of human-caused emissions, a subsidy whose strength is eroding as ecosystems are stressed.
How it is used
Climate science quantifies sources and sinks to construct the global carbon budget; carbon-accounting and offset methodologies distinguish fossil (slow-cycle) carbon from biogenic (fast-cycle) carbon; nature-based-solution policy rests on enhancing fast-cycle sinks.
Why it matters
Every climate target is ultimately a statement about rebalancing the carbon cycle; understanding which fluxes are fast, slow or effectively permanent defines what "removal" and "neutralisation" can credibly mean.