Introduction
Carbon is the fourth most abundant element in the universe and the biochemical backbone of all life on Earth. It cycles continuously between the atmosphere, ocean, land biosphere, and geological reservoirs through a complex network of biological, chemical, and physical processes operating on timescales from seconds to hundreds of millions of years. The carbon cycle is not a single cycle but a nested hierarchy of overlapping cycles with very different time constants — from the daily photosynthesis-respiration exchange of forests to the multi-million-year geological cycle of volcanic degassing and silicate rock weathering.
The fast carbon cycle (biological cycle) involves the rapid exchange of carbon between the atmosphere, land vegetation, soil, and the ocean surface. Gross terrestrial photosynthesis removes approximately 120 gigatonnes of carbon (GtC) from the atmosphere annually and converts it to organic matter; terrestrial respiration (plants and microbes decomposing organic material) returns approximately 115 GtC per year. The net land carbon sink — the small residual of photosynthesis minus respiration — absorbs approximately 2–3 GtC per year, roughly 25% of current human CO₂ emissions. The ocean surface exchanges approximately 90 GtC per year with the atmosphere, currently absorbing approximately 2.5–3 GtC per year as a net sink, again ~25% of human emissions.
The slow carbon cycle (geological cycle) involves the weathering of silicate rocks (a powerful CO₂ sink), the burial and lithification of organic matter to form fossil fuels and carbonate rocks (a very slow sink), and the return of CO₂ to the atmosphere through volcanic degassing (source). Natural geological carbon fluxes are approximately 0.1–0.3 GtC per year. Human fossil fuel combustion and land use change now emit approximately 10 GtC per year — 100 times the natural geological flux — effectively injecting fossil-derived carbon into the fast biological cycle at a rate the system has no precedent for managing. The result: atmospheric CO₂ has risen from ~280 ppm (pre-industrial) to over 420 ppm (2023), a ~50% increase in ~270 years, accumulating carbon faster than either the land biosphere or ocean can absorb it.
Key Terms
The annual accounting of carbon fluxes into and out of the atmosphere. For 2022: ~10.2 GtC/year emitted by fossil fuels + ~1.5 GtC/year from land use change = ~11.7 GtC/year gross emissions; ~3.3 GtC/year absorbed by ocean + ~3.5 GtC/year by land biosphere = ~6.8 GtC/year total sinks; residual ~4.7 GtC/year remaining in atmosphere → CO₂ concentration increase of ~2.4 ppm/year. Published annually by the Global Carbon Project, a consortium of climate research institutes.
The continuous record of atmospheric CO₂ concentration at Mauna Loa Observatory, Hawaii, begun by Charles David Keeling in 1958. The Keeling Curve shows both the long-term upward trend in CO₂ (from 315 ppm in 1958 to >420 ppm in 2023) and the regular seasonal oscillation (~6–8 ppm amplitude) caused by the Northern Hemisphere's annual photosynthesis cycle (CO₂ drops in Northern Hemisphere spring/summer as vegetation grows; rises in fall/winter as vegetation senesces and decomposes). The Keeling Curve is arguably the most important environmental dataset ever collected.
Any reservoir that absorbs more carbon from other reservoirs than it releases. Current major carbon sinks: terrestrial biosphere (~3.5 GtC/year net uptake, concentrated in boreal forests and regrowing Northern Hemisphere forests); ocean (~3.3 GtC/year, primarily through CO₂ dissolution and biological pump). Sinks are not static — warming reduces ocean CO₂ solubility (Henry's Law: warmer water holds less dissolved gas), and if terrestrial respiration increases faster than photosynthesis (as some regions warm and dry), land sinks could weaken or become sources.
The process by which photosynthetic organisms in the ocean surface (phytoplankton) fix dissolved CO₂ into organic matter, which then sinks to the deep ocean when the organisms die, effectively transporting carbon from the atmosphere-ocean surface to the deep ocean. The biological pump exports ~10 GtC/year from the surface to the deep ocean, of which ~1 GtC/year reaches the seafloor and is buried in sediments. Without the biological pump, atmospheric CO₂ would be ~200 ppm higher than it currently is. Ocean warming and acidification threaten to weaken the biological pump.
The geological thermostat: CO₂ from the atmosphere reacts with silicate minerals (e.g., CaSiO₃ + CO₂ → CaCO₃ + SiO₂) in a weathering reaction that removes CO₂ from the atmosphere. At higher CO₂ and temperature (which increases precipitation and chemical reaction rates), weathering rates increase, removing more CO₂ — a negative feedback that stabilises CO₂ on geological timescales (millions of years). This thermostat is why Earth has maintained habitable temperatures for 4 billion years despite a steadily brightening Sun (the "faint young Sun" paradox). On human timescales, it is far too slow (1–10 Myr) to mitigate modern CO₂ emissions.