Carbon cycle

The carbon cycle constitutes a fundamental biogeochemical process involving the exchange of carbon among Earth's biosphere, pedosphere, geosphere, hydrosphere, and atmosphere. Other significant biogeochemical cycles include the nitrogen cycle and the water cycle. Carbon is a primary constituent of biological compounds and a major component of numerous rocks, such as limestone. This cycle encompasses a series of events crucial for sustaining life on Earth, detailing the movement of carbon as it is recycled and reused throughout the biosphere, alongside long-term processes of carbon sequestration (storage) in and release from carbon sinks. In 2024, the global average atmospheric carbon dioxide concentration reached an unprecedented 422.7 parts per million (ppm).

The carbon cycle is a part of the biogeochemical cycle where carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth. Other major biogeochemical cycles include the nitrogen cycle and the water cycle. Carbon is the main component of biological compounds as well as a major component of many rocks such as limestone. The carbon cycle comprises a sequence of events that are key to making Earth capable of sustaining life. It describes the movement of carbon as it is recycled and reused throughout the biosphere, as well as long-term processes of carbon sequestration (storage) to and release from carbon sinks. At 422.7 parts per million (ppm), the global average atmospheric carbon dioxide has set a new record high in 2024.

To characterize the dynamics of the carbon cycle, a distinction is drawn between the fast and slow carbon cycles. The rapid cycle is also identified as the biological carbon cycle. Fast cycles typically conclude within years, facilitating the transfer of substances from the atmosphere to the biosphere and subsequently back to the atmosphere. Conversely, slow or geological cycles, also termed the deep carbon cycle, can span millions of years, moving substances through Earth's crust among rocks, soil, oceans, and the atmosphere.

Anthropogenic activities have disrupted the carbon cycle for many centuries, primarily through modifications in land use and the extraction and combustion of ancient organic carbon reserves (coal, petroleum, and natural gas). By 2020, atmospheric carbon dioxide had increased by nearly 52% over pre-industrial levels, contributing to global warming. This elevated carbon dioxide has also resulted in a reduction of oceanic pH, fundamentally altering marine chemistry. Carbon dioxide is essential for photosynthesis.

Principal Components

The carbon cycle was initially described by Antoine Lavoisier and Joseph Priestley, and subsequently popularized by Humphry Davy. Currently, the global carbon cycle is typically categorized into the following major reservoirs of carbon (also known as carbon pools), which are interconnected by various exchange pathways:

• Atmosphere
• Terrestrial Biosphere
• Ocean, encompassing dissolved inorganic carbon and both living and non-living marine biota.
• Sediments, including fossil fuels, freshwater systems, and non-living organic material.
• Earth's Interior (mantle and crust). These internal carbon stores interact with other components through geological processes.

Carbon exchanges among these reservoirs are driven by a variety of chemical, physical, geological, and biological processes. The ocean contains the largest active carbon pool near the Earth's surface. The natural fluxes of carbon among the atmosphere, ocean, terrestrial ecosystems, and sediments are relatively balanced, suggesting that carbon levels would remain approximately stable without human intervention.

Atmosphere

Carbon in the Earth's atmosphere exists predominantly in two forms: carbon dioxide and methane. Both gases absorb and retain heat within the atmosphere, contributing to the greenhouse effect. Methane exhibits a greater greenhouse effect per unit volume compared to carbon dioxide; however, its atmospheric concentrations are significantly lower, and its atmospheric lifetime is shorter. Consequently, carbon dioxide contributes more substantially to the global greenhouse effect than methane.

Carbon dioxide is primarily removed from the atmosphere through photosynthesis, subsequently entering the terrestrial and oceanic biospheres. It also dissolves directly from the atmosphere into aquatic bodies (e.g., oceans, lakes) and dissolves in precipitation as raindrops descend. When dissolved in water, carbon dioxide reacts with water molecules to form carbonic acid, which contributes to ocean acidity. This carbonic acid can then be absorbed by rocks through weathering processes, or it can acidify other surfaces it contacts or be transported into the ocean.

Over the past two centuries, human activities have increased the amount of atmospheric carbon by nearly 50% as of 2020, primarily in the form of carbon dioxide. This increase results from both the modification of ecosystems' capacity to extract atmospheric carbon dioxide and the direct emission of carbon dioxide, for instance, through the combustion of fossil fuels and the manufacturing of concrete.

Over the distant future, approximately 2 to 3 billion years from now, the absorption rate of atmospheric carbon dioxide into the soil through the carbonate–silicate cycle is projected to accelerate. This acceleration is attributed to anticipated changes in the Sun's aging process, specifically an increase in its luminosity, which will likely enhance surface weathering. Consequently, the majority of atmospheric carbon dioxide will eventually be sequestered into the Earth's crust as carbonate. When atmospheric carbon dioxide concentrations decline below approximately 50 parts per million (with species-specific tolerances), C₃ photosynthesis will cease. This critical threshold is predicted to be reached around 600 million years from the present, although model projections exhibit some variability.

Approximately 1.1 billion years in the future, the Earth's oceans are expected to evaporate, a phenomenon that will very likely halt plate tectonics due to the absence of water for lubrication. The subsequent cessation of volcanic activity, which typically releases carbon dioxide, is projected to terminate the carbon cycle within a timeframe of 1 to 2 billion years from the present.

Terrestrial Biosphere

The terrestrial biosphere encompasses the organic carbon present in all living and deceased land-dwelling organisms, alongside carbon sequestered within soils. Approximately 500 gigatons of carbon are stored above ground in plants and other biota, whereas soils contain an estimated 1,500 gigatons. While the majority of carbon within the terrestrial biosphere exists as organic carbon, roughly one-third of soil carbon is stored in inorganic compounds, such as calcium carbonate. Organic carbon constitutes a fundamental component of all Earth's organisms. Autotrophs assimilate carbon dioxide from the atmosphere, converting it into organic carbon, while heterotrophs acquire carbon through the consumption of other organisms.

Carbon uptake within the terrestrial biosphere exhibits diurnal and seasonal cycles, a characteristic directly attributable to its dependence on biotic factors. This cyclical pattern is distinctly observable in CO₂ measurements, particularly evident in the Keeling curve. The effect is most pronounced in the Northern Hemisphere, which possesses a greater landmass compared to the Southern Hemisphere, thereby offering more extensive areas for ecosystems to assimilate and release carbon.

Carbon exits the terrestrial biosphere through multiple mechanisms and across varying temporal scales. The rapid release of organic carbon into the atmosphere occurs via combustion or respiration. Alternatively, carbon can be transported to the ocean through fluvial systems or remain sequestered in soils as inert carbon. Soil-stored carbon can persist for millennia before being mobilized into rivers by erosion or emitted into the atmosphere through soil respiration. From 1989 to 2008, soil respiration exhibited an approximate annual increase of 0.1%. In 2008, the global CO₂ emissions from soil respiration totaled roughly 98 billion tonnes, a quantity approximately three times greater than the annual carbon input from human fossil fuel combustion (though this does not signify a net transfer from soil to atmosphere, as respiration is largely balanced by carbon inputs to soil). Several hypotheses explain this trend, with the most probable being that rising temperatures have accelerated the decomposition rates of soil organic matter, thereby augmenting CO₂ flux. The duration of carbon sequestration in soil is contingent upon local climatic conditions and is consequently subject to alteration during climate change.

Ocean

Conceptually, the ocean is delineated into a surface layer, where water frequently interacts with the atmosphere (on daily to annual timescales), and a deep layer, situated beneath the typical mixed layer depth of a few hundred meters or less, where the interval between successive atmospheric contacts can span centuries. Dissolved inorganic carbon (DIC) in the surface layer undergoes rapid exchange with the atmosphere, thereby maintaining equilibrium. The deep ocean harbors significantly more carbon, primarily due to its substantially larger volume and partly because its DIC concentration is approximately 15% higher. It represents the world's largest reservoir of actively cycled carbon, holding 50 times more than the atmosphere. However, the timescale required for the deep ocean to achieve equilibrium with the atmosphere extends over hundreds of years, as carbon exchange between these two layers, propelled by thermohaline circulation, proceeds slowly.

Carbon primarily enters the ocean via the dissolution of atmospheric carbon dioxide, a minor portion of which transforms into carbonate. Additionally, dissolved organic carbon can be introduced into the ocean through fluvial systems. Organisms convert this carbon into organic carbon through photosynthesis, subsequently facilitating its exchange within the food chain or its precipitation into the ocean's deeper, carbon-dense strata as decomposed soft tissue or calcium carbonate shells. This carbon then circulates within these layers for extended durations before either settling as sediment or ultimately returning to the surface waters through thermohaline circulation.

Oceans exhibit an alkaline nature, currently maintaining a pH range of 8.1 to 8.2. However, the escalating concentration of atmospheric CO₂ drives a shift in oceanic pH towards neutrality, a phenomenon termed ocean acidification. The absorption of CO₂ by oceans represents a critical mechanism for carbon sequestration. Projections indicate that the anticipated rate of pH decline may impede the biological precipitation of calcium carbonates, thereby diminishing the ocean's capacity to assimilate CO§4_5§.

Geosphere

The geological segment of the carbon cycle functions at a considerably slower pace compared to other global carbon cycle components. This segment significantly influences atmospheric carbon levels and, consequently, global temperatures.

The majority of Earth's carbon is inertly sequestered within the lithosphere. A substantial portion of the carbon residing in the Earth's mantle has been present since the planet's formation. Additionally, some carbon originated from the biosphere as deposited organic matter. Within the geosphere, approximately 80% of stored carbon comprises limestone and related compounds, which originate from the sedimentation of calcium carbonate found in marine organism shells. The remaining 20% exists as kerogens, formed through the sedimentation and subsequent burial of terrestrial organisms under conditions of elevated heat and pressure. Organic carbon within the geosphere can persist for millions of years.

Carbon exits the geosphere through various mechanisms. During the metamorphism of carbonate rocks subducted into the Earth's mantle, carbon dioxide is liberated. This CO2 can then be discharged into the atmosphere and ocean via volcanic activity and geological hotspots. Furthermore, anthropogenic activities contribute to carbon removal through the direct extraction of kerogens, which constitute fossil fuels. Following extraction, these fossil fuels are combusted to generate energy, thereby releasing their stored carbon into the atmosphere.

Dynamic Classifications

The carbon cycle encompasses both a rapid and a protracted cycle. The rapid cycle primarily functions within the biosphere, whereas the protracted cycle is associated with geological formations. The rapid, or biological, cycle can conclude within a few years, facilitating carbon transfer from the atmosphere to the biosphere and subsequently back to the atmosphere. Conversely, the protracted, or geological, cycle can span millions of years, extending into the Earth's mantle and involving carbon movement through the Earth's crust among rocks, soil, oceans, and the atmosphere.

The rapid carbon cycle entails relatively short-duration biogeochemical interactions occurring between the environment and living organisms within the biosphere. This includes the translocation of carbon between the atmosphere and both terrestrial and marine ecosystems, in addition to soils and seafloor sediments. The rapid cycle incorporates annual processes such as photosynthesis and decadal cycles involving vegetative development and decomposition. The responses of the rapid carbon cycle to anthropogenic activities will largely dictate many of the immediate consequences of climate change.

The slow (or deep) carbon cycle encompasses medium to long-term geochemical processes integral to the rock cycle. The oceanic-atmospheric carbon exchange can span centuries, while rock weathering processes may extend over millions of years. Oceanic carbon precipitates onto the seafloor, potentially forming sedimentary rock and subsequently undergoing subduction into the Earth's mantle. Orogenic processes facilitate the return of this geological carbon to the Earth's surface. Here, rocks undergo weathering, releasing carbon back into the atmosphere via degassing and into the ocean through fluvial transport. Additional geological carbon re-enters the ocean through the hydrothermal emission of calcium ions. Annually, the slow carbon cycle circulates between 10 and 100 million tonnes of carbon, including the direct atmospheric release of geological carbon as carbon dioxide from volcanic activity. Nevertheless, this amount constitutes less than one percent of the carbon dioxide introduced into the atmosphere by the combustion of fossil fuels.

Processes of the Fast Carbon Cycle

Terrestrial Carbon within the Hydrological Cycle

The movement of terrestrial carbon within the hydrological cycle is detailed as follows:

1. Atmospheric particulates serve as cloud condensation nuclei, thereby facilitating cloud genesis.
2. As raindrops descend, they assimilate both organic and inorganic carbon via particle scavenging and the adsorption of organic vapors.
3. Combustion and volcanic eruptions generate highly condensed polycyclic aromatic molecules, specifically black carbon, which are subsequently released into the atmosphere alongside greenhouse gases like CO₂.
4. Terrestrial flora assimilate atmospheric CO₂ via photosynthesis, with a portion subsequently returned to the atmosphere through respiration. Lignin and cellulose collectively constitute up to 80% of the organic carbon found in forests and 60% in grasslands.
5. Litterfall and root-derived organic carbon integrate with sedimentary material, forming organic soils where both plant-derived and petrogenic organic carbon are stored and biochemically transformed by microbial and fungal communities.
6. Water acquires dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) from plant surfaces and settled aerosols as it traverses forest canopies (throughfall) and descends along plant trunks/stems (stemflow). Biogeochemical alterations ensue as water infiltrates soil solutions and groundwater reservoirs. Overland flow manifests when soils reach complete saturation or when precipitation rates exceed the soil's infiltration capacity.
7. Organic carbon originating from the terrestrial biosphere and in situ primary production undergoes decomposition by microbial communities in lotic systems, complemented by physical decomposition processes such as photo-oxidation. This activity generates a flux of CO₂ from rivers to the atmosphere, which is comparable in magnitude to the annual carbon sequestration by the terrestrial biosphere. Terrestrially-derived macromolecules, including lignin and black carbon, are catabolized into smaller constituents and monomers, ultimately transforming into CO§4_{5§, metabolic intermediates, or new biomass.}
8. Lakes, reservoirs, and floodplains generally accumulate substantial quantities of organic carbon and sediments. However, their water columns often exhibit net heterotrophy, leading to a net atmospheric CO₂ flux approximately one order of magnitude lower than that from rivers. Furthermore, methane production is characteristically elevated in the anoxic sediments of these floodplains, lakes, and reservoirs.
9. Primary productivity is commonly augmented in river plumes, attributable to the export of fluvial nutrients. Despite this, estuarine waters globally contribute as a source of CO₂ to the atmosphere.
10. Coastal marshes serve as both reservoirs and exporters of blue carbon. Globally, marshes and wetlands are hypothesized to exhibit an atmospheric CO₂ flux comparable to that of rivers.
11. Continental shelves and the pelagic ocean generally assimilate atmospheric CO₂.
12. The marine biological pump sequesters a minor yet substantial proportion of the absorbed CO₂ as organic carbon within marine sediments.

Terrestrial Runoff into the Ocean

Riverine transport constitutes the primary connection between terrestrial and marine ecosystems, facilitating the influx of erosive, land-derived substances into oceanic systems. The regulation of ecosystem carbon and dioxygen (O₂) pools is collectively managed by material and energy exchanges between the terrestrial biosphere and the lithosphere, in conjunction with organic carbon fixation and oxidation processes.

As the principal conduit connecting these reservoirs, riverine transport facilitates the movement of net primary productivity, predominantly as dissolved organic carbon (DOC) and particulate organic carbon (POC), from terrestrial to oceanic environments. During this transport, a fraction of DOC undergoes rapid redox reactions, leading to its return to the atmosphere, a phenomenon termed 'carbon degassing' between land-atmosphere storage layers. The residual DOC and dissolved inorganic carbon (DIC) are subsequently exported to the ocean. In 2015, global riverine export fluxes for inorganic and organic carbon were estimated at 0.50–0.70 Pg C y⁻¹ and 0.15–0.35 Pg C y⁻¹, respectively. Conversely, POC exhibits the capacity for long-term burial within sediments, with the annual global terrestrial-to-oceanic POC flux estimated at 0.20 (+0.13,-0.07) Gg C y⁻¹.

The Ocean's Biological Pump

The oceanic biological pump represents the biologically mediated sequestration of carbon from the atmosphere and terrestrial runoff into the deep ocean and seafloor sediments. This mechanism is not attributable to a singular process but rather emerges from the cumulative effect of multiple processes, each contributing to biological carbon transfer. Annually, the pump facilitates the transfer of approximately 11 billion tonnes of carbon into the ocean's interior. Without the biological pump, atmospheric CO₂ concentrations would be approximately 400 ppm greater than current levels.

The majority of carbon integrated into organic and inorganic biological material originates at the sea surface, subsequently initiating its descent towards the ocean floor. The deep ocean receives most of its nutrient supply from the upper water column through the sinking of marine snow. Marine snow comprises a heterogeneous mixture of deceased or senescent organisms, microbial biomass, fecal matter, sand, and other inorganic constituents.

The biological pump facilitates the conversion of dissolved inorganic carbon (DIC) into organic biomass and its subsequent transport, in particulate or dissolved forms, into the deep ocean. During photosynthesis, phytoplankton assimilate inorganic nutrients and carbon dioxide, simultaneously releasing dissolved organic matter (DOM) and serving as a food source for herbivorous zooplankton. Larger zooplankton, such as copepods, produce fecal pellets that can be reingested, sink, or coalesce with other organic detritus to form larger, more rapidly sinking aggregates. DOM undergoes partial consumption and respiration by bacteria; the remaining refractory DOM is subsequently advected and mixed into the deep sea. Both DOM and aggregates transported into the deep water are consumed and respired, thereby reintroducing organic carbon into the vast deep-ocean DIC reservoir.

An individual phytoplankton cell exhibits a sinking rate of approximately one meter per day. Considering the average ocean depth of about four kilometers, these cells would require over a decade to reach the seafloor. Nevertheless, processes such as coagulation and expulsion within predator fecal pellets lead to the formation of aggregates from these cells. These aggregates possess sinking rates that are orders of magnitude higher than those of individual cells, enabling them to reach the deep ocean within days.

Approximately one percent of particles originating from the surface ocean ultimately reach the seabed, where they are either consumed, respired, or incorporated into sediments. The cumulative outcome of these processes is the removal of organic carbon from the surface and its conversion back to DIC at deeper levels, thereby sustaining a DIC gradient between the surface and deep ocean. Thermohaline circulation facilitates the return of deep-ocean DIC to the atmosphere over millennial timescales. Carbon sequestered within sediments can undergo subduction into the Earth's mantle, remaining stored for millions of years as an integral component of the slow carbon cycle.

Regulatory Role of Viruses

Viruses function as key regulators of the fast carbon cycle by influencing material cycling and energy transfer within food webs and the microbial loop. Globally, viruses contribute an average of 8.6% to the Earth's ecosystem carbon cycle. Their impact is notably lower in marine ecosystems (1.4%) compared to terrestrial (6.7%) and freshwater (17.8%) environments. Over the last two millennia, human activities and climate change have progressively modified the regulatory capacity of viruses in ecosystem carbon cycling. This alteration has been especially pronounced in the past two centuries, coinciding with rapid industrialization and subsequent population expansion.

Processes of the Slow Carbon Cycle

The slow, or deep, carbon cycle represents a crucial process, although its mechanisms are less comprehensively understood than the comparatively rapid carbon fluxes through the atmosphere, terrestrial biosphere, oceans, and geosphere. This deep carbon cycle is intrinsically linked to carbon's movement across the Earth's surface and into the atmosphere. Without this process, carbon would perpetually accumulate in the atmosphere, reaching exceptionally high concentrations over extended geological timescales. Consequently, by facilitating the return of carbon to the Earth's interior, the deep carbon cycle is indispensable for sustaining the planetary conditions essential for life.

Moreover, this process holds significant importance due to the immense volumes of carbon it circulates throughout the planet. Investigations into basaltic magma composition and measurements of volcanic carbon dioxide emissions indicate that the Earth's mantle contains approximately a thousand times more carbon than its surface. Direct observation and physical exploration of deep-Earth carbon processes are inherently challenging, given that the lower mantle spans depths from 660 to 2,891 km, and the core extends from 2,891 to 6,371 km below the Earth's surface. Consequently, definitive knowledge about carbon's role in the deep Earth remains limited. Nevertheless, various lines of evidence, frequently derived from laboratory simulations of deep-Earth conditions, have elucidated potential mechanisms for carbon's descent into the lower mantle and the specific forms it assumes under the extreme temperatures and pressures characteristic of that layer. Additionally, methodologies such as seismology have advanced our comprehension of carbon's potential existence within the Earth's core.

Carbon within the Lower Mantle

Carbon primarily enters the mantle as carbonate-rich sediments situated on oceanic crustal tectonic plates, which transport carbon into the mantle during subduction. While comprehensive knowledge regarding carbon circulation within the mantle, particularly in the deep Earth, is limited, numerous studies have aimed to enhance our understanding of the element's migration and speciation within this region. For example, a 2011 study revealed that carbon cycling extends into the lower mantle. This research involved analyzing rare, super-deep diamonds from a location in Juina, Brazil, and determined that the bulk composition of certain diamond inclusions corresponded to the anticipated outcome of basalt melting and crystallization under lower mantle temperatures and pressures. Consequently, the study's conclusions suggest that fragments of basaltic oceanic lithosphere serve as the primary transport mechanism for carbon into the Earth's deep interior. These subducted carbonates are capable of interacting with lower mantle silicates, ultimately leading to the formation of super-deep diamonds, such as the one examined.

Nevertheless, carbonates descending into the lower mantle can undergo various transformations beyond diamond formation. In 2011, carbonates were exposed to conditions mimicking those found at a depth of 1800 km within the Earth, firmly placing them in the lower mantle. This experimental setup led to the formation of magnesite, siderite, and multiple forms of graphite. Further experiments and petrological observations corroborate this finding, suggesting that magnesite constitutes the most stable carbonate phase across much of the mantle, primarily due to its elevated melting temperature. Accordingly, researchers have inferred that carbonates undergo reduction as they penetrate the mantle, subsequently stabilizing at depth within environments characterized by low oxygen fugacity. Magnesium, iron, and other metallic compounds function as buffering agents throughout this process. The detection of reduced, elemental carbon forms, such as graphite, further supports the hypothesis that carbon compounds are reduced during their descent into the mantle.

Polymorphism influences the stability of carbonate compounds at varying depths within the Earth. For instance, laboratory simulations and density functional theory calculations indicate that tetrahedrally coordinated carbonates exhibit maximum stability at depths nearing the core–mantle boundary. A 2015 study further suggests that the intense pressure in the lower mantle induces a transition in carbon bonds from sp₂ to sp₃ hybridised orbitals, leading to carbon forming tetrahedral bonds with oxygen. Unlike CO§45§ trigonal groups, which cannot form polymerisable networks, tetrahedral CO§6_{7§ structures can, thereby increasing carbon's coordination number and causing profound changes in the properties of carbonate compounds within the lower mantle. Specifically, preliminary theoretical investigations propose that elevated pressure enhances carbonate melt viscosity; this reduced mobility of the melts, resulting from increased viscosity, contributes to the formation of extensive carbon deposits deep within the mantle.}

Consequently, carbon can persist in the lower mantle for extended durations; however, significant concentrations of carbon frequently return to the lithosphere. This process, termed carbon outgassing, results from carbonated mantle undergoing decompression melting and from mantle plumes transporting carbon compounds upward to the crust. Carbon undergoes oxidation during its ascent toward volcanic hotspots, where it is subsequently released as CO₂. This phenomenon ensures that the carbon atom aligns with the oxidation state characteristic of basalts erupting in these regions.

Carbon in the core

While the presence of carbon in the Earth's core is recognized, recent investigations propose that substantial quantities of carbon might be stored within this region. Shear (S) waves propagating through the inner core exhibit velocities approximately fifty percent lower than anticipated for typical iron-rich alloys. Given that the core's composition is hypothesized to consist of an alloy of crystalline iron and a minor proportion of nickel, this seismic anomaly suggests the existence of light elements, such as carbon, within the core. Indeed, research employing diamond anvil cells to simulate Earth's core conditions demonstrates that iron carbide (Fe₇C₃) corresponds to the inner core's observed wave speed and density. Consequently, the iron carbide model provides evidence suggesting that the core may contain up to 67% of the Earth's total carbon. Moreover, a separate study revealed that under the pressure and temperature conditions characteristic of the Earth's inner core, carbon dissolved in iron, forming a stable phase with an identical Fe₇C§67§ composition, though possessing a distinct structure from the previously described one. In conclusion, while the precise quantity of carbon potentially sequestered within the Earth's core remains undetermined, contemporary research suggests that the existence of iron carbides can account for certain geophysical observations.

Human influence on fast carbon cycle

Since the onset of the Industrial Revolution, and particularly following the conclusion of World War II, human activities have significantly disrupted the global carbon cycle through the extensive redistribution of carbon from the geosphere. Furthermore, anthropogenic alterations to vegetation and other land uses have persistently modified the natural functional components of the terrestrial biosphere. Synthetic carbon compounds, specifically designed and mass-produced by humans, are known to persist as pollutants in air, water, and sediments for durations ranging from decades to millennia. Climate change itself is intensifying and compelling additional indirect anthropogenic modifications to the carbon cycle, driven by a range of positive and negative feedback mechanisms.

Climate change

Contemporary climate change trends contribute to elevated ocean temperatures and increased acidity, consequently altering marine ecosystems. Additionally, acid rain and contaminated runoff originating from agricultural and industrial activities modify the ocean's chemical composition. These alterations can profoundly impact highly sensitive ecosystems, including coral reefs, thereby regionally diminishing the ocean's capacity to absorb atmospheric carbon and globally reducing oceanic biodiversity.

The dynamic exchanges of carbon between the atmosphere and other Earth system components, collectively termed the carbon cycle, presently provide significant negative (dampening) feedbacks that mitigate the impact of anthropogenic carbon emissions on climate change. Terrestrial and oceanic carbon sinks each presently absorb approximately one-quarter of annual anthropogenic carbon emissions.

Future projections indicate a weakening of these feedbacks, which is anticipated to intensify the impact of anthropogenic carbon emissions on global climate change. Nevertheless, the extent of this weakening remains highly uncertain, as Earth system models forecast diverse land and ocean carbon sequestration rates, even when subjected to identical atmospheric concentration or emission scenarios. Furthermore, Arctic methane emissions, an indirect consequence of anthropogenic global warming, influence the carbon cycle and exacerbate global warming.

Fossil Carbon Extraction and Combustion

The most substantial and rapidly escalating anthropogenic influence on the carbon cycle and biosphere involves the extraction and combustion of fossil fuels, a process that directly transfers carbon from the geosphere into the atmosphere. Additionally, carbon dioxide is generated and emitted during the calcination of limestone, a critical step in clinker production, with clinker serving as an industrial precursor for cement.

By 2020, approximately 450 gigatons of fossil carbon had been extracted, a quantity nearly equivalent to the total carbon stored in Earth's living terrestrial biomass. Contemporary global emission rates into the atmosphere have surpassed the absorption capacities of vegetation and oceans. While these natural sinks have been anticipated and observed to sequester approximately half of the additional atmospheric carbon within a century, their saturation properties, particularly in oceanic sinks, are evolving. Consequently, a significant proportion (20–35%, according to coupled models) of the anthropogenic carbon is projected to persist in the atmosphere for hundreds to thousands of years.

Halocarbons

Halocarbons, though less abundant, are compounds synthesized for various industrial applications, including solvents and refrigerants. Despite their relatively low atmospheric concentrations (measured in parts per trillion), the accumulation of chlorofluorocarbon, hydrofluorocarbon, and perfluorocarbon gases accounts for approximately 10% of the total direct radiative forcing from all long-lived greenhouse gases as of 2019, a figure that includes the forcing from significantly higher concentrations of carbon dioxide and methane. Furthermore, chlorofluorocarbons are implicated in stratospheric ozone depletion. International initiatives, notably the Montreal Protocol and Kyoto Protocol, are actively working to regulate the rapid industrial production and deployment of these environmentally impactful gases. For specific uses, less harmful alternatives, such as hydrofluoroolefins, have been developed and are progressively being implemented.

Land Use Changes

Since the advent of agriculture, human activities have progressively and directly altered the carbon cycle over centuries by modifying the composition of terrestrial biosphere vegetation. Over recent centuries, both direct and indirect anthropogenic land use and land cover change (LUCC) have resulted in biodiversity loss, thereby diminishing ecosystem resilience to environmental stressors and impairing their capacity to sequester atmospheric carbon. More immediately, these changes frequently precipitate the release of carbon from terrestrial ecosystems into the atmosphere.

Deforestation, primarily for agricultural expansion, involves the removal of forests—significant carbon reservoirs—and their replacement, typically with agricultural or urban landscapes. Both of these substitute land cover types store considerably less carbon, leading to a net outcome where a greater quantity of carbon remains in the atmosphere. Nevertheless, these atmospheric and carbon cycle impacts can be deliberately or naturally mitigated through reforestation efforts.

References

References

Carbon Cycle Science Program – an interagency collaborative initiative.

• Carbon Cycle Science Program – an interagency partnership.
• NOAA's Carbon Cycle Greenhouse Gases Group
• Global Carbon Project – an initiative of the Earth System Science Partnership.
• UNEP – The current carbon cycle and its implications for climate change, detailing carbon levels and flows (Archived 15 September 2008).