Earth's carbon cycle

The Earth's carbon cycle is represented by reservoirs of carbon (atmosphere, land, oceans) which exchange carbon with each other. Global carbon reservoirs are measured in terms of petagrams of carbon. A petagram is 10**15 grams, or one trillion tonnes of carbon.

Global carbon cycle including reservoirs (PgC) and fluxes (PgC/yr)
Global carbon cycle including reservoirs (PgC) and fluxes (PgC/yr)
Figure: Global carbon cycle including reservoirs (PgC) and fluxes (PgC/yr). Black numbers and arrows indicate reservoir mass and exchange fluxes estimated for the time prior to the Industrial Era beginning around 1750. Red arrows and numbers indicate annual ‘anthropogenic’ fluxes averaged over the 2000–2009 time period. By convention, a positive cumulative change means that a reservoir has gained carbon since 1750. Individual gross fluxes and their changes since the beginning of the Industrial Era have typical uncertainties of more than 20%, while their differences (Net land flux and Net ocean flux in the figure) are determined from independent measurements with a much higher accuracy.

There are huge carbon stores in rocks and sediments which exchange carbon with the atmosphere, oceans, sea floor, and land. Volcanic emissions, chemical weathering, erosion and sediment formation are the primary mechanisms. These geologic carbon exchange processes occur over very long periods of 10,000 years or longer. The natural carbon exchange between the geologic carbon reservoirs and the atmosphere, oceans, land and sea floor are relatively small, less than 0.3 petagrams of carbon per year (PgC/yr) or 300 billion tonnes of carbon per year. The most important are volcanism and sedimentation.

Other carbon exchange processes are large and rapid, involving years to decades to millennia. The reservoirs of carbon that are affected by these rapid exchange processes are the atmosphere, the oceans, especially the surface ocean, and on land in vegetation, soils and freshwaters.

Antarctic ice core records from the past 800 thousand years show atmospheric CO2 varied from 180 ppm during glacial (cold) up to 300 ppm during interglacial (warm) periods. About 420 thousand years ago, interglacial CO2 values increased from 240 to 260 ppm to 270 to 290 ppm. During the 7000 years prior to 1750, atmospheric CO2 from ice cores shows a very slow increase from 260 ppm to 280 ppm.

The Paleoclimate recordGlacial CO2 concentration (ppm)Interglacial CO2 concentration (ppm)
800,000-420,000 years ago180Between 240 and 260
420,000-Preindustrial-Between 270 and 290
7,000 years before 1750AD-Gradual increase from 260 to 290
Preindustrial mean-278

Anthropogenic carbon

Global anthropogenic CO2 budget since 1750
Global anthropogenic CO2 budget since 1750
Since the beginning of the Industrial Era about 1750, humans have been producing energy by burning fossil fuels (coal,oil and gas) which release increasing amounts of carbon dioxide (CO2) into the atmosphere. The amount of fossil fuel CO2 emitted to the atmosphere can be estimated with an accuracy of about 5 to 10% for recent decades from statistics of fossil fuel use. Total cumulative emissions between 1750 and 2011 amount to 375 ± 30 PgC, including a contribution of 8 PgC from the production of cement.

Atmospheric carbon reservoirsPrior to 1750(PgC)Industrial Era(PgC)Total(PgC)
Carbon dioxide (CO2)589240828

The second major source of anthropogenic CO2 emissions to the atmosphere is caused by changes in land use (mainly deforestation), which causes a net reduction in land carbon storage. Estimation of the effect of land use change on atmospheric CO2 requires knowledge of changes in land area as well as estimates of the carbon stored per area before and after the land use change. Other effects such as the decomposition of soil organic matter after land use change, also have to be taken into account.

In 1750 the estimated cropland and pasture area amounted to 7.5 to 9 million square km. It is estimated that since 1750, anthropogenic land use changes have resulted into about 50 million square km being used for cropland and pasture, corresponding to about 38% of the total ice-free land area. The cumulative net CO2 emissions from land use changes between 1750 and 2011 are estimated at 180 ± 80 PgC.

Anthropogenic sources of CO2Cumulative 1750 to 2011 (PgC)
Fossil fuel combustion and cement375 ± 30
Net land use changes180 ± 80

The rising atmospheric CO2 content creates a disequilibrium in the exchange fluxes between the atmosphere and the land and oceans. The rising CO2 concentration implies a rising atmospheric CO2 partial pressure (pCO2) that induces a globally averaged net air-to-sea flux and thus an ocean sink for CO2. On land, the rising atmospheric CO2 concentration fosters photosynthesis via the CO2 fertilization effect.

Atmospheric carbon reservoirs

In the atmosphere, CO2 is the dominant trace gas containing carbon. In 2011 its concentration was 390.5 ppm, which corresponds to a mass of 828 PgC. Additional carbon containing trace gases include methane, carbon monoxide, and still smaller amounts of hydrocarbons, black carbon aerosols and organic compounds.

Atmospheric carbon reservoirs (in 2011)Size (PgC)
Carbon dioxide (CO2)828
Methane (CH4)~3.7
Carbon monoxide (CO)0.2
Hydrocarbons, black carbon aerosols and organic compoundstrace

Terrestrial carbon reservoirs

The terrestrial biosphere reservoir contains carbon in living vegetation and in dead organic matter in litter and soils. There is an additional amount of old soil carbon in wetland soils and in permafrost soils.

Land carbon reservoirsSize (PgC)
Living vegetation450 to 650
Dead organic matter in litter and soils1500 to 2400
Old soil carbon in wetland soils300 to 700
Old soil carbon in permafrost soils~1700

Atmosphere-land carbon exchange

The atmosphere exchanges CO2 with the land and the oceans. The most important land process for removing CO2 from the atmosphere is plant photosynthesis (Gross Primary Production). The process takes CO2, water and light as inputs and produces sugars and oxygen (CO2 + H2O + light -> sugar + O2). The sugar is then used by plants in a variety of organic chemical processes to produce plant structures and tissues. Ultimately carbon fixed in plant tissues becomes litter and soil carbon. Soil carbon is released back into the atmosphere by plant, soil microbial and animal respiration and by sporadic fires.

Plant respiration is the process of oxidizing organic compounds to produce CO2, water and energy (organic compounds + O2 -> CO2 + H2O + energy). Because CO2 uptake by photosynthesis occurs only during the growing season, whereas CO2 release by respiration occurs nearly year-round, the greater land mass in the Northern Hemisphere where growth is seasonal is responsible for the annual global ‘sawtooth’ pattern in atmospheric CO2.

The anthropogenic effect has resulted in increased CO2 in the atmosphere. More CO2 in the atmosphere results in more CO2 being taken up by the biosphere (CO2 fertilization) and concomitantly, more CO2 being released back into the atmosphere by respiration and fires. The pre-industrial land carbon balance was very close with a net flux of about 1.7 PgC/yr being transferred annually to land. The anthropogenic effect has increased that by 2.6 PgC/yr to 4.2 PgC/yr. However, this is not enough to offset increased anthropogenic emissions of 8.9 PgC/yr due to fossil fuel combustion and land use change. The disequilibrium results in an transfer of CO2 to the atmosphere.

Land-atmosphere carbon exchangeNatural(PgC/yr)Anthropogenic(PgC/yr)Total(PgC/yr)CO2 sink or source
Photosynthesis-108.9(-14.1 est)-123Sink
Respiration and fires107.2(11.6 est)118.7Source
Fossil fuel combustion and cement-7.87.8Source
Land use change-1.11.1Source
Fresh water outgassing1.0-1.0Source
Rock weathering-0.3--0.3Sink

Oceanic carbon reservoirs

In the ocean, carbon is available predominantly as Dissolved Inorganic Carbon (DIC) comprised of carbonic acid, bicarbonate and carbonate ions. In addition, the ocean contains a pool of Dissolved Organic Carbon (DOC) of which a substantial fraction remains dissolved for a very long time, with a turnover time of 1000 years or longer.

The marine biota, predominantly phytoplankton and other microorganisms, represent a small organic carbon pool, which is turned over very rapidly in days to a few weeks.

Oceanic carbon reservoirs (at present)Size (PgC)
Dissolved Inorganic Carbon (DIC) carbonic acid, bicarbonate and carbonate ions ~38,000
Dissolved Organic Carbon (DOC)~700
Marine biota, phytoplankton and other microorganisms~3
Marine methane hydrates1500 to 7000

Within the ocean, carbon is cycled through different forms by processes called marine pumps, the most important of which are the biological and carbonate pumps.

The marine biological pump is a process by which biological organisms form shells from calcium and carbonate. In forming shells, two bicarbonate ions are split into one carbonate and one dissolved CO2 molecules, which increases the partial CO2 pressure in surface waters and drives release of CO2 into the atmosphere.

The marine carbonate pump is a process in which shells created in surface waters by oceanic microorganisms sink and are re-mineralized back into DIC and calcium ions in the ocean depths. Only a small fraction (~0.2 PgC yr) of the carbon exported by biological and carbonate pumps from the surface reaches the sea floor to be captured in sediments for millennia and longer.

Atmosphere-ocean carbon exchange

Atmospheric CO2 dissolves into the ocean surface through gas exchange. This exchange is driven by the partial CO2 pressure difference between the air and the sea. In the pre-industrial era, the balance between the amount of CO2 being absorbed by the oceans and the CO2 released back into the atmosphere from the oceans was very close, with an estimated net of 0.7 PgC/yr released into the atmosphere from the oceans.

Ocean-atmosphere carbon exchange (at present)Natural(PgC/yr)Anthropogenic(PgC/yr)Total(PgC/yr)CO2 sink or source
Absorption-60(-20 est)-80Sink
Ventilation60.7(17.7 est)78.4Source

The anthropogenic effect has resulted in more CO2 being exchanged with the oceans with a net balance of 1.6 PgC/yr being transferred from the atmosphere to the oceans. The net anthropogenic effect is that since 1750 the oceans have become a carbon sink absorbing significant quantities of CO2 from the atmosphere.

Land-ocean carbon exchange

A significant amount of terrestrial carbon (1.7 PgC/yr) is transported from soils to streams and ultimately to rivers. A fraction of this carbon is outgassed as CO2 by rivers and lakes to the atmosphere, a fraction is buried in freshwater organic sediments and the remaining amount (~0.9 PgC/yr) is delivered by rivers to the coastal ocean as dissolved inorganic carbon, dissolved organic carbon and particulate organic carbon.

Net atmosphere and land + ocean carbon exchange

The net response to rising CO2 is to increase cumulative land and ocean uptake, regardless of the time scale. The increased takeup of CO2 by land and the oceans is not sufficient to compensate for fossil fuel combustion and land use change.

Atmospheric carbon exchangePrior to 1750(PgC/yr)Currently(PgC/yr)
Land net-1.7-4.2
Oceans net0.7-1.6
Fossil fuels-7.7
Land use change-1.1
Net to atmosphere-1.03.1


Methane (CH4) absorbs more heat (long wavelength radiation) per molecule than CO2 making it is a more effective greenhouse gas, but methane remains less than 10 years in the atmosphere compared to 100 years for CO2. Global methane mass is measured as teragrams of CH4 (TgCH4). One teragram is a billion tonnes of CH4.

The Earth contains vast reserves of methane, in the form of natural gas or as hydrates. The global size of these reservoirs is difficult to estimate.

Land and ocean methane reservoirs (at present)Size (TgCH4)
Gas reserves511,000-1,513,000
Permafrost hydrates530,000
Ocean hydrates2,000,000-8,000,000

A very large geological stock, globally 1500 to 7000 PgC, (with "low confidence in estimates") exists in the form of frozen hydrate deposits (‘clathrates’) in shallow ocean sediments and on the slopes of continental shelves, and permafrost soils. These CH4 hydrates are stable under conditions of low temperature and high pressure. Warming or changes in pressure could render some of these hydrates unstable with a potential release of CH4 to the overlying soil/ocean and/or

Global methane cycle
Global methane cycle
Figure Global methane cycle Annual fluxes in TgCH4/yr estimated for the time period 2000–2009 for the atmosphere and three geological reservoirs (hydrates on land and in the ocean floor and gas reserves). Black arrows denote ‘natural’ fluxes, that is, fluxes that are not directly caused by human activities since 1750, red arrows anthropogenic fluxes, and the light brown arrow denotes a combined natural + anthropogenic flux. The atmospheric inventories have been calculated using a conversion factor of 2.7476 TgCH per ppb. The assumed preindustrial annual mean globally averaged CH4 concentration was 722 ± 25 ppb taking the average of the Antarctic Law Dome ice core observations and the measurements from the GRIP ice core in Greenland. The atmospheric inventory in the year 2011 is based on an annual globally averaged CH4 concentration of 1803 ± 4 ppb in the year 2011. It is the sum of the atmospheric increase between 1750 and 2011 (in red) and of the pre-industrial inventory (in black). The average atmospheric increase each year, also called growth rate, is based on a measured concentration increase of 2.2 ppb/yr during the time period 2000–2009.

The sources of CH4 at the surface of the Earth include
  • Natural emissions of fossil CH4 and calcium carbonate (CaCO) of sea floor sediments
  • Emissions caused by leakages from fossil fuel extraction and use (natural gas, coal and oil industry).

CH4 is also generated by incomplete burning of fossil fuels and plant biomass (both natural and anthropogenic fires. Methane emissions also are generated from wetlands, fresh water bodies, and from termites. There are also very small emissions from the ocean. Anthropogenic methane emissions are generated by rice paddy agriculture, ruminant livestock, landfills, man-made lakes and wetlands and waste treatment.

Atmospheric CH4 is removed primarily by photochemistry, through atmospheric chemistry reactions with the OH radicals. Other smaller removal processes of atmospheric CH4 take place in the stratosphere through reaction with chlorine and oxygen radicals, by oxidation in well aerated soils, and possibly by reaction with chlorine in the marine boundary layer.

SourceMethane processesNatural(TgCH4/yr)Anthropogenic(TgCH4/yr)Total(TgCH4/yr)
WetlandsMethanogenesis177 to 188-177 to 188
FreshwatersMethanogenesis8 to 73-8 to 73
TermitesCellulose digestion2 to 22-2 to 22
Ocean clathratesDestabilization2 to 9-2 to 9
Geological sourcesVolcanism9 to 47-9 to 47
AtmosphereStratospheric hydroxl reaction-16 to -84--16 to -84
AtmosphereTropospheric hydroxyl reaction-454 to -617--454 to -617
AtmosphereTropospheric chlorine reaction-13 to -17--13 to -17
AtmosphereSoil oxidation-9 to -47--9 to -47
BiomassBurningtbdtbd32 to 39
DrillingFossil fuels-85 to 10585 to 105
Landfills and wasteMethanogenesis-67 to 9067 to 90
Ruminant livestockDigestion-87 to 9487 to 94
Wetland rice cultivationMethanogenesis-33 to 4033 to 40

Atmospheric methane observations from ice cores show methane concentration increased by about 100 ppb between 5000 years ago and around 1750. During the Industrial Era the amount of methane in the atmosphere has more than doubled.

Atmospheric carbon reservoirPrior to 1750(TgCH4)Industrial Era(TgCH4)Total(TgCH4)

Important conclusions from AR5 about the carbon cycle

With a "very high level of confidence", the increase in CO2 emissions from fossil fuel burning and from land use change are the dominant cause of the observed increase in atmospheric CO2 concentration. About half of the emissions have remained in the atmosphere (240 ± 10 PgC) since 1750. The rest has been removed from the atmosphere by sinks and stored in the natural carbon cycle reservoirs. The ocean reservoir stored 155 ± 30 PgC. Vegetation biomass and soils not affected by land use change stored 160±90 PgC.

Anthropogenic carbon reservoirsSize (PgC)
Atmospheric CO2240 ± 10
Oceans155 ± 30
Land (vegetation and soils)160 ± 90

During the past 800,000 years prior to 1750, atmospheric CO2 varied from 180 ppm during glacial (cold) up to 300 ppm during interglacial (warm) periods. This is "well established" from multiple ice core measurements.

Ice core records indicate CO2 values of ~180 to 200 ppm during glacial (cold) intervals. Interglacial CO2 values were 240 to 260 ppm until 420,000 years ago, after which they increased to 270 to 290 ppm.

Ice cores recovered from the Antarctic ice sheet reveal that the concentration of atmospheric CO2 at the Last Glacial Maximum (LGM) at 21,000 years ago was about one third lower than during the subsequent interglacial (Holocene) period which started about 11,700 years ago.

During the 7000 years prior to 1750, atmospheric CO2 from ice cores shows a very slow increase from 260 ppm to 280 ppm. The contribution of CO2 emissions from early anthropogenic land use is "unlikely" sufficient to explain the CO2 increase prior to 1750.

Atmospheric CH4 from ice cores increased by about 100 ppb between 5000 years ago and 1750. About as "likely as not", this increase can be attributed to early human activities involving livestock, human-caused fires and rice cultivation.

IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis Chapter 6 Carbon and Other Biogeochemical Cycles