The atmospheric carbon cycle accounts for the exchange of gaseous

carbon compounds Carbon compounds are defined as chemical substances containing carbon. More compounds of carbon exist than any other chemical element except for hydrogen. Organic carbon compounds are far more numerous than inorganic carbon compounds. In general ...

, primarily

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

(), between Earth's atmosphere, the oceans, and the terrestrial

biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be ...

. It is one of the faster components of the planet's overall

carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...

, supporting the exchange of more than 200 billion tons of carbon (i.e.

gigatons The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States c ...

carbon or GtC) in and out of the atmosphere throughout the course of each year. Atmospheric concentrations of remain stable over longer timescales only when there exists a balance between these two flows.

Methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...

(),

Carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

(CO), and other man-made compounds are present in smaller concentrations and are also part of the atmospheric carbon cycle. Human activities, primarily the extraction and burning of fossil carbon from Earth's lithosphere starting with the

industrial revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...

, have disturbed the previous balance of the atmospheric carbon cycle and have been mostly responsible for the ongoing rapid growth in and concentrations. As of year 2019, annual

emission Emission may refer to: Chemical products * Emission of air pollutants, notably: **Flue gas, gas exiting to the atmosphere via a flue ** Exhaust gas, flue gas generated by fuel combustion ** Emission of greenhouse gases, which absorb and emit radi ...

s grew to 10 GtC/year, with a cumulative total of about 450 GtC injected into the cycle. The terrestrial and ocean

sink A sink is a bowl-shaped plumbing fixture for washing hands, dishwashing, and other purposes. Sinks have a tap (faucet) that supply hot and cold water and may include a spray feature to be used for faster rinsing. They also include a drain to ...

s have thus far absorbed half of the added carbon, and half has remained in the atmosphere primarily as . Assuming the growth trend in emissions continues, the concentration is on a path to at least double by the latter half of this century. The atmospheric carbon cycle also strongly influences

Earth's energy balance Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but ma ...

through the

greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...

, and affects the

acidity or alkalinity A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH ( acidity or basicity) of the solution can be determined visually or spectroscopically by changes in absorption and/or emission properties. Henc ...

of the planet's surface waters and soils. Despite comprising less than 0.05% of all atmospheric gases by

mole fraction In chemistry, the mole fraction or molar fraction (''xi'' or ) is defined as unit of the amount of a constituent (expressed in moles), ''ni'', divided by the total amount of all constituents in a mixture (also expressed in moles), ''n''tot. This ex ...

, the recent rise in carbon concentrations has caused substantial

global heating Global means of or referring to a globe and may also refer to: Entertainment * ''Global'' (Paul van Dyk album), 2003 * ''Global'' (Bunji Garlin album), 2007 * ''Global'' (Humanoid album), 1989 * ''Global'' (Todd Rundgren album), 2015 * Bruno ...

and

ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...

. Such effects are generally projected to accelerate further until net emissions are stabilized and reduced.

Relevant gases

The atmosphere is one of the Earth's major carbon reservoirs and holds approximately 720 gigatons of carbon as of year 2000. The concentration of mostly carbon-based greenhouse gases has increased dramatically since the onset of the

industrial era The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...

. This makes an understanding of the carbon component of the atmosphere highly important. The two main carbon greenhouse gases are methane and carbon dioxide.

Methane

Methane (CH₄) is one of the more potent greenhouse gases and is mainly produced by the digestion or decay of biological organisms. It is considered the second most important greenhouse gas, yet the methane cycle in the atmosphere is currently only poorly understood. The amount of methane produced and absorbed yearly varies widely. Large stores of methane can be found in the form of

methane ice Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...

under permafrost and on continental shelves. Additional methane is produced by the anaerobic decay of organic material and is produced in organisms' digestive tracts, soil, etc. Natural methane production accounts 10-30% of global methane sources. Anthropogenic methane is produced in various ways, e.g. by raising cattle or through the decay of trash in landfills. It is also produced by several industrial sources, including the mining and distribution of fossil fuels. More than 70% of atmospheric methane comes from

biogenic A biogenic substance is a product made by or of life forms. While the term originally was specific to metabolite compounds that had toxic effects on other organisms, it has developed to encompass any constituents, secretions, and metabolites of p ...

sources. Methane levels have risen gradually since the onset of the industrial era, from ~700 ppb in 1750 to ~1775 ppb in 2005. Methane can be removed from the atmosphere through a reaction of the photochemically produced

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

free radical (OH). It can also leave the atmosphere by entering the stratosphere, where it is destroyed, or by being absorbed into soil sinks. Because methane reacts fairly quickly with other compounds, it does not stay in the atmosphere as long as many other greenhouse gases, e.g. carbon dioxide. It has an atmospheric lifetime of about eight years. This keeps the concentration of methane in the atmosphere relatively low and is the reason that it currently plays a secondary role in the greenhouse effect to carbon dioxide, despite the fact that it produces a much more powerful greenhouse effect per volume.

Carbon dioxide

Carbon dioxide () has a large warming effect on global temperatures through the

. Although individual CO₂ molecules have a short

residence time The residence time of a fluid parcel is the total time that the parcel has spent inside a control volume (e.g.: a chemical reactor, a lake, a human body). The residence time of a set of parcels is quantified in terms of the frequency distribution ...

in the atmosphere, it takes an extremely long time for carbon dioxide levels to sink after sudden rises, due to e.g. volcanic eruptions or human activity and among the many long-lasting greenhouse gases, it is the most important because it makes up the largest fraction of the atmosphere. Since the

, the CO₂ concentration in the atmosphere has risen from about 280 ppm to almost 400 ppm. Although the amount of CO₂ introduced makes up only a small portion of the global carbon cycle, carbon dioxide's long residence time makes these emissions relevant for the total carbon balance. The increased carbon dioxide concentration strengthens the greenhouse effect, causing changes to the global

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...

. Of the increased amounts of carbon dioxide that are introduced to the atmosphere each year, approximately 80% are from the combustion of fossil fuels and cement production. The other ~20% originate from land use change and deforestation. Because gaseous carbon dioxide does not react quickly with other chemicals, the main processes that change the carbon dioxide content of the atmosphere involve exchanges with the earth's other carbon reservoirs, as explained in the following sections.

Interactions with other systems

Atmospheric carbon is exchanged quickly between the oceans and the terrestrial biosphere. This means that at times the atmosphere acts as a sink, and at other times as a source of carbon. The following section introduces exchanges between the atmospheric and other components of the global carbon cycle.

Terrestrial biosphere

Carbon is exchanged with varying speed with the terrestrial biosphere. It is absorbed in the form of carbon dioxide by

autotrophs An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...

and converted into

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...

s. Carbon is also released from the biosphere into the atmosphere in the course of biological processes.

Aerobic respiration Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...

converts organic carbon into carbon dioxide and a particular type of

anaerobic respiration Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. In aerobic organisms undergoing re ...

converts it into methane. After respiration, both carbon dioxide and methane are typically emitted into the atmosphere. Organic carbon is also released into the atmosphere during burning. The residence time of carbon in the terrestrial biosphere varies and is dependent on a large number of factors. The uptake of carbon into the biosphere occurs on various time scales. Carbon is absorbed primarily during plant growth. A pattern of increased carbon uptake is observable both over the course of the day (less carbon is absorbed at night) and over the course of the year (less carbon is absorbed in winter). While organic matter in animals generally decays quickly, releasing much of its carbon into the atmosphere through respiration, carbon stored as dead plant matter can stay in the biosphere for as much as a decade or more. Different plant types of plant matter decay at different rates - for example, woody substances retain their carbon longer than soft, leafy material. Active carbon in soils can stay sequestered for up to a thousand years, while inert carbon in soils can stay sequestered for more than a millennium.

Oceans

Large amounts of carbon are exchanged each year between the ocean and the atmosphere. A major controlling factor in oceanic-atmospheric carbon exchange is

thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temper ...

. In regions of ocean upwelling, carbon-rich water from the deep ocean comes to the surface and releases carbon into the atmosphere as carbon dioxide. Large amounts of carbon dioxide are dissolved in cold water in higher latitudes. This water sinks down and brings the carbon into the deeper ocean levels, where it can stay for anywhere between decades and several centuries. Ocean circulation events cause this process to be variable. For example, during

El Nino EL, El or el may refer to: Religion * El (deity), a Semitic word for "God" People * EL (rapper) (born 1983), stage name of Elorm Adablah, a Ghanaian rapper and sound engineer * El DeBarge, music artist * El Franco Lee (1949–2016), American po ...

events there is less deep ocean upwelling, leading to lower outgassing of carbon dioxide into the atmosphere. Biological processes also lead to ocean-atmosphere carbon exchange. Carbon dioxide equilibrates between the atmosphere and the ocean's surface layers. As

autotroph An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...

s add or subtract carbon dioxide from the water through

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellula ...

, they modify this balance, allowing the water to absorb more carbon dioxide or causing it to emit carbon dioxide into the atmosphere.

Geosphere

Carbon is generally exchanged very slowly between the atmosphere and geosphere. Two exceptions are

volcanic eruptions Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...

and the combustion of

fossil fuels A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ...

, both of which release high amounts carbon into the atmosphere very quickly. Fresh silicate rock that is exposed through geological processes absorbs carbon from the atmosphere when it is exposed to air by the processes of

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...

and

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...

Anthropogenic sources

Human activities change the amount of carbon in the atmosphere directly through the burning of fossil fuels and other organic material, thus

oxidizing Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

the organic carbon and producing carbon dioxide. Another human-caused source of carbon dioxide is

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mix ...

production. The burning of fossil fuels and cement production are the main reasons for the increase in atmospheric CO₂ since the beginning of the industrial era. Other human-caused changes in the atmospheric carbon cycle are due to anthropogenic changes to carbon reservoirs. Deforestation, for example, decreases the biosphere's ability to absorb carbon, thus increasing the amount of carbon in the atmosphere. As the industrial use of carbon by humans is a very new dynamic on a geologic scale, it is important to be able to track sources and sinks of carbon in the atmosphere. One way of doing so is by observing the proportion of stable carbon

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

s present in the atmosphere. The two main carbon isotopes are ¹²C and ¹³C. Plants absorb the lighter isotope, ¹²C, more readily than ¹³C. Because fossil fuels originate mainly from plant matter, the ¹³C/¹²C ratio in the atmosphere falls when large amounts of fossil fuels are burned, releasing ¹²C. Conversely, an increase in the ¹³C/¹²C in the atmosphere suggests a higher biospheric carbon uptake. The ratio of the annual increase in atmospheric CO₂ compared to CO₂ emissions from fossil fuel and cement manufactured is called the "airborne fraction.". The airborne fraction has been around 60% since the 1950s, indicating that about 60% of the new carbon dioxide in the atmosphere each year originated from human sources. For clarity, this is not meant to suggest that 60% of the uptake of carbon dioxide into the atmosphere comes from human activity. It means that the atmosphere exchanges around 210 gigatonnes of carbon annually, but absorbs between 6 and 10 gigatonnes more than it loses. Of this net gain, about 60% is attributable to the burning of fossil fuels.

Gallery

File:Carbon Dioxide 800kyr.svg , Atmospheric concentrations over the last 800,000 years as measured from ice cores (blue/green) and directly (black). File:Carbon dioxide emissions global carbon cycle.jpg , Ocean and land sinks have taken up about half of fossil carbon emissions into the atmosphere. It's uncertain how long this will continue. File:Global carbon budget components.png , flows from human activity (left) into atmosphere, land, and ocean sinks (right).Friedlingstein, P., Jones, M., O'Sullivan, M., Andrew, R., Hauck, J., Peters, G., Peters, W., Pongratz, J., Sitch, S., Le Quéré, C. and 66 others (2019) "Global carbon budget 2019". ''Earth System Science Data'', 11(4): 1783–1838. . CC-BY icon

Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
File:Anthropogenic carbon flows 1850-2018.png , Anthropogenic carbon flows during years 1850-2018 (left) and 2009-2018 (right).

References

External links

Carbon Dioxide - Global Circulation
(

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

; 13 December 2016)
Carbon Dioxide - Information Analysis Center

World Data Centre for Greenhouse Gases
{{Webarchive, url=https://web.archive.org/web/20160406090043/http://ds.data.jma.go.jp/gmd/wdcgg/ , date=2016-04-06 Atmospheric chemistry Carbon cycle Climate change and the environment