Atmospheric carbon cycle

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Schematic representation of the overall perturbation of the global carbon cycle caused by anthropogenic activities, averaged from 2010 to 2019. Anthropogenic changes in the global carbon cycle.png
Schematic representation of the overall perturbation of the global carbon cycle caused by anthropogenic activities, averaged from 2010 to 2019.

The atmospheric carbon cycle accounts for the exchange of gaseous carbon compounds, primarily carbon dioxide (CO2), between Earth's atmosphere, the oceans, and the terrestrial biosphere. It is one of the faster components of the planet's overall carbon cycle, supporting the exchange of more than 200 billion tons of carbon (i.e. gigatons carbon or GtC) in and out of the atmosphere throughout the course of each year. [2] Atmospheric concentrations of CO2 remain stable over longer timescales only when there exists a balance between these two flows. Methane (CH4), Carbon monoxide (CO), and other human-made compounds are present in smaller concentrations and are also part of the atmospheric carbon cycle. [3]

Contents

Human activities, primarily the extraction and burning of fossil carbon from Earth's lithosphere starting with the Industrial Revolution, have disturbed the previous balance of the atmospheric carbon cycle and have been mostly responsible for the ongoing rapid growth in CO2 and CH4 concentrations. [4] As of year 2019, annual emissions grew to 10 GtC/year, with a cumulative total of about 450 GtC injected into the cycle. [5] The terrestrial and ocean sinks have thus far absorbed half of the added carbon, and half has remained in the atmosphere primarily as CO2. Assuming the growth trend in emissions continues, the CO2 concentration is on a path to at least double by the latter half of this century. [6]

The atmospheric carbon cycle also strongly influences Earth's energy balance through the greenhouse effect, and affects the acidity or alkalinity of the planet's surface waters and soils. Despite comprising less than 0.05% of all atmospheric gases by mole fraction, [7] the recent rise in carbon concentrations has caused substantial global heating and ocean acidification. [8] Such effects are generally projected to accelerate further until net emissions are stabilized and reduced. [6]

Relevant gases

Computer model showing a year in the life of atmospheric carbon dioxide and how it travels around the globe  [9]

The atmosphere is one of the Earth's major carbon reservoirs and holds approximately 720 gigatons of carbon as of year 2000. [2] The concentration of mostly carbon-based greenhouse gases has increased dramatically since the onset of the industrial era. This makes an understanding of the carbon component of the atmosphere highly important. The two main carbon greenhouse gases are methane and carbon dioxide. [10]

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, [10] yet the methane cycle in the atmosphere is currently only poorly understood. [11] The amount of methane produced and absorbed yearly varies widely. [10]

Large stores of methane can be found in the form of methane ice 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. [12]

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. [11] More than 70% of atmospheric methane comes from biogenic sources. Methane levels have risen gradually since the onset of the industrial era, [13] from ~700 ppb in 1750 to ~1775 ppb in 2005. [10]

Methane can be removed from the atmosphere through a reaction of the photochemically produced hydroxyl free radical (OH). [14] [15] It can also leave the atmosphere by entering the stratosphere, where it is destroyed, or by being absorbed into soil sinks. [16] 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. [13] 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. [11]

Carbon dioxide

2011 carbon dioxide mole fraction in the troposphere AIRS Carbon Dioxide.png
2011 carbon dioxide mole fraction in the troposphere

Carbon dioxide (CO2) has a large warming effect on global temperatures through the greenhouse effect. Although individual CO2 molecules have a short residence time 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 [17] and among the many long-lasting greenhouse gases, it is the most important because it makes up the largest fraction of the atmosphere. [10] Since the Industrial Revolution, the CO2 concentration in the atmosphere has risen from about 280 ppm to almost 400 ppm. [7] 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. 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. [18] 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

Major global carbon reservoirs and flows between them. Carbon cycle - Main components.png
Major global carbon reservoirs and flows between them.

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. [2] 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 and converted into organic compounds. Carbon is also released from the biosphere into the atmosphere in the course of biological processes. Aerobic respiration converts organic carbon into carbon dioxide and a particular type of anaerobic respiration 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. [19]

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). [10] 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. [20] 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. [19]

Oceans

Each year, the ocean and atmosphere exchange large amounts of carbon. A major controlling factor in oceanic-atmospheric carbon exchange is thermohaline circulation. 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. [2] Ocean circulation events cause this process to be variable. For example, during El Nino events there is less deep ocean upwelling, leading to lower outgassing of carbon dioxide into the atmosphere. [18]

Biological processes also lead to ocean-atmosphere carbon exchange. Carbon dioxide equilibrates between the atmosphere and the ocean's surface layers. As autotrophs add or subtract carbon dioxide from the water through photosynthesis or respiration, they modify this balance, allowing the water to absorb more carbon dioxide or causing it to emit carbon dioxide into the atmosphere. [2]

Geosphere

Carbon is generally exchanged very slowly between the atmosphere and geosphere. Two exceptions are volcanic eruptions and the combustion of fossil fuels, both of which release high amounts carbon into the atmosphere very quickly. [21] 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 and erosion.[ citation needed ]

Anthropogenic sources

Carbon dioxide emissions and partitioning
CO2 Emissions by Source Since 1880.svg
Emissions of CO2 have been caused by different sources ramping up one after the other (Global Carbon Project)
Carbon Dioxide Partitioning.svg
Partitioning of CO2 emissions show that most emissions have been absorbed by carbon sinks, including plant growth, soil uptake, and ocean uptake (Global Carbon Project)

Human activities change the amount of carbon in the atmosphere directly through the burning of fossil fuels and other organic material, thus oxidizing the organic carbon and producing carbon dioxide. [22] [23] Another human-caused source of carbon dioxide is cement 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. [10]

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. [24]

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 isotopes present in the atmosphere. The two main carbon isotopes are 12C and 13C. Plants absorb the lighter isotope, 12C, more readily than 13C. [25] 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. [19] The ratio of the annual increase in atmospheric CO2 compared to CO2 emissions from fossil fuel and cement manufactured is called the "airborne fraction.". [26] 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. [10] 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.

Related Research Articles

<span class="mw-page-title-main">Causes of climate change</span> Effort to scientifically ascertain mechanisms responsible for recent global warming

The scientific community has been investigating the causes of climate change for decades. After thousands of studies, it came to a consensus, where it is "unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times." This consensus is supported by around 200 scientific organizations worldwide, The dominant role in this climate change has been played by the direct emissions of carbon dioxide from the burning of fossil fuels. Indirect CO2 emissions from land use change, and the emissions of methane, nitrous oxide and other greenhouse gases play major supporting roles.

<span class="mw-page-title-main">Carbon dioxide</span> Chemical compound with formula CO₂

Carbon dioxide is a chemical compound with the chemical formula CO2. It is 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, and as the source of available carbon in the carbon cycle, atmospheric CO2 is the primary carbon source for life on Earth. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When carbon dioxide dissolves in water, it forms carbonate and mainly bicarbonate, which causes ocean acidification as atmospheric CO2 levels increase.

<span class="mw-page-title-main">Global warming potential</span> Potential heat absorbed by a greenhouse gas

Global warming potential (GWP) is an index to measure how much infrared thermal radiation a greenhouse gas would absorb over a given time frame after it has been added to the atmosphere. The GWP makes different greenhouse gases comparable with regard to their "effectiveness in causing radiative forcing". It is expressed as a multiple of the radiation that would be absorbed by the same mass of added carbon dioxide, which is taken as a reference gas. Therefore, the GWP has a value of 1 for CO2. For other gases it depends on how strongly the gas absorbs infrared thermal radiation, how quickly the gas leaves the atmosphere, and the time frame being considered.

<span class="mw-page-title-main">Carbon cycle</span> Natural processes of carbon exchange

The carbon cycle is that part of the biogeochemical cycle by which 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 minerals 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.

This glossary of climate change is a list of definitions of terms and concepts relevant to climate change, global warming, and related topics.

Trace gases are gases that are present in small amounts within an environment such as a planet's atmosphere. Trace gases in Earth's atmosphere are gases other than nitrogen (78.1%), oxygen (20.9%), and argon (0.934%) which, in combination, make up 99.934% of its atmosphere.

<span class="mw-page-title-main">Clathrate gun hypothesis</span> Meteorological hypothesis

The clathrate gun hypothesis is a proposed explanation for the periods of rapid warming during the Quaternary. The hypothesis is that changes in fluxes in upper intermediate waters in the ocean caused temperature fluctuations that alternately accumulated and occasionally released methane clathrate on upper continental slopes. This would have had an immediate impact on the global temperature, as methane is a much more powerful greenhouse gas than carbon dioxide. Despite its atmospheric lifetime of around 12 years, methane's global warming potential is 72 times greater than that of carbon dioxide over 20 years, and 25 times over 100 years. It is further proposed that these warming events caused the Bond Cycles and individual interstadial events, such as the Dansgaard–Oeschger interstadials.

<span class="mw-page-title-main">Greenhouse gas emissions</span> Sources and amounts of greenhouse gases emitted to the atmosphere from human activities

Greenhouse gas (GHG) emissions from human activities intensify the greenhouse effect. This contributes to climate change. Carbon dioxide, from burning fossil fuels such as coal, oil, and natural gas, is one of the most important factors in causing climate change. The largest emitters are China followed by the United States. The United States has higher emissions per capita. The main producers fueling the emissions globally are large oil and gas companies. Emissions from human activities have increased atmospheric carbon dioxide by about 50% over pre-industrial levels. The growing levels of emissions have varied, but have been consistent among all greenhouse gases. Emissions in the 2010s averaged 56 billion tons a year, higher than any decade before. Total cumulative emissions from 1870 to 2022 were 703 GtC, of which 484±20 GtC from fossil fuels and industry, and 219±60 GtC from land use change. Land-use change, such as deforestation, caused about 31% of cumulative emissions over 1870–2022, coal 32%, oil 24%, and gas 10%.

<span class="mw-page-title-main">Carbon dioxide in Earth's atmosphere</span> Atmospheric constituent and greenhouse gas

In Earth's atmosphere, carbon dioxide is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis and oceanic carbon cycle. It is one of several greenhouse gases in the atmosphere of Earth. The current global average concentration of carbon dioxide in the atmosphere is 421 ppm (0.04%) as of May 2022. This is an increase of 50% since the start of the Industrial Revolution, up from 280 ppm during the 10,000 years prior to the mid-18th century. The increase is due to human activity.

<span class="mw-page-title-main">Global Carbon Project</span>

The Global Carbon Project (GCP) is an organisation that seeks to quantify global greenhouse gas emissions and their causes. Established in 2001, its projects include global budgets for three dominant greenhouse gases—carbon dioxide, methane, and nitrous oxide —and complementary efforts in urban, regional, cumulative, and negative emissions.

Carbon monitoring as part of greenhouse gas monitoring refers to tracking how much carbon dioxide or methane is produced by a particular activity at a particular time. For example, it may refer to tracking methane emissions from agriculture, or carbon dioxide emissions from land use changes, such as deforestation, or from burning fossil fuels, whether in a power plant, automobile, or other device. Because carbon dioxide is the greenhouse gas emitted in the largest quantities, and methane is an even more potent greenhouse gas, monitoring carbon emissions is widely seen as crucial to any effort to reduce emissions and thereby slow climate change.

The Suess effect is a change in the ratio of the atmospheric concentrations of heavy isotopes of carbon (13C and 14C) by the admixture of large amounts of fossil-fuel derived CO2, which contains no 14CO2 and is depleted in 13CO2 relative to CO2 in the atmosphere and carbon in the upper ocean and the terrestrial biosphere. It was discovered by and is named for the Austrian chemist Hans Suess, who noted the influence of this effect on the accuracy of radiocarbon dating. More recently, the Suess effect has been used in studies of climate change. The term originally referred only to dilution of atmospheric 14CO2 relative to 12CO2. The concept was later extended to dilution of 13CO2 and to other reservoirs of carbon such as the oceans and soils, again relative to 12C.

<span class="mw-page-title-main">Airborne fraction</span>

The airborne fraction is a scaling factor defined as the ratio of the annual increase in atmospheric CO
2 to the CO
2 emissions from human sources. It represents the proportion of human emitted CO2 that remains in the atmosphere. Observations over the past six decades show that the airborne fraction has remained relatively stable at around 45%. This indicates that the land and ocean's capacity to absorb CO2 has kept up with the rise in human CO2 emissions, despite the occurrence of notable interannual and sub-decadal variability, which is predominantly driven by the land's ability to absorb CO2. There is some evidence for a recent increase in airborne fraction, which would imply a faster increase in atmospheric CO
2 for a given rate of human fossil-fuel burning. Changes in carbon sinks can affect the airborne fraction as well.

<span class="mw-page-title-main">Greenhouse gas</span> Gas in an atmosphere with certain absorption characteristics

Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. What distinguishes them from other gases is that they absorb the wavelengths of radiation that a planet emits, resulting in the greenhouse effect. The Earth is warmed by sunlight, causing its surface to radiate heat, which is then mostly absorbed by greenhouse gases. Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F).

<span class="mw-page-title-main">Atmospheric methane</span> Methane in Earths atmosphere

Atmospheric methane is the methane present in Earth's atmosphere. The concentration of atmospheric methane is increasing due to methane emissions, and is causing climate change. Methane is one of the most potent greenhouse gases. Methane's radiative forcing (RF) of climate is direct, and it is the second largest contributor to human-caused climate forcing in the historical period. Methane is a major source of water vapour in the stratosphere through oxidation; and water vapour adds about 15% to methane's radiative forcing effect. The global warming potential (GWP) for methane is about 84 in terms of its impact over a 20-year timeframe, and 28 in terms of its impact over a 100-year timeframe.

<span class="mw-page-title-main">Climate change feedbacks</span> Feedback related to climate change

Climate change feedbacks are natural processes which impact how much global temperatures will increase for a given amount of greenhouse gas emissions. Positive feedbacks amplify global warming while negative feedbacks diminish it. Feedbacks influence both the amount of greenhouse gases in the atmosphere and the amount of temperature change that happens in response. While emissions are the forcing that causes climate change, feedbacks combine to control climate sensitivity to that forcing.

Carbon rift is a theory attributing the input and output of carbon into the environment to human capitalistic systems. This is a derivative of Karl Marx's concept of metabolic rift. In practical terms, increased commodity production demands that greater levels of carbon dioxide (or CO2) be emitted into the biosphere via fossil fuel consumption. Carbon rift theory states that this ultimately disrupts the natural carbon cycle and that this "rift" has adverse effects on nearly every aspect of life. Many of the specifics regarding how this metabolic carbon rift interacts with capitalism are proposed by Brett Clark and Richard York in a 2005 article titled "Carbon Metabolism: Global capitalism, climate change, and the biospheric rift" in the journal Theory and Society. Researchers such as Jean P. Sapinski of the University of Oregon claim that, despite increased interest in closing the carbon rift, it is projected that as long as capitalism continues, there is little hope of reducing the rift.

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