Atmospheric carbon cycle

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 (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.

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]

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]

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.

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.