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3.1.1.3 The carbon cycle
Global distribution, and size of major stores of carbon – lithosphere, hydrosphere, cryosphere, biosphere, atmosphere.
Factors driving change in the magnitude of these stores over time and space, including flows and transfers at plant, sere and continental scales. Photosynthesis, respiration, decomposition, combustion, carbon sequestration in oceans and sediments, weathering.
3.1.1.3 The carbon cycle
Global distribution, and size of major stores of carbon – lithosphere, hydrosphere, cryosphere, biosphere, atmosphere.
Factors driving change in the magnitude of these stores over time and space, including flows and transfers at plant, sere and continental scales. Photosynthesis, respiration, decomposition, combustion, carbon sequestration in oceans and sediments, weathering.
1. Where is all of the carbon?
NASA writes that "Carbon flows between each reservoir in an exchange called the carbon cycle, which has slow and fast components. Any change in the cycle that shifts carbon out of one reservoir puts more carbon in the other reservoirs. Changes that put carbon gases into the atmosphere result in warmer temperatures on Earth."
If you find the carbon cycle tricky, have a look at the BBC Bitesize page using the button below:
If you find the carbon cycle tricky, have a look at the BBC Bitesize page using the button below:
Carbon exists in all of the earths spheres. If you think back to the Gaia hypothesis it is clear that without the combination of the water and carbon cycles living things could not exist on Earth as we see them today, if at all.
Carbon is critical to sustain a huge range of Earth’s functions. Not only is it present in all living beings, it is a major component of a number of minerals (e.g. graphite and diamond). Each sphere acts as storage areas of ‘reservoirs’ of carbon, either in the short-term (a few minutes) or long-term (millions of years). As the Earth is such a dynamic environment, processes such as erosion, evaporation, photosynthesis, respiration, and decomposition constantly move carbon between these reservoirs. Carbon enters, is stored, and leaves the different spheres of the Earth through different methods, and in different quantities:
Atmosphere.
As the ice sheets added loess to the soil, the soil got thicker. As the soil built up, the active layer on top stayed the same thickness. The active layer freezes and thaws each year, and plants can grow in it. But underneath the active layer, roots and other organic matter were frozen into the permafrost, where they can't decay.
Most of the organic matter consists of partially decayed roots, whole roots, and other plant material. However, there are also animals and animal material frozen in the ground--sometimes people find entire mastodons or other animals frozen in the permafrost (Figure 3). Significant deposits of carbon-rich permafrost, or yedoma, have been found in Russia.
How much carbon is stored in frozen ground?There is a huge amount of carbon stored in permafrost. Right now, the Earth's atmosphere contains about 850 gigatons of carbon. (A gigaton is one billion tons—about the weight of one hundred thousand school buses). We estimate that there are about 1,400 gigatons of carbon frozen in permafrost. So the carbon frozen in permafrost is greater than the amount of carbon that is already in the atmosphere today. That doesn't mean that all of the carbon will decay and end up in the atmosphere. The trick is to find out how much of the frozen carbon is going to decay, how fast, and where.
What will happen to the frozen carbon if permafrost thaws?
When permafrost thaws, the frozen organic matter inside it will thaw out, too, and begin to decay. It's like taking a bag of frozen broccoli out of the freezer and putting it into the refrigerator. Once it thaws, it will eventually decay and break down.
As organic matter decays, it gets eaten up and digested by microbes. The bacteria that eat it produce either carbon dioxide or methane as waste. If there is oxygen available, the microbes make carbon dioxide. But if there is no oxygen available, they make methane. Most of the places where methane would form are the swamps and wetlands. And there are many miles of wetlands in the Arctic. When you walk around in the Arctic tundra, it's like sloshing through a giant sponge.
When permafrost carbon turns into methane, it bubbles up through soil and water. On the way, other microorganisms eat some of it. But some methane makes it to the surface and escapes into the air.
How will additional methane from permafrost affect global warming?There are several opposing processes at work, which make this a hard question to answer. Warmer temperatures mean that permafrost can thaw and release methane to the atmosphere. But warming also means that the growing seasons in Arctic latitudes will last longer. When the growing season is longer, plants have more time to suck up carbon from the atmosphere. Since carbon in the air is what plants use to grow, it can also act as a sort of fertilizer under certain conditions. Then plants to grow faster and take up even more carbon. Right now, the Arctic takes up more carbon than it releases. This means that plants take up carbon during the growing season, but do not release as much carbon through decay. So we say that the Arctic acts as a carbon sink.
But if the Earth continues to warm, and a lot of permafrost thaws out, the Arctic could become an overall source of carbon to the atmosphere, instead of a sink. This is what scientists refer to as a "tipping point." We say that something has reached a tipping point when it switches from a relatively stable state to an unstoppable cycle. In this case, the Arctic would change from a carbon sink to a carbon source. If the Arctic permafrost releases more carbon than it absorbs, it would start a cycle where the extra carbon in the atmosphere leads to increased warming. The increased warming means more permafrost thawing and methane release.
(Source: https://nsidc.org/cryosphere/frozenground/methane.html)
Where is carbon stored in the Arctic?
Lithosphere.
Carbon is critical to sustain a huge range of Earth’s functions. Not only is it present in all living beings, it is a major component of a number of minerals (e.g. graphite and diamond). Each sphere acts as storage areas of ‘reservoirs’ of carbon, either in the short-term (a few minutes) or long-term (millions of years). As the Earth is such a dynamic environment, processes such as erosion, evaporation, photosynthesis, respiration, and decomposition constantly move carbon between these reservoirs. Carbon enters, is stored, and leaves the different spheres of the Earth through different methods, and in different quantities:
Atmosphere.
- carbon is stored in the atmosphere as both methane (CH4) and carbon dioxide (CO2); which are greenhouse gases which absorb and retain heat. CO2 is released into the atmosphere through respiration by living organisms, volcanic eruptions, weathering, and human activity. It is removed from the atmosphere by dissolution into water and through photosynthesis by plants. CH4 is released into the atmosphere through animal emissions, decomposition, and burning of fossil fuels.
- all living and deceased organisms contain organic carbon. Organisms gain carbon by either extracting it from CO2 in the atmosphere through photosynthesis, or by consuming other organisms and therefore receiving their carbon. Carbon remains in an organism until it decomposes sufficiently to release carbon to the atmosphere or lithosphere.
- the upper layers of oceans hold a vast amount of dissolved organic carbon, and the lower ocean waters are rich in dissolved inorganic carbon. Dissolved organic carbon in the surface layers is rapidly exchanged with the atmosphere because they are constantly in contact with each other. In contrast, the dissolved inorganic carbon is much deeper in the water column, and remains stored for longer periods of time – up to thousands of years. It is the thermohaline circulation, which leads to the large scale mixing of ocean waters, which allows exchange between the upper and lower ocean layers.
- the cryosphere contains less than 0.01% of Earth's carbon.
- most of the carbon that is stored is within permafrost. This is where plants and animals which are in the process of decomposing are trapped in the permanently frozen soil.
As the ice sheets added loess to the soil, the soil got thicker. As the soil built up, the active layer on top stayed the same thickness. The active layer freezes and thaws each year, and plants can grow in it. But underneath the active layer, roots and other organic matter were frozen into the permafrost, where they can't decay.
Most of the organic matter consists of partially decayed roots, whole roots, and other plant material. However, there are also animals and animal material frozen in the ground--sometimes people find entire mastodons or other animals frozen in the permafrost (Figure 3). Significant deposits of carbon-rich permafrost, or yedoma, have been found in Russia.
How much carbon is stored in frozen ground?There is a huge amount of carbon stored in permafrost. Right now, the Earth's atmosphere contains about 850 gigatons of carbon. (A gigaton is one billion tons—about the weight of one hundred thousand school buses). We estimate that there are about 1,400 gigatons of carbon frozen in permafrost. So the carbon frozen in permafrost is greater than the amount of carbon that is already in the atmosphere today. That doesn't mean that all of the carbon will decay and end up in the atmosphere. The trick is to find out how much of the frozen carbon is going to decay, how fast, and where.
What will happen to the frozen carbon if permafrost thaws?
When permafrost thaws, the frozen organic matter inside it will thaw out, too, and begin to decay. It's like taking a bag of frozen broccoli out of the freezer and putting it into the refrigerator. Once it thaws, it will eventually decay and break down.
As organic matter decays, it gets eaten up and digested by microbes. The bacteria that eat it produce either carbon dioxide or methane as waste. If there is oxygen available, the microbes make carbon dioxide. But if there is no oxygen available, they make methane. Most of the places where methane would form are the swamps and wetlands. And there are many miles of wetlands in the Arctic. When you walk around in the Arctic tundra, it's like sloshing through a giant sponge.
When permafrost carbon turns into methane, it bubbles up through soil and water. On the way, other microorganisms eat some of it. But some methane makes it to the surface and escapes into the air.
How will additional methane from permafrost affect global warming?There are several opposing processes at work, which make this a hard question to answer. Warmer temperatures mean that permafrost can thaw and release methane to the atmosphere. But warming also means that the growing seasons in Arctic latitudes will last longer. When the growing season is longer, plants have more time to suck up carbon from the atmosphere. Since carbon in the air is what plants use to grow, it can also act as a sort of fertilizer under certain conditions. Then plants to grow faster and take up even more carbon. Right now, the Arctic takes up more carbon than it releases. This means that plants take up carbon during the growing season, but do not release as much carbon through decay. So we say that the Arctic acts as a carbon sink.
But if the Earth continues to warm, and a lot of permafrost thaws out, the Arctic could become an overall source of carbon to the atmosphere, instead of a sink. This is what scientists refer to as a "tipping point." We say that something has reached a tipping point when it switches from a relatively stable state to an unstoppable cycle. In this case, the Arctic would change from a carbon sink to a carbon source. If the Arctic permafrost releases more carbon than it absorbs, it would start a cycle where the extra carbon in the atmosphere leads to increased warming. The increased warming means more permafrost thawing and methane release.
(Source: https://nsidc.org/cryosphere/frozenground/methane.html)
Where is carbon stored in the Arctic?
- Atmosphere: ~750 Pg globally and ~125 Pg north of 60°N
- Ocean: Cold temperatures in the Arctic increase the efficiency of carbon transfer from the atmosphere to the ocean
- Arctic Ocean is 3% of the Earth’s oceans, but removes 5 to 14% of the Earth’s ocean carbon uptake
- Land: Boreal forest store carbon as plant material, Sequester about 1.3±0.5 Pg/yr ,
- Arctic tundra stores carbon as soil carbon: ~1672 Pg in northern permafrost regions
Lithosphere.
- over 99.9% of the earths carbon is stored in the lithosphere predominantely in sedimentary rocks such as limestone.
- 0.004% is stored in fossil fuels.
- carbon in the lithosphere is held in soil in the form of both organic and inorganic carbon (often as calcium carbonate). Carbon can leave the soil through soil respiration – which releases CO2, or by erosion – which can carry it into rivers or the ocean, where it then enters the hydrosphere. Within the Earth’s crust a large amount of carbon is stored in limestone and kerogens (the term given to organic matter held within sedimentary rocks). These organics are made of decomposed and highly compressed living matter. Once they become lithified (transformed into rock), some of the kerogens can become crude oil or natural gas – these are a source of fossil fuels. These forms of carbon are highly stable and can remain in the lithosphere for millions of years. If rock is subducted into the Earth’s mantle, it will melt, and the CO2 it contains is released into the atmosphere via subsequent volcanic eruptions. Alternatively, the extraction and burning of fossil fuels by human activity can release carbon into the atmosphere.
2. How do carbon stores change over time?
The Fast Carbon cycle
The time it takes carbon to move through the fast carbon cycle is measured in a lifespan. The fast carbon cycle is largely the movement of carbon through life forms on Earth, or the biosphere. Between 1015 and 1017 grams (1,000 to 100,000 million metric tons) of carbon move through the fast carbon cycle every year.
Carbon plays an essential role in biology because of its ability to form many bonds—up to four per atom—in a seemingly endless variety of complex organic molecules. Many organic molecules contain carbon atoms that have formed strong bonds to other carbon atoms, combining into long chains and rings. Such carbon chains and rings are the basis of living cells. For instance, DNA is made of two intertwined molecules built around a carbon chain.
The bonds in the long carbon chains contain a lot of energy. When the chains break apart, the stored energy is released. This energy makes carbon molecules an excellent source of fuel for all living things.
Photosynthesis
The Slow Carbon Cycle
Chemical Weathering
Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. On average, 1013 to 1014 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year. In comparison, human emissions of carbon to the atmosphere are on the order of 1015 grams, whereas the fast carbon cycle moves 1016 to 1017 grams of carbon per year.
The movement of carbon from the atmosphere to the lithosphere (rocks) begins with rain. Atmospheric carbon combines with water to form a weak acid—carbonic acid—that falls to the surface in rain. The acid dissolves rocks—a process called chemical weathering—and releases calcium, magnesium, potassium, or sodium ions. Rivers carry the ions to the ocean.
Rivers carry calcium ions—the result of chemical weathering of rocks—into the ocean, where they react with carbonate dissolved in the water. The product of that reaction, calcium carbonate, is then deposited onto the ocean floor, where it becomes limestone.
In the ocean, the calcium ions combine with bicarbonate ions to form calcium carbonate, the active ingredient in antacids and the chalky white substance that dries on your faucet if you live in an area with hard water. In the modern ocean, most of the calcium carbonate is made by shell-building (calcifying) organisms (such as corals) and plankton (like coccolithophores and foraminifera). After the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented together and turn to rock, storing the carbon in stone—limestone and its derivatives. Only 80 percent of carbon-containing rock is currently made this way. The remaining 20 percent contain carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In special cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale.
Tectonic Activity
The slow cycle returns carbon to the atmosphere through volcanoes. Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide.
When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin the cycle again. At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year—100–300 times more than volcanoes—by burning fossil fuels.
Chemistry regulates this dance between ocean, land, and atmosphere. If carbon dioxide rises in the atmosphere because of an increase in volcanic activity, for example, temperatures rise, leading to more rain, which dissolves more rock, creating more ions that will eventually deposit more carbon on the ocean floor. It takes a few hundred thousand years to rebalance the slow carbon cycle through chemical weathering.
However, the slow carbon cycle also contains a slightly faster component: the ocean. At the surface, where air meets water, carbon dioxide gas dissolves in and ventilates out of the ocean in a steady exchange with the atmosphere. Once in the ocean, carbon dioxide gas reacts with water molecules to release hydrogen, making the ocean more acidic. The hydrogen reacts with carbonate from rock weathering to produce bicarbonate ions.
Before the industrial age, the ocean vented carbon dioxide to the atmosphere in balance with the carbon the ocean received during rock weathering. However, since carbon concentrations in the atmosphere have increased, the ocean now takes more carbon from the atmosphere than it releases. Over millennia, the ocean will absorb up to 85 percent of the extra carbon people have put into the atmosphere by burning fossil fuels, but the process is slow because it is tied to the movement of water from the ocean’s surface to its depths.
In the meantime, winds, currents, and temperature control the rate at which the ocean takes carbon dioxide from the atmosphere. It is likely that changes in ocean temperatures and currents helped remove carbon from and then restore carbon to the atmosphere over the few thousand years in which the ice ages began and ended.
The time it takes carbon to move through the fast carbon cycle is measured in a lifespan. The fast carbon cycle is largely the movement of carbon through life forms on Earth, or the biosphere. Between 1015 and 1017 grams (1,000 to 100,000 million metric tons) of carbon move through the fast carbon cycle every year.
Carbon plays an essential role in biology because of its ability to form many bonds—up to four per atom—in a seemingly endless variety of complex organic molecules. Many organic molecules contain carbon atoms that have formed strong bonds to other carbon atoms, combining into long chains and rings. Such carbon chains and rings are the basis of living cells. For instance, DNA is made of two intertwined molecules built around a carbon chain.
The bonds in the long carbon chains contain a lot of energy. When the chains break apart, the stored energy is released. This energy makes carbon molecules an excellent source of fuel for all living things.
Photosynthesis
- During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel—glucose and other sugars—for building plant structures. This process forms the foundation of the fast (biological) carbon cycle.
- Plants and phytoplankton are the main components of the fast carbon cycle. Phytoplankton (microscopic organisms in the ocean) and plants take carbon dioxide from the atmosphere by absorbing it into their cells. Using energy from the Sun, both plants and plankton combine carbon dioxide (CO2) and water to form sugar (CH2O) and oxygen. The chemical reaction looks like this:
- CO2 + H2O + energy = CH2O + O2
- Four things can happen to move carbon from a plant and return it to the atmosphere, but all involve the same chemical reaction. Plants break down the sugar to get the energy they need to grow. Animals (including people) eat the plants or plankton, and break down the plant sugar to get energy. Plants and plankton die and decay (are eaten by bacteria) at the end of the growing season. Or fire consumes plants. In each case, oxygen combines with sugar to release water, carbon dioxide, and energy. The basic chemical reaction looks like this:
- CH2O + O2 = CO2 + H2O + energy
- In all four processes, the carbon dioxide released in the reaction usually ends up in the atmosphere. The fast carbon cycle is so tightly tied to plant life that the growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the Northern Hemisphere winter, when few land plants are growing and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop. It is as if the Earth is breathing.
The Slow Carbon Cycle
Chemical Weathering
Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. On average, 1013 to 1014 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year. In comparison, human emissions of carbon to the atmosphere are on the order of 1015 grams, whereas the fast carbon cycle moves 1016 to 1017 grams of carbon per year.
The movement of carbon from the atmosphere to the lithosphere (rocks) begins with rain. Atmospheric carbon combines with water to form a weak acid—carbonic acid—that falls to the surface in rain. The acid dissolves rocks—a process called chemical weathering—and releases calcium, magnesium, potassium, or sodium ions. Rivers carry the ions to the ocean.
Rivers carry calcium ions—the result of chemical weathering of rocks—into the ocean, where they react with carbonate dissolved in the water. The product of that reaction, calcium carbonate, is then deposited onto the ocean floor, where it becomes limestone.
In the ocean, the calcium ions combine with bicarbonate ions to form calcium carbonate, the active ingredient in antacids and the chalky white substance that dries on your faucet if you live in an area with hard water. In the modern ocean, most of the calcium carbonate is made by shell-building (calcifying) organisms (such as corals) and plankton (like coccolithophores and foraminifera). After the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented together and turn to rock, storing the carbon in stone—limestone and its derivatives. Only 80 percent of carbon-containing rock is currently made this way. The remaining 20 percent contain carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In special cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale.
Tectonic Activity
The slow cycle returns carbon to the atmosphere through volcanoes. Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide.
When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin the cycle again. At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year—100–300 times more than volcanoes—by burning fossil fuels.
Chemistry regulates this dance between ocean, land, and atmosphere. If carbon dioxide rises in the atmosphere because of an increase in volcanic activity, for example, temperatures rise, leading to more rain, which dissolves more rock, creating more ions that will eventually deposit more carbon on the ocean floor. It takes a few hundred thousand years to rebalance the slow carbon cycle through chemical weathering.
However, the slow carbon cycle also contains a slightly faster component: the ocean. At the surface, where air meets water, carbon dioxide gas dissolves in and ventilates out of the ocean in a steady exchange with the atmosphere. Once in the ocean, carbon dioxide gas reacts with water molecules to release hydrogen, making the ocean more acidic. The hydrogen reacts with carbonate from rock weathering to produce bicarbonate ions.
Before the industrial age, the ocean vented carbon dioxide to the atmosphere in balance with the carbon the ocean received during rock weathering. However, since carbon concentrations in the atmosphere have increased, the ocean now takes more carbon from the atmosphere than it releases. Over millennia, the ocean will absorb up to 85 percent of the extra carbon people have put into the atmosphere by burning fossil fuels, but the process is slow because it is tied to the movement of water from the ocean’s surface to its depths.
In the meantime, winds, currents, and temperature control the rate at which the ocean takes carbon dioxide from the atmosphere. It is likely that changes in ocean temperatures and currents helped remove carbon from and then restore carbon to the atmosphere over the few thousand years in which the ice ages began and ended.
3. How does carbon move between the stores?
We call the movement a "flux" as it moves from one store to another. The movement from one pool to another may happen though many different fluxes leading to different stores. The main ways that carbon moves from pool to pool are:
- Photosynthesis: plants use energy from the sunlight, carbon dioxide from the atmosphere and water to create carbohydrates. This means that carbon is transferred into the biosphere. This can be an example of carbon sequestration- where carbon is removed from the atmosphere and stored in the biosphere for long periods of time.
- Respiration: all organisms respire. This process occurs with plants, animals and soils. Plants release carbon back into the atmosphere. It occurs as a by product as plants are using their energy stored in carbohydrates. Soil is full of organic matter. As it decomposes, it releases carbon into the atmosphere. This process can take months or it may take many years.
- Litterfall: Living plants shed leaves which then decompose into the soil. Different ecosystems will experience this in different volumes and timescales. For example, the rainforest will have much litterfall, and the climate will mean that it rapidly decomposes. However, the nature of the system means that although lots of carbon will be released, lots will also be taken up by the green plants during photosynthesis. This is an example of how different parts of the world will cycle carbon at varying rates.#
- Ocean-Atmosphere Exchange: carbon is both absorbed by the ocean and released from the ocean through the process of diffusion. Once the CO2 is dissolved, it reacts with the water as a carbonate reaction. In this process water and carbon combine to form carbonic acid, of which the anion is carbonate. This carbonate is what helps marine life to build shells and to form the skeleton of coral reefs. Please see the page on the oceans being a source and store, and the physical and biological pumps. The oceans, lakes and ponds of planet Earth are important as they absorb and ‘lock away’ over a quarter of the carbon dioxide that humans emit into the atmosphere. The process by which they absorb and lock away the carbon dioxide is known as sequestration. This occurs due to:
- Carbon dioxide being soluble and dissolving directly in the water.
- Phytoplankton performing photosynthesis which absorbs carbon dioxide, trapping the carbon within their biomass.
- This sequestering plays an important role in removing carbon dioxide from the atmosphere. As carbon dioxide levels in the atmosphere rise, it is likely that more would be sequestered in the oceans, rivers and ponds.
- Burning fossil fuels: man has continued to release carbon into the atmosphere through the process of burning fossil fuels. This is one of the fast fluxes of carbon. Coal, oil and natural gas all contain carbon and when burnt this is released into the atmosphere. Since the industrial revolution socieites have been mining and burning fossil fuels at an increasingly rapid rate. This has meant that the flux involved is very fast, although the carbon that is sequestered into the atmosphere in this way may remain in the atmosphere for many years.
- Deforestation and other land use change: urbanisation, agricultural practices, mining and logging have all led to the degradation of the rainforest. Forest contain much carbon both in the plants and animals, but also in the soils. Because these sinks are not replaced these areas will also affect the local atmospheric volumes. In areas of afforestation this can represent a new sink being formed, but currently it isn't enough to balance out the losses that have already taken place.
- Geological processes: this is part of the slow carbon cycle as it takes millions of years (see above). Sediments accumulate in layers at the bottom of oceans, trapping organic matter within the deposits, and over millions of years layers of sediment builds up trapping the carbon. This may be sequestered through extracting of fossil fuels or may be brought to the earths surface through tectonic processes as the rocks are melted at subduction margins and then carbon is emitted during volcanic eruptions. Weathering is also a key process that moves carbon around. Chemical weathering dissolves carbon in water, and physical weathering may break down sedimentary rocks moving the carbon from a solid state to soils, or into the oceanic system to be deposited on the ocean floor.