Carbon cycle

Under the carbon cycle or the carbon cycle refers to the system of chemical transformations of carbon-containing compounds in the global systems lithosphere, hydrosphere, atmosphere and biosphere, and the exchange of these compounds between this Erdsphären. Knowledge of this cycle, including its sub-processes allows, among other things, human intervention in the climate and thus to estimate their impact on global warming and to respond appropriately.

3.1 atmosphere
3.2 hydrosphere 3.2.1 Transport Processes
3.2.2 Chemical reactions and equilibria

3.3.1 sedimentation
3.3.2 diagenesis
3.3.3 metamorphosis

4.1 Long-term inorganic cycle 4.1.1 Mechanical weathering
4.1.2 Chemical weathering
4.1.3 precipitation

5.1 causes an increase in carbon dioxide concentration of the atmosphere
5.2 Estimates of potential new carbon sinks 5.2.1 afforestation
5.2.2 Sustainable use of wood

5.3.1 Effects on Photosynthesis
5.3.2 disruption of circuits

6.1 Problems of technical solutions

System approach

The system "Earth" is considered as a closed system. Supply of carbon, for example by meteorites or nuclear- chemical processes and loss of carbon, for example through space are thereby ignored. At the macro level of the system "Earth" the total carbon content is constant. Each of the four sub- systems, is characterized by storage capacity, length of inflow and outflow ( flow rate ) and storage forms of carbon.

Carbon storage

The global amount of carbon 75 million Gt.

Atmosphere

In the atmosphere were provided by IPCC 2007 765 Gt C. The carbon content of the atmosphere is growing annually by about 3 Gt. Main carbon compound is carbon dioxide ( CO2). Its concentration is 390 ml / m³ ( ppmv ), which corresponds to an amount of about 800 Gt C. This is around 0.001 % of the global total carbon. The atmosphere and the biosphere are the smallest carbon storage. The carbon content of the atmosphere that is responsive to change in the flow rates particularly sensitive. On the basis of biochemical processes, however, the atmosphere has the highest carbon flow rates and is thus part of the short-term cycles.

In addition to carbon dioxide even trace gases and impurities come in the atmosphere:

A = years, d = days

1) For CO2 can no definite residence time are specified as different, sometimes strongly dependent on the boundary conditions sink processes play a role IPCC, S. 38 and S. 18 of the PDF file.

Hydrosphere

To hydrosphere, all waters and the polar ice caps, ice sheets and glaciers counted ( cryosphere ). The hydrosphere contains 38,000 Gt C in the form of dissolved CO2, bicarbonate and carbonate ions. This corresponds to 0.045 % of global carbon content. In addition there are traces of dissolved methane and organic suspended solids.

The trapped carbon dioxide in the ice does not anticipate the rapid exchange processes with the atmosphere part.

Lithosphere

The lithosphere includes the outer solid rock layers of the earth. With 99.95% share of global total carbon, the lithosphere of the largest carbon storage dar. However, the flow rates are low. It is thus part of the long-term carbon cycles.

Sediments and carbonate rocks resulting from:

Coal, natural gas, petroleum 4,100 Gt C
Pedosphere with humus, peat, sediments, minerals 1,500 Gt
Graphite

Gas hydrates are under "normal" conditions, gaseous substances, on whose molecules with weak binding forces water molecules are attached in a regular arrangement. The addition of water molecules occurs under certain conditions: solution in water, low temperature, high pressure. The resulting hydrates are mostly solid. Especially methane hydrates are important for the carbon cycle. The methane molecules are trapped with them in cavities of the crystal lattice (see clathrate ). They are found in marine sediments and in permafrost. The methane produced by anaerobic methane hydrates microbial decomposition of organic matter. When supersaturation of the water with methane and at temperatures just above freezing and at high pressure ( in the sea from 500 m depth), the methane hydrates form. By changing the pressure and temperature conditions, larger amounts of methane can be short-term free and escape into the atmosphere.

The outgassing from the deposits of methane can be used under anoxic conditions chemoautotrophic archaea: Obligate anaerobic methanoxidiernde Methanosarcinales form acetic acid ( ethanoic acid ) of methane:

This Ethan acid is used by the bacterium Desulfosarcina used ( in a symbiosis with the mentioned Methanosarcinales ) for energy in the so-called sulfate respiration:

It is estimated that 300 million tons of methane are consumed annually by this symbiosis, which is more than 80 % of the methane produced by archaea in the sediment. Under oxic conditions, methane may be oxidized to carbon dioxide and water by aerobic methane-oxidizing bacteria in full with elemental oxygen ( O2):

Biosphere

Carbon is in the universe and the earth is a relatively rare element ( percentages mean atom number ratios ):

Most common elements in the universe: hydrogen ( 92.7 %) and helium ( 7.2%), (carbon, however, only 0.008 %)
Most elements of the earth's crust: oxygen 49%, Iron 19%, 14% silicon, magnesium, 12.5 % (carbon, however, only 0.099 %)
Most common elements in the human body: hydrogen ( 60.6 % ), oxygen ( 25.7 % ) and carbon ( 10.7%)

A development of carbon-based life is therefore only possible if the species make global carbon cycles advantage and even re- create a closed carbon cycle.

Storage forms of carbon in the biosphere to an organic material, on the other carbonates (typically calcium carbonate CaCO3). Of particular importance are the building materials for skeletons, so outer skeletons of organic substances: chitin in arthropods ( crustaceans, arachnids, insects), exterior skeletons of carbonates in molluscs, foraminifera and Coccolithophoridae, internal skeletons of carbonates with corals, these form a yearly average of 640 million tonnes of reef limestone.

Terrestrial ecosystems contain 800 Gt C, navy 3 Gt C in the biosphere, which corresponds to a total of a share of 0.001 % of the global total carbon. Thus the biosphere is one like the atmosphere of the smallest carbon stores, but is the driver of short-term cycles.

Pedosphere

In the bottom there are an estimated 1100 to 1600 Petagramm carbon. This is twice as much as in living plants (560 Petagramm ) and in the atmosphere ( 750 Petagramm ) is located.

Processes within the systems

Atmosphere

Within the atmosphere take place physical transport processes mainly. Since wind takes place a constant mixing, the concentration of CO2 in the lower layers of the atmosphere is the same everywhere.

Only in places that are protected for a long time against wind, to CO2 can accumulate in the bottom. Example: Kohlenstoffdioxidseen in mines or caves that lie in volcanically active areas.

Methane is oxidized in the course of time into CO2 ( and water).

Hydrosphere

Transport processes

Physical carbon pump: In the sea is by sinking water masses a short-term transport of 33 Gt C per year to great depths of the ocean instead.
Biological carbon pump: Sinking marine organisms transported long term 11 Gt C per year to the bottom of the ocean.

Chemical reactions and equilibria

A chemical equilibrium exists between the various forms of inorganic carbon (the percentages are valid for the conditions T = 10 ° C, pH = 8, salinity 34.3 ‰ - as for example, in large parts of the oceans prevail):

Changes in the conditions and concentrations also change the equilibrium position. Thus, an increase in the CO2 concentration in the atmosphere would shift the equilibrium to the right, the hydrosphere would therefore absorb increased carbon dioxide. On the other hand, would shift the equilibrium to the left global warming.

Lithosphere

Sedimentation

With sedimentation sparingly soluble inorganic and organic substances slowly fall to the ground. The settling velocity depends on the particle size and density of the water and can be very low in undisturbed water. In the carbon cycle, the sedimentation of the calcium carbonate skeletons of Coccolithophoridae plays a major role.

Diagenesis

Diagenesis is the long -term consolidation of loose sediments by biological, chemical and physical transformations. Here, for example, the lime skeletons of microorganisms limestone. Organic deposits are gradually converted into inorganic or other organic substances, under certain conditions, such as prevail in oxygen-poor, warm, shallow seas. There arise kerogens ( for example, in oil shale), tars (bitumen ), charcoal, graphite and oil as well as methane. The Diageneserate is 0.2 Gt C per year.

Metamorphosis

Metamorphosis is the long -term conversion of solid rock on the basis of elevated pressure and temperature: Determined by subduction of sediments of the seafloor pressure and temperature are increased. At the interface of lime and Silicatsedimenten (sand ) following chemical conversions take place:

The thus released CO2 dissolves in the liquid magma and then when a volcano erupts or escapes freely through the same fissures or volcanoes. Tectonic change the resulting silicate will be transported to the surface and exposed to weathering.

Biosphere

Within the biosphere is a carbon flow instead of producing the organic substances autotrophic organisms to organic matter consuming heterotrophic organisms. Through wind and animals organic material is transported. A closed circuit is only possible through the mediation of atmosphere and hydrosphere.

Carbon component circuits

A memory is both source and sink for carbon fluxes.

Between the carbon saving is a constant exchange takes place through chemical, physical, geological and biological processes.

Long-term inorganic cycle

This is to geochemical processes that can take place over a period of several thousand to billions of years.

Mechanical weathering

Due to thermal stresses ( eg frost damage ), pressure ( eg glaciers ), and wind and water erosion large rock blocks can be divided into smaller and smaller portions. By this river crushed material is transported and deposited back into the estuaries. These sediments can be re- subjected to subduction of metamorphosis.

Chemical weathering

Weathering of limestone and silicate rock escapes through the mediation of water CO2 from the atmosphere. The resulting hydrogen is soluble and remains in the hydrosphere.

Calcitverwitterung: ( See also: karst, sinkhole, cave )

Dolomite weathering:

Silicate weathering:

By entering subduction SiO2 ( quartz sand) and CaCO3 (lime) below the earth's crust. There they are fused by the heat and react to silicate and CO2 which then in turn pass through volcanoes on the earth's surface. This cycle is called the carbonate silicate cycle. It is tied more CO2 than is emitted, so that the CO2 content of the atmosphere is reduced.

If the weathering of limestone by other acids, for example sulfuric acid, which can arise from volcanoes emitted hydrogen sulfide and sulfur dioxide by oxidation and reaction with water, CO2 is emitted to the atmosphere:

Precipitation

From a saturated calcium bicarbonate solution is precipitated by increasing the pH of calcite, in which CO2 is released:

This reaction is enhanced in particular by increasing the pH ( basic) due to CO2 consumption ( autotrophic organisms! ) And by high water evaporation. ( See also: stalactites, stalagmites, sinter terrace)

Organisms such as mussels, snails and protozoa also perform a calcite precipitation in order to build skeletons, shells and housing. Of particular importance are small marine organisms ( foraminifera and coccolithophores ) whose exoskeletons sediment after the death of the organisms to form carbonate sediments, and corals that build coral sticks of calcium carbonate. About coral reefs, the CO2 concentration is significantly increased. All reefs of the Earth ( 285,000 km ²) cases estimated from 0.64 Gt of calcium carbonate per year. Here, about 0.28 Gt CO2 are released. Of these, however, only a portion is discharged to the atmosphere (see also: Climate history ).

The cycle is closed in two ways again:

Long-term organic cycle

This is to biochemical processes that initially run quickly, but are coupled with long-term geological processes. It sedimented organic material is not completely degraded under anoxic conditions. Only a small part is converted by anaerobic bacteria in CO2. By overlaying with other sediment blankets and sinking into greater depths the pressure and temperature increase. Thus, the organic biomolecules are hermetically sealed in kerogen ( limited to: hydrocarbons) or carbon ( coal ) is converted.

Crude Oil: From the kerogen of the rocks (petroleum source rocks ) may be caused by further conversion of oil. Traversal ( " migration " ) arise from oil reservoirs. The oldest petroleum deposits are believed to be 3 billion years old. Main time of origin of oil was above 500 to 1000 million years ago. It originated in lagoon, warm shallow seas from sinking down dead plants and animals. Through cracks and fissures in the rock, the gaseous hydrocarbons, primarily methane (CH4), occur at the Earth's surface. In the sea bacteria can use this gas as an energy source by oxidizing it to CO2:

Trespassing on the surface of oil loses the volatile compounds and solidified to viscous asphalt, bitumen or mineral wax (see: asphalt lake ).

Coal: coal deposits originated from the forest bogs of the Carboniferous period, about 350-290 million years ago. If shipped by tectonic processes coal to the surface, it may be oxidized to CO2 by bacteria.

Short-term organic cycle

This is to biochemical processes of assimilation and dissimilation, which can quickly run and are subject to seasonal fluctuations.

By the photosynthesis of plants, algae, bacteria and organic substances are produced from CO2 by using the light energy.
Through cellular respiration is carbon from these materials using oxygen again oxidized to CO2. Many organisms operate under oxygen-deficient fermentation, where the organic materials are mineralized to CO2 or incompletely broken down to other organic substances such as methane.

In the North Atlantic occur in swarms of salps ( Salpa asperia ), which can be spread over an area of up to 100,000 square kilometers. They feed on phytoplankton, which covers its carbon demand from the atmosphere. The salps excrete the waste from the beads that fall at a speed of 1000 meters per day to the bottom of the sea, and thus settle the fixed atmospheric carbon. It is estimated that several thousand tons of carbon per swarm the atmosphere each day be deprived in this way.

Human intervention in the carbon cycle

Causes of the increase of the carbon dioxide concentration of the atmosphere

From the analysis of holes in the Antarctic ice (blue curve) shows that the global carbon dioxide concentration by volume of the atmosphere has never exceeded 300 ppm for at least the last 650,000 years. During the ice ages it was lower than during the warm periods with 180 ppm. With the beginning of industrialization, the concentration increased exponentially. ( The red curve is derived from continuous measurements of the GAW station Mauna Loa in Hawaii since 1958. )

From these measurements is currently an increase in the CO2 content of the atmosphere results corresponding to 3.2 Gt C per year, which contributes to global warming.

By burning fossil fuels containing carbon (oil, gas, coal) and by the production of cement produced carbon dioxide corresponding to 7.1 Gt C per year (figures for 1980 ). Meanwhile, the annual CO2 emissions have increased to about 8.7 Gt. Of the 7.1 Gt C, the oceans absorb 2 ± 1 Gt C per year, as the increased CO2 concentration in the atmosphere shifts the diffusion equilibrium to the side of the dissolved carbon. The amplified by the same effects photosynthesis of land plants from the atmosphere 1.5 ± 0.7 Gt C per year, so remain 3.2 Gt C in the atmosphere and lead to an increase in CO2 concentration (3.2 Gt C correspond approximately to a increase in CO2 concentration by 1.5 ppm, the value currently stands at 3-4 ppm).

The combustion of recent carbonaceous fuels ( canola oil, timber, slash and burn ) should not contribute to an increase than surgery in the short-term biochemical cycle.

In cement production, calcium carbonate reacts with clay ( aluminum silicate) to a calcium silicate. Here, carbon dioxide is released. Of these, only a part of the bonding by the formation of calcium carbonate is again removed from the air.

The lime mortar used previously represented a closed CO2 cycle: The CO2 formed during the firing was tied again upon curing.

Also in glass manufacturing is carbon dioxide released (see metamorphosis ):

Estimates of potential new carbon sinks

Reafforestation

Afforestation and better management (erosion control, selection of species, changes of use in plantations, conversion of fields into pasture and other measures ) increase the effectiveness of CO2 - consumption by the photosynthesis of crops. This results in a consumption from 1.202 to 1.589 Gt C per year. ( The range of estimates arises from the uncertainty in the estimate of the effect of newly reforested woods, hitting with 0.197 to 0.584 Gt C per year to book. ), However, is offset by a release of 1,788 Gt C per year by burning. The role of the oceans in the global carbon cycle, particularly in their role as a carbon sink, has been studied among others in the international research project 1990-2002 JGOFS.

Sustainable use of wood

Through sustainable use of timber, the increase in carbon dioxide concentration in the atmosphere and thus the greenhouse effect can be mitigated. Through the use of wood in a sustainably managed forest of carbon stored in the wood is removed from the atmosphere over a long period. Without wood use, for example in a natural or jungle, the stored carbon is released through decomposition of the trees again as carbon dioxide to the atmosphere. The forest in Germany stores carbon equivalent to approximately 8 % of the annual carbon dioxide emissions. Thus, by increased use of wood, such as construction or in furniture construction, long-term carbon stored in the atmosphere. A wooden desk saves about 23 kg of carbon equivalent to about 83 kg of carbon dioxide. A garden bench made from local wood about half. For comparison, 83 kg of carbon dioxide equal to the amount that ejects a German passenger cars in the middle, to travel a distance of around 520 km. After the useful life of the wood can by recycling continue to be held as a carbon sink, or the stored energy can be used by combustion, in which the same amount of carbon dioxide produced, was withdrawn as before the formation of the atmosphere.

Follow

Effects on photosynthesis

For the photosynthesis of land plants, a carbon dioxide - volume concentration of 1 ‰ would be optimal. The increase in the rate of photosynthesis, however, falls less than expected, as the company responsible for the carboxylation enzyme ( Rubisco ) reacts temperature dependent. As a result of rising temperatures will reduce the carboxylation rate of Rubisco. This applies to the C3 photosynthesis. Assuming that the rate of oxidation of organic matter increases according to the thermo-chemical principle with a temperature increase, there is a dynamic positive feedback both processes with the result of faster rising atmospheric CO2 levels: rising CO2 content of the atmosphere - temperature rise - increase in the oxidation rate at lower or not adequately increasing carboxylation rate - rising CO2 content of the atmosphere. This momentum is slowing, carboxylation and oxidation rate when approaching more and more. If the oxidation rate under the carboxylation rate, sink of atmospheric CO2 concentration and temperature. The Rubisco responds to falling temperatures with higher Carboxylierungsraten. Again, there is again a positive feedback with then quickly falling temperatures and CO2 levels. The cause of the dynamics can be found in the temperature-dependent bifunctionality of Rubisco. It is for the current situation of climate change to the following conclusion: If the anthropogenic dynamics of rising temperature and CO2 content of the atmosphere once recorded "Ride ", a momentum will develop that will be unstoppable by human hands.

Disruption of circuits

The increase in carbon dioxide in the atmosphere leads to increased dissolution of CO2 in seawater. By the formation of carbonic acid, the pH value of the water is lowered (acidic ). Thus, the biogenic and abiogenic precipitation of lime is disabled. As a result, should the amount of phytoplankton decrease and decrease the rate of photosynthesis.

By lowering the pH of rain and water would have the weathering of limestone and thus the consumption of CO2 increase. Geochemical as flow rates are very low, this effect is not important in the short term.

Implications for climate change

Without human intervention in the course of Earth's development is a relatively stable steady state has been reached. Each participant of the circuit emits carbon and takes on what, without this resulting in significant changes in the distribution of carbon.

By burning fossil fuels enters carbon that has been stored for millennia, in the form of CO2 in the atmosphere. Overall, " produced" humanity at the dawn of the 21st century, about 8.7 Gt C per year. The delicate balance is disturbed. The result is global warming, to which contributes significantly to the growing share of the greenhouse gas CO2 in the Earth's atmosphere.

To counteract this, an attempt is made to develop procedures to withdraw the excess carbon from the atmosphere and store it in the reservoir sediments (CO2 sequestration ).

Problems of technical solutions

Currently, solutions of the CO2 problem are discussed, which are already technically feasible but can not be controlled and can not be estimated in the environmental damages:

Liquefying CO2 and incorporation into underground gas-tight storage as mineable coal seams not, degraded salt deposits, deep water-bearing rock layers and depleted oil and gas fields. The latter is already used in " Sleipner ", a gas and oil platform of the Norwegian company Statoil, where weekly incurred 20,000 tonnes of CO2 will be pumped back into the exploited deposits. This method is considered relatively safe because the sand layers of the reservoir crude oil and natural gas have held over millions of years.
Dumping of frozen CO2 ( so-called dry ice) in the sea. During lowering, the dry ice dissolves, whereby the concentration of CO2 increases and the pH of the water is lowered, which leads to local poisoning organisms.
Liquefying and injecting into the sea at a depth of 600 m. There should form gas hydrates, which would fall due to their higher density to the ground. Tests showed that while the formation of gas hydrates, but they rose to the surface again.
Liquefying and injecting into the deep ocean below 3000 m. Here, the CO2 would remain liquid and form large Kohlenstoffdioxidseen in sinks. Tests showed that the CO2 did not remain liquid, but large-volume gas hydrates formed, which could rise again. Since the formation of gas hydrates increases the concentration of the salt content of the surrounding water by freezing of Süßwassereis, here all organisms would be affected. In addition, the CO2 at this depth was not controlled: it unexpectedly took a large volume and could not be collected in sinks, as even the slightest currents distributed CO2 drops. This version is favored by the Government of the United States. A large-scale trial in the summer of 2002 off the coast of Hawaii and Norway was stopped by the massive resistance of the inhabitants, and several environmental groups for the time being.

Comments

The figures are estimates and may vary greatly depending on the literature used. It is not always clear what is summarized under the respective carbon fluxes. Not always the information is complete. This causes problems such as in the carbon balance of the terrestrial biosphere: the inflow of 120 Gt C per year through the assimilation is offset by an outflow of only a total of 116 Gt C per year by dissimilation and detritus formation. This lack of balance 4 Gt C per year.

Abbreviations

GAW = Global Atmosphere Watch (see: gaw.web.psi.ch! )
IPCC = Intergovernmental Panel on Climate Change (see: www.ipcc.ch! )
JGOFS Joint Global Ocean Flux Study = (see: wdc mare.org! )

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